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Waves-ropes-lobes, fluidization
and deformation
at laccolith-host contacts
(Elba Island, Italy)
David S. Westerman – Norwich University, VT, USA
Andrea Dini – IGG-CNR, Pisa
Sergio Rocchi - Università di Pisa, Pisa
Emanuele Roni - Università di Pisa, Pisa
Ethan J. Thomas - Norwich University, VT, USA
5
Reconstructed laccolithic shape (diameter ~ 8.3 km)
3 tabular layers (thickn. = 700 m)
3 identified dikes
Inferred filling time: <100 years
Emplacement: depths 2-3.5 km
Filling and growth modalities (geometric study):
2 stage filling: sill + inflation (Rocchi et al., 2002, Geology)
Layer 1
Layer 2
Layer 3
SAN MARTINO LACCOLITH (2)(Dini et al., 2006)
geometric forms
• Waves – simple crests and troughs ranging from strongly pointed crests with open troughs to rounded crests and V-shaped troughs
• Ropes – extreme example of rounded crests and V-shaped troughs with wave heights and lengths of similar dimensions
• Lobes – similar in form to load casts and pillows
large waves against fluidized serpentinite
(“fluidized” is used to describe the complete loss of competency in water-rich host rock rock)
small waves against fluidized shale
phenocrysts stretched perpendicular to rounded crests and troughs
lobes – wave interference?(“load casts”; “pillows”)
basal contact – roof of cave
ramp contact against fluidized flysch
basal “load casts”
deformation structures
• linear strain – stretched quartz and feldspars
• disruption structures
– porphyry breccia-mylonite
– host breccia
– breccia dikes in poprhyry
– breccia dikes in host
linear strain at contacts
- lineations very common when waves are present- characteristically oriented perpendicular to crests and troughs- both quartz and feldspar phenocrysts are stretched, often by a factor of 10x!- this exceeds realistic stretching from wave growth by inflation
ex: a side of a square stretched to a 90° arc adds only 11%
1.00
linear strain parallel waves & decreases inward
sense of strain consistent – inflation would produce opposite sense on opposing limbs of
crests
summary
• waves, set up by two fluids flowing past each other, are the rule where host material is fluidized
• extreme fluidization leads to breccia contacts and, ultimately, to breccia dikes both in porphyries and their host rocks
• strong linear fabrics develop parallel flow in chilled porphyry and adjacent host rock, with decreasing strain away from the contact
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