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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS013 Poster presentation 2239
A highly variable depth of the brittle/ductile transition in the lithosphere
along the Andes Region
Dr. Miguel Muoz
IASPEI
Thermal and gravity data, together with rheological properties of rocks, are used to determine the
brittle/ductile transition in the lithosphere along the Andes region. In the area of the Oca-Ancon fault
system in northern Venezuela (heat flow (Q)=60-70 mWm-2), for acceptable thermal and rheological
model parametrizations, the crustal brittle layer is not thicker than 11-18 km, in contrast to a maximum
thickness of 23 km as suggested by the location of hypocentres. In the southern termination of the
Bocono fault zone, where Q is of about 50 mWm-2, the crustal seismogenic layer is not thicker than 20
km, and in models assuming a dry dunite upper mantle, a brittle layer is obtained beneath the
crust/mantle boundary (CMB) down to about 55-60 km, in accordance to depth solutions of Kafka and
Weidner (1981) where seismic activity at these depths is interpreted to be within the continental
lithosphere and not related to the subducted ancient Farallon plate. In the Central Cordillera of
Colombia and in the central northernmost area of Ecuador, the temperature at the CMB is close to 1000
C, and tectonic seismic activity can be expected to be generated only in the upper 13 km of the crust; in
northeastern Ecuador, Kawakatsu and Proao-Cadena (1991) determined focal depths suggesting a
seismogenic zone of 15 km thickness, and in the sub-Andes of Ecuador, focal depths larger than 30 km
were relocated at 16 km and 10 km (Surez et al., 1983). In the Central Andes plateau, thermal models
suggest that the crust below 15-20 km is in the ductile regime, in accordance with magnetotelluric (MT)
observations and Qp tomography; these last studies show no indication of a fluid curtain from the
descending slab in the fore-arc region (Schilling et al., 2006). Thermal and rheological zonation in areas
of generally low heat flow in Ecuador, Peru, Argentina and Chile indicate that earthquakes occurring at
depths of 70-110 km are generated within the subcontinental upper mantle, and thus they are not
associated to flat subduction of the oceanic lithosphere; in these areas there is no obvious correlation
between the subduction of oceanic ridges a main subject for the flat-slab hypothesis and deformation
processes and structural styles of the orogenic phases (Michaud et al., 2006; Aleman, 2006; Creixell et
al., 2006). In the main Cordillera of central Chile (33 S-35 S), the strength envelope describes a brittle
domain down to about 17-22 km, in accordance with crustal seismic activity. In southern Chile, in the
south volcanic zone, only shallow tectonic seismic activity could be generated; most of the crust is
ductile, and the temperature at the CMB is of about 880-1100 C. MT observations at latitudes of about
39 S show that high electrical conducting zones are encountered only beneath the volcanic front and
eastwards from it, and also linked to some fault systems, the electrical conductance decreasing strongly
towards de coastal range (Muoz et al., 1990; Brasse and Soyer, 2001); a down-going slab is not
resolved by the MT soundings.
Keywords: andes, lithosphere, rheology