HR: 16:20h
AN: V24A-01 INVITED     [Abstracts]
TI: Plate tectonic constraints on the cessation of subduction beneath the Baja California peninsula, Mexico
AU: * Stock, J M
EM: jstock@gps.caltech.edu
AF: Seismo Lab, Caltech 252-21, Pasadena, CA 91125, United States
AB: I review published models, existing global plate tectonic data and published marine geophysical observations west of Baja California to assess the timing and conditions under which subduction ceased along the W margin of Baja California. The relative motion of the Farallon microplate fragments can be reconstructed using Pacific- North America global plate motions (from the Pacific-Antarctica-Nubia-North America plate circuit) added to the local velocities of the microplates with respect to the Pacific plate. Because the Pacific plate was moving obliquely away from North America, the time at which subduction stopped has often been taken to be the time at which the microplates joined the Pacific plate (the ages of dead spreading centers preserved west of North America on the Pacific plate). The timing of cessation of subduction west of what is now northern Baja California is not recorded by a dead ridge offshore but is inferred to be coincident with extension and rotation in the continental borderland (early-middle Miocene). The Arguello microplate stopped spreading relative to the Pacific plate at about 13 Ma, providing a younger age limit on the cessation of subduction in the sector N of the Shirley transform fault. The time of cessation of spreading of the Magdalena-Pacific (M-P) ridge has been proposed by Michaud et al. (2006 Geology) to be as young as 8 Ma. However, the clockwise rotation of the M-P ridge before it ceased, and its inferred slow spreading rate away from the Pacific plate implies transcurrent motion with virtually no convergence between the Magdalena microplate and the North America plate during the last stages of activity of the M-P ridge. Subduction can occur by motion of forearc fragments without any convergence of the major bounding plates (e.g., the modern South Shetland Trench), but this may be ruled out for Baja California due to the small spatial scale of the microplates compared to the scale of the stable Baja California peninsula block. Due to the progressively slower convergence rates in this region since 14 Ma, the formation of asthenospheric windows during waning subduction is likely to have been extremely important in the change from subduction-related to "post-subduction" magmatism and in its variability along strike in Baja California.
DE: 8157 Plate motions: past (3040)
DE: 8178 Tectonics and magmatism
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly