Waves-ropes-lobes, fluidization
and deformation
at laccolith-host contacts
(Elba Island, Italy)
David S. Westerman – Norwi...
low-angle gravitational-tectonic
faulting
1 km
1 km
complex V
complex IV
CEF
5
Reconstructed laccolithic shape (diameter ~ 8.3 km)
3 tabular layers (thickn. = 700 m)
3 identified dikes
Inferred f...
San Martino Porphyry
Portoferraio Porphyry
layers map out as sheets
geometric forms
• Waves – simple crests and troughs ranging from
strongly pointed crests with open troughs to rounded
cres...
large waves against fluidized serpentinite
(“fluidized” is used to describe the complete loss of competency in water-rich ...
small waves against fluidized shale
phenocrysts stretched perpendicular to rounded crests and troughs
multiple waves patterns
Rounded crests and V-shaped troughs
small pointed crests against fluidized flysch
ropes – the extreme V-troughs
lobes – wave interference?
(“load casts”; “pillows”)
basal contact – roof of cave
ramp contact against fluidized flysch
ba...
deformation structures
• linear strain – stretched quartz and feldspars
• disruption structures
– porphyry breccia-mylonit...
linear strain at contacts
- lineations very common when waves are present
- characteristically oriented perpendicular to c...
linear strain parallel waves & decreases inward
sense of strain consistent – inflation would
produce opposite sense on opp...
mylonitic fabrics
most brittle at contact, grading to plastic toward magma
disruption structures
fluidized breccia invading porphyry
fluidized contact breccia at porphyry base
fluidized contact breccia
ss
Pp
fluidized host dike in porphyry
mixed porphyry-ophiolite-flysch-
breccia dike invading flysch host
summary
• waves, set up by two fluids flowing past each
other, are the rule where host material is
fluidized
• extreme flu...
Westerman lasi iv
Westerman lasi iv
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Westerman lasi iv

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Fall 2010

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Westerman lasi iv

  1. 1. 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
  2. 2. low-angle gravitational-tectonic faulting 1 km 1 km complex V complex IV CEF
  3. 3. 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)
  4. 4. San Martino Porphyry Portoferraio Porphyry layers map out as sheets
  5. 5. 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
  6. 6. large waves against fluidized serpentinite (“fluidized” is used to describe the complete loss of competency in water-rich host rock rock)
  7. 7. small waves against fluidized shale phenocrysts stretched perpendicular to rounded crests and troughs
  8. 8. multiple waves patterns Rounded crests and V-shaped troughs
  9. 9. small pointed crests against fluidized flysch
  10. 10. ropes – the extreme V-troughs
  11. 11. lobes – wave interference? (“load casts”; “pillows”) basal contact – roof of cave ramp contact against fluidized flysch basal “load casts”
  12. 12. deformation structures • linear strain – stretched quartz and feldspars • disruption structures – porphyry breccia-mylonite – host breccia – breccia dikes in poprhyry – breccia dikes in host
  13. 13. 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
  14. 14. linear strain parallel waves & decreases inward sense of strain consistent – inflation would produce opposite sense on opposing limbs of crests
  15. 15. mylonitic fabrics most brittle at contact, grading to plastic toward magma
  16. 16. disruption structures fluidized breccia invading porphyry
  17. 17. fluidized contact breccia at porphyry base
  18. 18. fluidized contact breccia ss Pp
  19. 19. fluidized host dike in porphyry
  20. 20. mixed porphyry-ophiolite-flysch- breccia dike invading flysch host
  21. 21. 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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