HR: 0800h
AN: U41A-0703    [Abstracts]
TI: Numerical thermo-mechanical model predicts new observable magmatic and tectonic signatures of lithospheric delamination at active continental margins
AU: * Sobolev, S V
EM: stephan@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg E3, Potsdam, D-14473 Germany
AU: Babeyko, A A
EM: babeyko@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg E3, Potsdam, D-14473 Germany
AU: Oncken, O
EM: oncken@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg E3, Potsdam, D-14473 Germany
AU: Trumbull, R
EM: bobby@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg E3, Potsdam, D-14473 Germany
AB: Major modification of the continental crust occur at active continental margins. An outstanding but poorly understood modification process is delamination of lower crust and mantle lithosphere, which can be expected if the overriding continental plate is subjected to compression and tectonic shortening. Delamination was previously suggested beneath the Southern Puna plateau in the Central Andes during the last 3-5 Myr, based on the occurrence of distinctive back-arc magmas, rapid recent uplift and high seismic attenuation in the upper mantle. These features are absent in the main plateau region of the Central Andes, the Altiplano and northern Puna; however, unusually low seismic velocities in the crust and the lack of a thick mantle lithosphere to match the double-thick crust beneath the Altiplano suggests that delamination may have been active in this region in the geologic past. We examine the physical conditions for lithospheric delamination and search for possible signatures of the process in the geologic record based on coupled thermo-mechanical modelling of the dynamic interaction between subducting slab and overriding continental plate. The models predict that, when delamination takes place, corner flow in the mantle wedge can move the relatively cool delaminating material toward the trench. Eventually, the delaminating material blocks the corner flow, reduces the temperatures in the mantle wedge and impedes slab roll-back. The consequence of these processes is a simultaneous reduction of arc magmatism and an increase in the rate of tectonic shortening in the overriding plate. The time scale for these effects is a few to several Myr, depending on the volume of the delaminating material. Some 5-10 Myr later, the models predict melting in the middle crust in the delamination region, with a beginning and then intensification of crustally-derived magmatic activity in the back arc. From a compilation of tectonic shortening rates and a statistical analysis of Cenozoic arc productivity for the Altiplano region at 19-22 œS, we show that the predicted sequence of delamination signatures may indeed show up in the geologic record, suggesting an episode of major delamination in the Central Andes at ca. 20-25 Ma.
DE: 8102 Continental contractional orogenic belts
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 3640 Igneous petrology
DE: 3210 Modeling
DE: 3230 Numerical solutions
SC: Union [U]
MN: 2004 AGU Fall Meeting