Subduction Dynamics in Northwestern South America from SKS and Local S-wave Splitting Measurements

Abstract:

In order to investigate subduction dynamics between the South America continent and the oceanic Nazca and Caribbean plates, we have obtained 257 high-quality splitting measurements from teleseismic SKS phases and 85 splitting results from slab-related local Sphases arriving at 38 seismic stations in northwestern South America. We used data primarily from the Colombia National Seismological Network. Our first observation is that delay times obtained from SKS splitting (mean value of 1.15 s) are consistently greater than those from the local S splitting (mean value of 0.31 s). This implies that most of the SKSsplitting is due to upper mantle anisotropy beneath or within the subducting slabs. In contrast, the polarizations of local S-waves are consistently aligned with crustal-scale fault systems, which implies that the anisotropy above the slabs is dominated by tectonic features in the overriding crust or lithosphere, and that the mantle wedge is not contributing significantly to the measured splitting. We identify a clear change in SKS fast directions at the trace of the so-called Caldas Tear (~5°N), which represents a boundary in the style of subduction. To the north of Caldas Tear fast directions are consistently parallel to the direction of convergence of the Caribbean plate and Panama arc with South America. Our results support the idea of Caribbean subduction beneath northwestern Colombia, a hotly debated topic in recent years. Fast directions south of the Caldas tear are subparallel to the direction of Nazca-South America convergence. Entrained mantle flow and deformation beneath the subducting Nazca and Caribbean plates are the main cause of the measured SKS splitting. Further inland, the SKS splitting is consistent with mantle flow parallel to the trend of the Eastern Cordillera. We speculate that a change in slab dip may lead to a change in mantle flow and may be responsible for the intermediate-depth nests of seismicity observed beneath Eastern Cordillera.