This document describes geological features observed at contacts between laccolith intrusions and their sedimentary host rocks on Elba Island, Italy. Various deformation structures were observed including waves, ropes, and lobes formed by the interaction of the magma and fluidized host rock. Samples showed evidence of fluidization such as stretched crystals perpendicular to wave crests and troughs. Breccia dikes and disrupted zones also indicated fluidization at contacts between the intrusions and more competent host rocks.

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

3. low-angle gravitational-tectonic
faulting
1 km
1 km
complex V
complex IV
CEF

5. 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)

6. San Martino Porphyry
Portoferraio Porphyry
layers map out as sheets

7. 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

8. large waves against fluidized serpentinite
(“fluidized” is used to describe the complete loss of competency in water-rich host rock rock)

9. small waves against fluidized shale
phenocrysts stretched perpendicular to rounded crests and troughs

10. multiple waves patterns
Rounded crests and V-shaped troughs

11. small pointed crests against fluidized flysch

12. ropes – the extreme V-troughs

13. lobes – wave interference?
(“load casts”; “pillows”)
basal contact – roof of cave
ramp contact against fluidized flysch
basal “load casts”

14. deformation structures
• linear strain – stretched quartz and feldspars
• disruption structures
– porphyry breccia-mylonite
– host breccia
– breccia dikes in poprhyry
– breccia dikes in host

15. 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

16. linear strain parallel waves & decreases inward
sense of strain consistent – inflation would
produce opposite sense on opposing limbs of
crests

17. mylonitic fabrics
most brittle at contact, grading to plastic toward magma

18. disruption structures
fluidized breccia invading porphyry

19. fluidized contact breccia at porphyry base

20. fluidized contact breccia
ss
Pp

21. fluidized host dike in porphyry

22. mixed porphyry-ophiolite-flysch-
breccia dike invading flysch host

23. 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