HR: 1340h
AN: V23A-0690    [Abstracts]
TI: Cracking the Lid - Dike Emplacement Above Large Sills of the Ferrar Province, Antarctica
AU: White, J D
EM: james.white@otago.ac.nz
AF: University of Otago, Geology Department, Leith Street, Dunedin, 9015 New Zealand
AU: * Thordarson, T
EM: moinui@soest.hawaii.edu
AF: University of Hawaii SOEST, Dept. Geology & Geophysics POST 614, 1680 East-West Road, Honolulu, HI 96822
AU: McClintock, M M
EM: murray.mcclintock@otago.ac.nz
AF: University of Otago, Geology Department, Leith Street, Dunedin, 9015 New Zealand
AU: Ross, P
EM: p_s_ross@hotmail.com
AF: University of Otago, Geology Department, Leith Street, Dunedin, 9015 New Zealand
AB: Using a combination of photographs at different scales and GPS points, the shape and extent of single intrusions and the nature of their relationships to adjacent intrusions and country rock were studied at Mt Gran and Terra Cotta Mountain. We found that many intrusions change their shape and orientation along their length; horizontal sills locally feed into subvertical dikes, dikes change strike abruptly, and the form of the fractures occupied by magma varies widely across small areas. These features are all to some degree unexpected, because magma transport at more than a kilometre below the surface (the depth of intrusion at these sites) has been treated as being controlled predominantly by the regional stress field. The rather chaotic pattern we see is more consistent with country rock being a 'floated' lid on top of or partly within a fluid magma, with cracks forming in response to very local stresses rather than regional ones. This conclusion gains further support from the contact geometries of some dikes, which show irregular buds and extensions that indicate different directions of magma flow and/or of dike propagation among closely spaced dikes. Local regularities in pattern may reflect intrusion dynamics, with initial sill propagation causing cracking of the floated lid. This working hypothesis will be tested by determining the AMS (anisotropy of magnetic susceptibility) of oriented samples from the dikes and sills to interpret directions of magma flow. If our hypothesis is correct, it implies for areas with thick sills that (1) dike orientations mapped from aerial photographs or satellite imagery cannot be used to infer basement stress regimes, and (2) local regularity of dike alignments near and above sills may be useful in inferring magmatic processes, such as the azimuth of sill-front propagation.
DE: 6207 Comparative planetology
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
DE: 8434 Magma migration and fragmentation
DE: 9310 Antarctica (4207)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005