 |
| Titel |
The transition zone below the Chile-Argentina flat subduction region |
| VerfasserIn |
Luciana Bonatto, Claudia Piromallo, Gabriela Badi |
| Konferenz |
EGU General Assembly 2017
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250152774
|
| Publikation (Nr.) |
 EGU/EGU2017-17656.pdf |
|
|
|
|
|
| Zusammenfassung |
| We study the fine structure of the upper mantle (below 200 km depth) beneath the western
margin of South America, within an area known as the Chile-Argentina flat subduction
zone (between 26∘S and 36∘S). Unlike what happens in most subduction zones, in
this region the Nazca Plate subducts with an angle close to the horizontal -initially
dips underneath the continent and flattens at a depth of approximately 100 km,
remaining almost horizontal for about 300 km before descending more steeply
into the mantle. Moreover, the flat slab follows the path of the subducting Juan
Fernández Ridge, a hot spot seamount chain on the Nazca Plate. The complex tectonic
setting makes this region an excellent laboratory to explore and quantify the relative
contributions of thermal and compositional heterogeneities to the mantle discontinuity
structure.
In this study we combine data available from four past temporary experiments: 18 seismic
stations from CHARGE; 43 from SIEMBRA, 12 from ESP and 30 from PUDEL. The
research tools are the Pds phases (the direct P wave converted to an S wave while
passing through a seismic discontinuity at depth d). These signals arrive in the coda
of the P-phase in the radial component and are expected to be coherent with the
waveform of the first arrival for conversion at discontinuities thinner than one half of the
P-wavelength. In order to extract these converted phases by means of waveform
similarity, we use the receiver function (RF) technique, i.e. the deconvolution of the
vertical from the radial component in the frequency domain. The Pds phases are then
detected on stacked RF (globally and by common conversion point) in the relative
time-slowness domain. Since the incidence angle of converted phases is larger than the
incidence angle of the P phase, they are expected with negative slowness. This
permits to separate them from the multiples, which are instead expected with positive
slowness.
We measure amplitudes and arrival times for the converted phases at the well-known 410
and 660 discontinuities and at a discontinuity at a depth of about 210 km, which we interpret
as the Lehmann discontinuity. The abrupt amplitude decrease for the P660s phase at
frequencies larger than 0.12 Hz indicates that the velocity jump at 660 km occurs in a depth
interval as wide as 40 km. Besides, the amplitudes of P410s and P660s are similar
at the lowest frequency (0.08 Hz). This analysis suggests that the velocity jump
at both discontinuities is similar or, alternatively that the 660 may not occur as a
discontinuity but as a gradual transition across a layer of about 40 km. We also identify a
negative amplitude signal between P410s and P660s arrival times, with negative
slowness, which we interpret as a converted phase at a negative discontinuity (a
decrease in velocity with depth) at a depth of about 590 km. We also present a map
of the Transition Zone Thickness (TZT) showing lateral variations in the study
area. |
|
|
|
|
|
|