SEISMIC IMAGING OF A MODERN CORDILLERAN BATHOLITH IN THE CENTRAL ANDES (Invited Presentation)

Cordilleran mountain belts, such as the modern central Andes and Mesozoic western U.S. formed in regions of significant upper plate compression and were punctuated by high flux magmatic events that coalesced into large composite batholiths like the Mesozoic Sierra Nevada Batholith in the western US. Unlike the western US, compressive mountain building is still active in the central Andes and any large modern batholith still resides at depth and its location and structure must be inferred from surface volcanics and geophysical data. In the Andes it is widely presumed that a modern batholith exists beneath the Western Cordillera, the location of the mostly andesitic active arc; however, this “batholith” has only been geophysically investigated in a few widely spaced locations. Combining data from an international suite of broadband seismic stations operating in phased deployments over the past 18 years, we have recently completed an ambient noise tomography (ANT) study of the central Andes from 12° to 32° S latitude. This study provides for the first time uniformly high-resolution 3-d images of the vertically-polarized shear-wave velocity (Sv) of the crust from the surface to a depth of ~70 km. A major feature in this model is a nearly continuous low-velocity body (LVB) located between depths of ~10 to 30 km and geographically situated along the western margin of the Central Andean Plateau (CAP). The active arc composed of mostly andesitic stratovolcanoes is consistently located on the margins of this LVB. In areas where late Cenozoic silicic ignimbrites and their corresponding caldera structures are well mapped there is excellent agreement between the locations of the caldera complexes and the shape of the LVB. In addition, there is general agreement between the location of the LVB and isostatic gravity low anomalies. Based on these correlations, we suggest the LVB is a composite granitoid batholith built in Late Cenozoic time. The identification of a modern batholith closely associated with Neogene ignimbrite fields supports the idea that these ignimbrites are the surface manifestation of batholith formation at depth. The large volume of this batholith and its location on the western margin of the high-standing plateau also supports the idea that magmatic addition has played a major role in the uplift of the CAP.