Small-scale convection during continental rifting: Evidence from the Rio Grande rift
Abstract
Recent seismic imaging across the Rio Grande rift, western UnitedStates, revealed unexpected structures in the underlying mantle.Low seismic wave velocity anomalies below the Rio Grande rifthave been interpreted as being partially of melt origin, andhigh-velocity structures below the western Great Plains havebeen proposed to be the result of small-scale convection, i.e.,cold downwelling lithospheric material with probably a compositionalcontribution. We perform a dynamic test of these interpretationsusing a passive rift model for isochemical convection. The modelsself-consistently produce a rift localized at approximatelythe right distance from the border to the nearby thicker GreatPlains lithosphere. With realistic upper mantle rheologies,small-scale convection forms, aided by the lithospheric step.The resulting thermal anomalies produce seismic low-velocityanomalies below the rift of amplitudes similar to those imagedseismically, requiring the presence of only small amounts ofmelt. The lateral extent of the observed low velocities belowthe Rio Grande rift is as in the models, where it is controlledby the spacing between downwelling limbs of the small-scaleconvection. The fast velocity structure below the western GreatPlains can be produced by cold downwelling lithosphere. Thethermal rifting models can predict the amplitudes and size ofthe main seismic anomalies; compositional heterogeneity maycontribute to some of the smaller features observed.
- Publication:
-
Geology
- Pub Date:
- July 2008
- DOI:
- 10.1130/G24691A.1
- Bibcode:
- 2008Geo....36..575V