125 Special Session: GSA Geophysics Division 40th Anniversary

Monday, 10 October 2011
1:30 PM - 5:30 PM
Room 101H-J

FOSSIL SLABS ATTACHED TO UNSUBDUCTED FRAGMENTS OF THE FARALLON PLATE

Paper number: 125-8
Program: Special Sessions
Time: 3:50 PM – 4:15 PM

Authors

Abstract

As the Pacific-Farallon spreading center approached North America, the Farallon plate fragmented into a number of small microplates. Fraqmentation continues today in the form of the Explorer, Juan de Fuca, Gorda, and Rivera microplates. Some of the microplates were never subducted as spreading between the microplates and the Paciific plate slowed, then stopped, and the microplates were incorporated into the Pacific plate. Spreading and subduction probably ceased because the sinking Farallon slab detached from the surface plate, removing the driving force, but it has been unclear where or at what depth detachment occurred.

New seismic tomography shows that there are fossil slabs attached to the Monterrey, Guadalupe and Magdalena remnant microplates off the coast of California and Baja California. The Monterrey slab is the well known Isabella velocity anomaly which has previously been interpreted as a lithospheric drip or attributed to delamination of the Sierra Nevada lithospheric root. The fossil slabs extend to a depth of 150 km or more.

A simple geodynamic model illustrates the conditions required for the Monterrey slab to remain attached to the microplate remnant without significant distortion as it moved along with the Pacific plate in the 19 to 20 Ma since subduction ceased. We find that the higher viscosity due to the combined effects of dehydration of the oceanic lithosphere and the cooler temperatures still expected to remain are sufficient to keep the fossil slab intact.