Imaging a Precambrian Mantle Suture in the Subandean Lithosphere by Surface Wave Tomography

Abstract:

The Central Andean Plateau (CAP), as defined by the 3 km elevation contour, extends over 1,800 km along the active South American Cordilleran margin reaching a maxim width around 400 km near the Bolivian Orocline making it second only to the Tibetan Plateau in geographic extent. First-order questions about principle segments of the lithospheric structure of the northern CAP remain underdeveloped. As part of the seismological component of the Central Andean Uplift and the Geodynamics of High Topography (CAUGHT) project, we image the crustal and lithospheric structure beneath the northern Central Andean Plateau by jointly inverting surface wave dispersion measured from ambient noise and two-plane wave tomography for a S-wave velocity model of the northern CAP. First-order robust features of our S-wave velocity model include a low-velocity (<4.4 km/s) upper mantle wedge under the Subandes extending east thickening towards the Chaco. Directly under the low-velocity wedge, a high-velocity (>4.5 km/s) eastward dipping feature in contact with the Moho below the eastern-most Altiplano reaches a maximum depth of ~160 km. Previous tomography studies have interpreted this high-velocity feature as the western extent of the Brazilian Craton. Additionally, we observe a strong inverse correlation with topography and mean lithospheric Vs east of the Eastern Cordillera excluding asthenosphere inflow from a foundering lithosphere as a possible explanation for the low-velocity wedge. We combine our model with geochemical studies that delineate two lithospheric mantle blocks and suggest the transition from the eastward dipping high-velocity feature to the low-velocity wedge is the mantle signature of suturing the Arequuipa/Antofalla terrane to Amazonia during the Grenville-Sunsás Orogeny. This interpretation of an intact upper plate lithosphere is inconsistent with recent lithospheric foundering in this region.