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SUMMARY:Up the down escalator: Formation and exhumation of (ultra) high pr
 essure rocks - Clare Warren\, Open University
DTSTART:20091110T163000Z
DTEND:20091110T173000Z
UID:TALK20168@talks.cam.ac.uk
CONTACT:John Maclennan
DESCRIPTION:Ultra-high-pressure (UHP) rocks\, exposed in many Phanerozoic 
 collisional orogens\, are formed and exhumed during the later stages of oc
 ean basin closure and the early stages of continental collision.    The ge
 odynamic processes and mechanisms driving their detachment from the subduc
 ting slab and subsequent exhumation are poorly understood.  Thermal-mechan
 ical\, upper-mantle-scale numerical models provide insight into the detach
 ment and exhumation of UHP material in the subduction channel.  The models
  show that exhumation potential is affected by the competition between dow
 n-channel shear traction and up-channel buoyancy forces.  The results are 
 be interpreted in terms of the “exhumation number”\, E\, which express
 es the relative contributions of these forces. The value of E depends on t
 he pressure gradient (dominated by density contrasts)\, channel thickness\
 , effective viscosity\, and subduction velocity\, and varies with both tim
 e and distance along the channel. Although buoyancy is the main driving fo
 rce for exhumation\, viscous strain weakening plays a critical role in the
  models by first reducing the downward drag and then promoting upward flow
  of low-viscosity UHP material. In natural systems\, other weakening mecha
 nisms may also play a role.  During the later stages of exhumation\, the m
 odels predict the formation of prominent low-angle extensional shear zones
  at high structural levels\, consistent with observations from many natura
 l examples.  The models predict pressure-temperature-time paths which are 
 consistent with natural examples.\n\n•	Beaumont\, C.\, Jamieson\, R.A.\,
  Butler J.P.\, Warren\, C.J.\, 2009\, Crustal structure: a key constraint 
 on the mechanisms of ultra-high-pressure rock exhumation. Earth and Planet
 ary Science Letters 287\, 116-129. doi: 10.1016/j.epsl.2009.08.001\n•	Wa
 rren\, C.J.\, Beaumont\, C.\, Jamieson\, R.A.\, 2008\, Deep subduction and
  rapid exhumation: the role of crustal strength and strain weakening in co
 ntinental subduction and ultra-high pressure rock exhumation. Tectonics 27
 \, TC6002. doi: 10.1029/2008TC002292\n•	Warren\, C.J.\, Beaumont\, C.\, 
 Jamieson\, R.A.\, 2008\, Formation and exhumation of ultra-high-pressure r
 ocks during continental collision: role of detachment in the subduction ch
 annel.  Geochemistry\, Geophysics\, Geosystems\, Vol. 9\, Q04019\, doi: 10
 29/2007GC001839.\n
LOCATION:Tilley Lecture Theater\, Department of Earth Sciences
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