IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS014 Poster presentation 2327
Spatial pattern similarities between small magnitude seismicity in the
argentinian backarc continental crust (m < 3.5) and in the Nazca Plate (M
< 4.5) at its transition from flat to normal subduccion
Prof. Enrique Triep
Subcom. de Sismologa y Fs. del Int. de la Tierra Member
Renzo Furlani
It is generally accepted that the flat subduction process between latitudes ~28-33S is at least in part a
consequence of the thickened oceanic crust of the Juan Fernndez ridge, and that it is related to the high
crustal upper plate seismicity and to the broad scale uplift process of the Precordillera and Sierras
Pampeanas. Small earthquakes from a three months broadband experiment deployed mainly in the
backarc of west-central Argentina, between latitudes 31.5-33.5S, show pattern similarities between the
seismicities in the double lithosphere: the one in the subducted Nazca plate within the ~100-250 km
depth range and the one in the continental crust of the South America plate. Clusters and alignments in
both seismicities have spatial correspondences. Two neighboring zones are distinguished: 1) North of
~31.7S, just above the Juan Fernndez ridge trace, the superficial seismicity is displaced eastward with
respect to deeper one due to the predominant direction progression of the ridge. Vertical stresses are
favourably transferred by a continental upper mantle without an asthenospheric wedge. The
consequence is the high seismicity of the zone. Also, as one example, the vertical component of
accumulated stresses could be linked to the uplift of the Sierra de Pie de Palo. 2) Between ~31.7-33.5S,
where there is the major transition from flat to normal subduction, the superficial seismicity is displaced
southeastward with respect to the deeper one. Both seismicities have lined shapes branches that
embrace the base and top of an aseismic continental lithospheric volume, some what like an inclined
cylinder, that help to visualize their spatial connection. The base of the volume at 105 km depth is on
the strong bending of the contorted plate, and the top includes some crustal regions of Precordillera,
Cerrillada Pedemontana, Huayqueras and eastern plains. The volume coincides with a feature
determined by tomography and characterized by high Vp, high Vs velocity values and relatively high
Vp/Vs ratio, which are consistent with upper mantle mineralogies no longer hydrated or melted to any
significant extent. In these conditions the volume seams to perform as an efficient stress guide. The
stresses can be originated at the deep boundary between the two lithospheres and/or from the
deformation process in the continental upper mantle that fills the space pinched out by the Nazca plate
strong bend. Then, at least for the region and the earthquake range magnitude here considered and
besides the horizontal stresses coming from the relative plate movement, the spatial pattern similarities
between the seismicities in the subducted plate and continental crust show a case of a noticeable effect
of the vertical stress components coming from the deep boundary of the two involved lithospheres
and/or the deformation process of the continental upper mantle produced by the contorted plate.
Keywords: lithosphere, boundary, interaction