Publications

The central Andes are formed by a complex interaction of both subduction related and tectonic processes on a lithospheric scale. In this thesis, the deep structure of the entire mountain range and underlying subduction zone has been investigated with local earthquake data. Seismologic data from the PUNA experiment, a temporary array of 60 seismographs deployed across the Andes at approximately 23.5°S, were analysed. P and S arrival times of 426 intermediate depth earthquakes were inverted for 1-D average velocity structure and hypocentral coordinates. Average velocities and Vp/Vs in the crust are low, indicating a felsic composition. Average mantle velocities are high but difficult to interpret because of the presence of a fast velocity slab at depth. Although the hypocenters sharply define a 35° dipping Benioff zone, seismicity in the slab is not continuous. The spatial clustering of earthquakes is thought to reflect inherited heterogeneities of the subducted oceanic lithosphere. Additionally, 57 crustal earthquakes were located. Seismicity concentrates in the fold and thrust belts of the foreland and Eastern Cordillera, and along and south of the El Toro-Olacapato-Calama Lineament (TOCL), a major shear zone cutting though the entire Andes. Focal mechanisms of two earthquakes at this structure exhibit left lateral strike-slip mechanisms similar to the suggested kinematics of the TOCL,. Further seismicity in the Puna is restricted to the area south of the TOCL. Data from the PUNA network were merged with data from the earlier PISCO and ANCORP experiments for a combined tomographic inversion. Although the deployments did not coincide in time, spatial overlap was achieved by re-occupying existing sites. Travel times and t* operators from P wave spectra of about 1600 earthquakes were inverted for 3-D models of Vp, Vp/Vs, and P wave attenuation. All three attributes provide a consistent image of the entire subduction zone on a lithospheric scale. The tomographic images reveal low velocities and high attenuation in crust and mantle underlying the Western Cordillera and most of the Puna plateau, indicative of weak rheology and mostly asthenospheric mantle. In contrast, forearc and eastern foreland are characterized by high Qp values, in accordance with thermal models. The lithosphere beneath the Atacama depression was found to be seismically very strong. It is believed to be an old, cold structure, that displaces hot isotherms and also the volcanic front a 100 km to the east. Although the block has clearly subsided, its seismic properties are hard to reconcile with extension and crustal thinning as has been suggested in previous studies. Instead, it is proposed that the block subsides as a whole along weak zones, characterized by low velocities and low Qp values, surrounding it. Low Qp regions in mantle and crust are interpreted as indicating presence of aqueous fluids and partial melts. Continuous anomalies connect the different earthquake clusters in 100 and 200 km depth and the active volcanoes in the Western Cordillera and in the backarc. It is suggested that water is conveyed into the mantle wedge due to earthquake ruptures that may trigger hydro-fracturing. Water then fluxes melting in the hot mantle wedge. Melt ascent ways, as imaged by seismic Qp, are not straight up to the base of volcanoes, as is often implicitly assumed. Instead, melt sources are located on significantly different depth levels, and ascent ways follow different patterns and cover significant horizontal distance. Between 23°S and 24°S, a high velocity, high Qp structure beneath the Eastern Cordillera and eastern Puna is interpreted as delaminating and detaching continental lithosphere that has been thickened in the orogenic process. South of this structure, the mantle is characterized by low velocities, high Vp/Vs ratios, and low Qp values. It is believed that here lithosphere originally underlying Andean crust has already been removed. This is supported by new estimates of crustal thickness and volcanic activity. It is proposed that lithospheric Delamination beneath the Puna is sanctioned by inherited weak lithospheric rheology and weakening due to pure shear deformation.

---

Schurr, B. (2001): Seismic Structure of the Central Andean Subduction Zone from Local Earthquake Data. Scientific Technical Report ; 01/01, GeoForschungsZentrum GFZ, 125.