HR: 13:55h
AN: T43E-02    [Abstracts]
TI: The role of hydration in the eclogitization of the lower crust of central Chile and Argentina
AU: * Gilbert, H
EM: hgilbert@geo.arizona.edu
AF: University of Arizona, Department of Geosciences 1040 E. 4th Street, Tucson, AZ 85721 United States
AU: Beck, S
EM: beck@geo.arizona.edu
AF: University of Arizona, Department of Geosciences 1040 E. 4th Street, Tucson, AZ 85721 United States
AU: Zandt, G
EM: zandt@geo.arizona.edu
AF: University of Arizona, Department of Geosciences 1040 E. 4th Street, Tucson, AZ 85721 United States
AU: Sakaguchi, K
EM: ksa@geo.arizona.edu
AF: University of Arizona, Department of Geosciences 1040 E. 4th Street, Tucson, AZ 85721 United States
AU: CHARGE working group, t
AF: University of Arizona, Department of Geosciences 1040 E. 4th Street, Tucson, AZ 85721 United States
AB: Instead of marking a sharp boundary between the crust and mantle, the Moho beneath the western Sierras Pampeanas, Precordillera, and Andes in the flat slab region of central Chile and Argentina appears as a diffuse boundary that has been difficult to detect on receiver functions. This observation differs from the clear strong Moho signal detected by both P and S wave receiver functions across much of the eastern Sierras Pampeanas by the CHARGE PASSCAL array. The differences in crustal signature appear to correlate with a major suture zone between the eastern and western Sierras Pampeanas; to the east of which, a strong Moho can be clearly detected near 40 km depth, however to the west, we find only low-amplitude broad arrivals around 50-60 km depth and the presence of mid-crustal arrivals. Interestingly, this change in crustal thickness is not accompanied by a similar change in surface elevations. Numerous recent findings of weak, or absent, Moho signals have been attributed to hydration of the mantle wedge reducing mantle velocities and thereby diminishing the impedance contrast across the Moho. Our findings differ from other places where the Moho has been found to be weak as the anomalous region extends over 400 km to the east of the trench compared to less that 100 km found elsewhere. Ascribing the expansive weak Moho to more extensive serpentization in the flat slab region is inconsistent with detailed tomographic images of the upper mantle that identify increased shear wave speeds and low Vp/Vs, inconsistent with extensive hydration. It instead appears that the diminished impedance contrast across the Moho is more likely to result from increased lower crustal velocities. The increased thickness of the crust below the western Sierras Pampeanas, Precordillera, and Andes results in the lower crust reaching the eclogite stability field. Yet, as others have demonstrated elsewhere, rocks under these P-T conditions will metastably remain as granulite until exposed to hydrous fluids, at which point they undergo rapid eclogitization. The fluids present in the sediments and crust associated with the Juan Fernandez ridge, the subduction of which has been proposed to initiate the current flat subduction geometry, could provide the needed hydration to transform the metastable granulite into eclogite. The low Vp/Vs anomaly in the mantle is consistent with a dehydrated serpentinite, suggesting fluids transited through the mantle to reach the crust but no longer reside there. A dense lower crust composed of eclogite can readily explain the lack of a sharp Moho signal as well as account for the lack of correlation between crustal thickness and surface elevation.
DE: 7218 Lithosphere and upper mantle
DE: 8110 Continental tectonics--general (0905)
DE: 8159 Rheology--crust and lithosphere
DE: 7205 Continental crust (1242)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting