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

low-angle gravitational-tectonicfaulting

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 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)

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 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

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

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

mylonitic fabricsmost 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 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