BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Talks.cam//talks.cam.ac.uk//
X-WR-CALNAME:Talks.cam
BEGIN:VEVENT
SUMMARY:Structure and Development of Oceanic Core Complexes - Roger C. Sea
 rle\, Durham University
DTSTART:20091020T153000Z
DTEND:20091020T163000Z
UID:TALK19407@talks.cam.ac.uk
CONTACT:Alex Piotrowski
DESCRIPTION:Oceanic core complexes are the footwalls of low-angle detachme
 nt fault systems that exhume lower crust and mantle rocks at slow-spreadin
 g mid-ocean ridges.  However\, relatively little is known about how they i
 nitiate or become inactive\, and their lithological structure is uncertain
 . I shall present results from a recent study of active and recently defun
 ct core complexes on the Mid-Atlantic Ridge near 13°N.  High-resolution s
 onar images and limited sampling are consistent with a model in which the 
 smooth\, domed and striated part of the footwall represents the top of the
  mantle section and is connected to a rugged\, deformed high massif repres
 enting the mafic crust.  \nThe detachments initiated as high-angle normal 
 faults similar to other typical valley-wall normal faults.  Like these\, t
 hey rapidly flatten by >30° in response to flexural unloading.  Local var
 iations in magma supply appear to play a critical role in determining the 
 fate of ridge-axis normal faults. When most plate separation is accommodat
 ed by axial intrusion of magma (classic sea-floor spreading)\, normal faul
 ts play a minor role. They take up less than 20% of plate extension\, slip
 ping for a few hundred thousand years until lithospheric thickening and st
 rengthening make continued fault slip harder than the creation of a new fa
 ult.  However\, if magma supply to the ridge axis is significantly reduced
 \, most plate extension must be taken up by faulting.  In areas of reduced
  magma\, peridotite is more accessible to normal faults and penetrating wa
 ter\, leading to creation of talc and serpentine.  These extremely weak mi
 nerals allow continued slip and runaway displacement on pre-existing fault
 s\, leading to core complex formation.\nNow the effective plate boundary  
 has jumped from the magmatic axis to an off-axis detachment fault.  In thi
 s configuration plate accretion is highly asymmetric and unstable: the det
 achment is forced to migrate back towards the magmatic axis.  Our imaging 
 data confirm that ridge-axis volcanism is absent opposite actively extendi
 ng detachment faults.  It appears that renewed axial volcanism propagates 
 along axis and eventually cuts off the migrating detachment footwall at de
 pth\, thus terminating its activity.\n\nReference:\nMacLeod\, C.J.\, R.C. 
 Searle\, B.J. Murton\, et al.\, Life cycle of oceanic core complexes\, Ear
 th and Planetary Science Letters\, under review\, 2009.\n
LOCATION:Tilley Lecture Theater\, Department of Earth Sciences
END:VEVENT
END:VCALENDAR