PhD thesis,
3-D Lokalbebentomographie der südlichen Anden zwischen 36 und 40S
Freie Universität Berlin, Berlin, Germany, (2004)
Abstract
The Southern Andes are part of the South American active continental
margin, which was built by a combination of subduction related processes.
The Andean mountain chain covers the whole South American continent
from north to south, but varies remarkeably in elevation and crustal
thickness. Whereas the Central Andes, above all the deeper structures
as far as the upper mantle, are studied quite intensively (SFB267),
so far little is known about the geophysical constitution of the
Southern Andes. For this reason the seismic experiment ISSA2000 was
carried out in the Southern Andes. In this study a 3-D model of the
seismic structure was derived using local earthquake tomography.
The simultaneous localisation of the registrated earthquakes results
also in a detailed picture of the distribution of seismicity in the
area under investigation. Within the framework of the ISSA2000 experiment
a seismological network with 62 seismographs was operated, covering
the subduction zone from the coast to the backarc between 36 and
40S. The hypocenters of 440 local earthquakes show a \~30 deg dipping
Wadati-Benioff zone with continuous seismicity until 120km depth,
which can be observed partly until 200km depth. Both the general
low seismicity in the area under investigation, the low depth extent
of the Wadati-Benioff zone (especially when compared to the Central
Andes) and the concentration of the intermediate depth seismicity
in 60km depth reflect most probably the influence of theyoung age
and therefore high thermal state of the Nazca plate to the subduction
regime. The tomographic Vp model shows for the first time the velocity
structure of the Southern Andes until a depth of 100km. The downgoing
Nazca plate is defined by the location of the earthquakes and characterized
by high Vp velocities (7.2-8.3km/s). The mean velocities in the continental
crust are relatively high throughout the whole Vp model (5.5-6.5km/s).
The lower structures can be divided into three subregions. (1) Low
Vp velocities in 30-50km depth beneath the Coastal Cordillera and
above the downgoing plate indicate a possible hydratization and serpentinization
of the continental mantle wedge. The concentration of earthquakes
in 60km depth, which can be attributed to dehydration processes within
the subducted oceanic lithosphere, could be connected to the observed
velocity anomaly. Mantle velocities are not obeserved in this area.
(2) The area beneath the Longitudinal Valley shows a most prominent
anomaly of increased Vp velocities, which can be observed in the
whole area under investigation from north to south. Mantle velocities
are reached here in 40km depth.(3) Further east crustal thickness
increases and reaches 50-55km below the Main Cordillera and active
volcanic arc. Low velocities, an increased Vp/Vs ratio and a zone
of increased seismic attenuation which can be seen in the spectral
analysis suggest the existence of partial melts and its possible
attachment below the crust (magmatic underplating). urn:nbn:de:kobv:188-2005000078
