DRIPABILITY: DOCUMENTING A HETEROGENEOUS VISCOSITY STRUCTURE IN AND BENEATH POTENTIALLY FOUNDERING ARC LITHOSPHERE, NORTHERN VOLCANIC ZONE, ANDES

The creation, refinement, and destruction of lithospheric material in the deep arc roots of continental subduction zones is as challenging to study as it is critical to understanding cordilleran systems. The removal of dense, mafic lower crust due to foundering from gravitational instability has been used as a mechanical explanation for the evolved geochemistry of Earth’s crust. This removal, or ‘lithospheric drip,’ has been proposed globally in ~23 localities including 12 sites in the American Cordillera.

The Northern Volcanic Zone (NVZ) of the Colombian Andes has been proposed as a site of lithospheric foundering. Recent Rayleigh-Taylor Instability (RTI) modeling of the gravitationally unstable root under the NVZ suggests the dense lower crust should not grow thicker than 1 km before foundering within ~1 kyr [1]. However, geophysical and petrological studies reveal there is currently a 10-14 km thick intact deep crustal root in this location.

Young xenoliths (erupted ~280 ka) from the Granatifera Tuff sampled at the Mercaderes locality in the Andean NVZ present a direct, near-modern window into this thick intact arc root that, within our current geodynamic framework, should founder rapidly. Variations from assumed ‘typical’ viscosity structures may be responsible for this root’s stability. We investigate a subset of lower crustal and mantle xenoliths including moderately to highly deformed garnet-bearing gabbros, hornblendites, garnet clinopyroxenites, and peridotites.

Petrologic and microstructural analysis of eight lower crustal xenoliths which equilibrated at 1.2-2.2 GPa and 920-1290 C, when combined with experimentally derived flow laws yield effective viscosities between 10¹⁸-10³³ Pa s. Highly deformed shallow mantle xenoliths from the Mercaderes suite record exceptionally low viscosities of 10⁹-10¹¹ Pa s, while slightly deeper mantle xenoliths are not nearly as deformed and reflect higher viscosities. First order observations from Stoke’s Sinkers and RTIs paired with conceptual frameworks for lower crustal structure inform models for root stability under the Andean NVZ. We suggest a heterogeneous viscosity structure, melt buoyancy, and varying stress and strain rates may be important.

[1] Zieman et al., 2023. Geology, DOI: 10.1130/G50973.1