TY - JOUR
T1 - Depleted lithosphere, cold, trapped asthenosphere, and frozen melt puddles above the flat slab in central Chile and Argentina
AU - Wagner, Lara S.
AU - Beck, Susan
AU - Zandt, George
AU - Ducea, Mihai N.
N1 - Funding Information:
This material is based on work supported by the National Science Foundation under Grant No. EAR9811870 at the University of Arizona. Seismic data was acquired with instrumentation provided by the IRIS/PASSCAL program. We acknowledge INPRES (Argentina), the University of Chile, and the University of San Juan for their logistical support and assistance in the field. We thank the entire CHARGE working group for their many efforts and contributions. We would also like to thank our anonymous reviewer and Sue Kay for their insightful and helpful comments. This material is based upon work supported under a National Science Foundation Graduate Research Fellowship to L. Wagner.
PY - 2006/5/15
Y1 - 2006/5/15
N2 - Recent studies of the upper mantle above the flat slab in central Chile and Argentina indicate the seismic velocity structures in this area are very different from those found in subduction zones with "normal," steeper slab geometries. The mantle above the horizontal section of the flat slab is characterized by low P-wave velocities, high S-wave velocities, and low Vp / Vs ratios. As the slab begins to transition to a more normal dip to the south, the mantle above it changes as well. Above this "transition zone", the mantle is characterized by high P-wave velocities, high S-wave velocities, and generally high Vp / Vs ratios. Above this high velocity anomaly, in the corner between the south-dipping slab of the transition zone and the east-dipping slab of the normal subduction zone, lies a small shallow anomaly distinguished by its very low P- and S-wave velocities and its moderately high Vp / Vs ratio. We interpret these anomalies to represent very depleted cold lithosphere, cooled trapped asthenosphere, and frozen pooled melt, respectively. Recent research looking at the elastic properties of cratonic xenoliths indicate that a low Vp, high Vs, and low Vp / Vs signature is indicative of cratonic lithosphere. This suggests that the material above the flat slab may be ancient Laurentian lithosphere, or other melt-depleted, dry lithospheric material. The presence of frozen asthenosphere to the south of the flat slab may indicate that large volumes of asthenosphere were displaced in advance of the eastward progression of the flattening slab between 10 and 2 Ma, forcing a change in flow patterns from trench normal to SSW around the south eastern corner of the flattening slab. As flattening progressed this asthenosphere became trapped above the transition zone. Eventually, temperatures dropped sufficiently to freeze the last remaining pocket of extracted melt which had collected in the corner of the descending slab.
AB - Recent studies of the upper mantle above the flat slab in central Chile and Argentina indicate the seismic velocity structures in this area are very different from those found in subduction zones with "normal," steeper slab geometries. The mantle above the horizontal section of the flat slab is characterized by low P-wave velocities, high S-wave velocities, and low Vp / Vs ratios. As the slab begins to transition to a more normal dip to the south, the mantle above it changes as well. Above this "transition zone", the mantle is characterized by high P-wave velocities, high S-wave velocities, and generally high Vp / Vs ratios. Above this high velocity anomaly, in the corner between the south-dipping slab of the transition zone and the east-dipping slab of the normal subduction zone, lies a small shallow anomaly distinguished by its very low P- and S-wave velocities and its moderately high Vp / Vs ratio. We interpret these anomalies to represent very depleted cold lithosphere, cooled trapped asthenosphere, and frozen pooled melt, respectively. Recent research looking at the elastic properties of cratonic xenoliths indicate that a low Vp, high Vs, and low Vp / Vs signature is indicative of cratonic lithosphere. This suggests that the material above the flat slab may be ancient Laurentian lithosphere, or other melt-depleted, dry lithospheric material. The presence of frozen asthenosphere to the south of the flat slab may indicate that large volumes of asthenosphere were displaced in advance of the eastward progression of the flattening slab between 10 and 2 Ma, forcing a change in flow patterns from trench normal to SSW around the south eastern corner of the flattening slab. As flattening progressed this asthenosphere became trapped above the transition zone. Eventually, temperatures dropped sufficiently to freeze the last remaining pocket of extracted melt which had collected in the corner of the descending slab.
KW - Argentina
KW - Chile
KW - flat-slab
KW - harzburgite
KW - tomography
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U2 - 10.1016/j.epsl.2006.02.014
DO - 10.1016/j.epsl.2006.02.014
M3 - Article
AN - SCOPUS:33646072766
SN - 0012-821X
VL - 245
SP - 289
EP - 301
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
IS - 1-2
ER -