Abstract
We analyse mantle structure under South America in the DETOX-P1 seismic
tomography model, a global-scale, multi-frequency inversion of
teleseismic P-waves. DETOX-P1 inverts the most extensive data set of
broadband, waveform-based traveltime measurements to date, complemented
by analyst-picked traveltimes from the ISC-EHB catalogue. The mantle
under South America is sampled by ~665,000
cross-correlation traveltimes measured on 529 South American broadband
stations and on 5389 stations elsewhere. By their locations, depths, and
geometries, we distinguish four high-velocity provinces under South
America, interpreted as subducted lithosphere (’slabs’). The deepest
(~1800-1200 km depth) and shallowest (<600 km)
slab provinces are observed beneath the Andean Cordillera near the
continent’s west coast. At intermediate depths (1200-900 km, 900-600
km), two slab provinces are observed farther east, under Brazil, Bolivia
and Venezuela, with links to the Caribbean. We interpret the slabs
relative to South America’s paleo-position over time, exploring the
hypothesis that slabs sank essentially vertically after widening by
viscous deformation in the mantle transition zone. The shallowest slab
province carries the geometric imprint of the continental margin and
represents ocean-beneath-continent subduction during Cenozoic times. The
deepest, far-westerly slab complex formed under intra-oceanic trenches
during late Jurassic and Cretaceous times, far west of South America’s
paleo-position adjoined to Africa. The two intermediate slab complexes
record the Cretaceous transition from westward intra-oceanic subduction
to eastward subduction beneath South America. This geophysical inference
matches geologic records of the transition from Jura-Cretaceous,
extensional “intra-arc” basins to basin inversion and onset of the
modern Andean arc ~85 Ma.