Galápagos Rise
The Galápagos Rise is an extinct spreading centre located about 2,000 km west of Peru, which was active in the Miocene period between about 17.0 Ma (million years ago) and 5.8 Ma. It developed shortly after the Farallon plate broke into two parts, forming the Cocos and Nazca plates. During its existence the Galápagos Rise formed the eastern boundary of the Bauer microplate.[1]
Extent
[edit]The rise is made up of a series of inactive ridge and transform segments, currently within the northwestern part of the Nazca plate, southwest of the Galápagos islands, between about 8° South and 17° South latitude. The northern end of the rise is marked by two fracture zones, the North and South Gallego fracture zones. The southern end of the rise approaches close to the South Marquesas/Mendana fracture zone.[1]
History
[edit]Towards the end of the Oligocene period, the large Farallon plate, which had long been subducting beneath the western margins of the North American plate, the Caribbean plate and the South American plate, began to break up, forming the Cocos and Nazca plates, separated by the Cocos–Nazca spreading centre. Following this plate reorganisation, spreading along the East Pacific Rise was disrupted due to a 20° rotation in the spreading direction, causing locally chaotic spreading and the formation of large offset right-stepping transform faults. The existing 700km long Marquesas/Mendana transform zone was incorporated into this new pattern. New spreading centres were formed from elements of this transfer zone, a new part of the East Pacific Rise propagating to the north and a new rise, the Galápagos Rise, propagated to the south. Eventually these two ridges isolated a piece of older oceanic lithosphere, forming the Bauer microplate that began to rotate anti-clockwise. In its southward propagation, the Galápagos Rise appears to have been unable to cross the South Marquesas/Mendana fracture zone, possibly because the oceanic lithosphere to the south was older and stronger.[1]
At about 8 Ma, the western boundary of the microplate moved west with part of the Pacific plate becoming annexed to the microplate. This change also added another area to the microplate to the north, bounded to the east by the Annexe Rise. Between 6.5 to 5.8 Ma, motion on the Galápagos Rise progressively transferred over to the East Pacific Rise, ceasing at the end of this period. At this time, the Bauer microplate was effectively captured by the Nazca plate.[1]
After spreading ceased along the Galápagos Rise, there is evidence that magmatism continued along parts of the rise for at least a further 2 Ma. These post-spreading lavas are richer in alkalis (sodium and/or potassium) than those extruded before spreading stopped.[2]
References
[edit]- ^ a b c d Eakins, B.W.; Lonsdale, P.F. (2003). "Structural patterns and tectonic history of the Bauer microplate, Eastern Tropical Pacific". Marine Geophysical Researches. 24: 171–205. doi:10.1007/s11001-004-5882-4.
- ^ Haase, K.M.; Regelous, M.; Duncan, R.A.; Brandl, P.A.; Stroncik, N.; Grevemeyer, I. (2011). "Insights into mantle composition and mantle melting beneath mid-ocean ridges from postspreading volcanism on the fossil Galapagos Rise". Geochemistry, Geophysics, Geosystems. 12 (5). doi:10.1029/2010GC003482.