Subduction and collision processes in the Central Andes constrained by converted seismic phases

Abstract

The Central Andes are the Earth's highest mountain belt formed by ocean-continent collision. Most of this uplift is thought to have occurred in the past 20 Myr, owing mainly to thickening of the continental crust, dominated by tectonic shortening. Here we use P-to-S (compressional-to-shear) converted teleseismic waves observed on several temporary networks in the Central Andes to image the deep structure associated with these tectonic processes. We find that the Moho (the Mohorovicić discontinuity--generally thought to separate crust from mantle) ranges from a depth of 75 km under the Altiplano plateau to 50 km beneath the 4-km-high Puna plateau. This relatively thin crust below such a high-elevation region indicates that thinning of the lithospheric mantle may have contributed to the uplift of the Puna plateau. We have also imaged the subducted crust of the Nazca oceanic plate down to 120 km depth, where it becomes invisible to converted teleseismic waves, probably owing to completion of the gabbro-eclogite transformation; this is direct evidence for the presence of kinetically delayed metamorphic reactions in subducting plates. Most of the intermediate-depth seismicity in the subducting plate stops at 120 km depth as well, suggesting a relation with this transformation. We see an intracrustal low-velocity zone, 10-20 km thick, below the entire Altiplano and Puna plateaux, which we interpret as a zone of continuing metamorphism and partial melting that decouples upper-crustal imbrication from lower-crustal thickening.

More constraints to determine the seismic structure beneath the Central Andes at 21°S using teleseismic tomography analysis

“…Fluids have also been detected under the active volcanic front and below the Altiplano-Puna plateau and are thought to follow different pathways in the lithosphere . On the eastern border of the Altiplano, Wigger et al (1994) identified west-dipping structures in the crust, also confirmed by Yuan et al (2000) that are possibly related to the underthrusting of the Brazilian Shield beneath the Subandean and the Eastern Cordillera provinces. This situation has also been suggested by other authors (e.g.…”

“…Beneath the Altiplano-Puna region, a lowvelocity layer has been detected and explained as the possible presence of partial melting material (e.g. Chmielowski et al, 1999;Yuan et al, 2000;Brasse et al, 2002;Zandt et al, 2003). Fluids have also been detected under the active volcanic front and below the Altiplano-Puna plateau and are thought to follow different pathways in the lithosphere .…”

“…The results obtained express clear associations between the passive and active seismic experiments and geological observations. Related to the subduction zone, teleseismic receiver function analysis imaged the oceanic Moho down to a depth of 120 km (Yuan et al, 2000) coinciding with a reduction in seismic activity in the slab at this depth. Short period conversions from local earthquakes extend to depths of 160 km (Bock et al, 2000).…”

“…In our case, the Moho depth along 21˚ is from receiver function data (Yuan et al, 2000 andHeit et al, 2007 andWoelbern et al, 2007).…”

Migrating deformation in the Central Andes from enhanced orographic rainfall

“…Th e current topographic slope on the eastern side is ca 2.2 -2.6 ° , and is maintained by an actively deforming wedge that is detached above the < 4 ° dipping ALVZ 39 ( Fig. 1c ) within the critical to subcritical fi eld ( Fig.…”