Download 530, calderas 2

Collapsing calderas
Field evidence, experiments,
modeling
Caldera faulting and flexure
From Roche et al 2000
Glencoe and Valles
These two examples
highlight the important point
of simple surface structure
contrasting with complex
subsurface structure
Etive Rhyolites, Glencoe caldera
•Lava-like ignimbrites
near-vent
ponded by faults
hot
low fountains
no associated breccia – why ?
•Basal phreatomagmatic layers
suggestive of lacustrine environments
•Pre-eruptive fault bounded lakes
faults active to form lakes?
possible cause-effect relationship between
tectonics and eruptions?
Caldera faulting vs tectonic faulting
How does basin development occur at Glencoe ?
Do the basins develop as a result of tectonic faulting?
Or do they develop as true caldera basins?
Or a combination of the two processes?
Caldera faulting
•Thick ignimbrite
•Megabreccias and mesobreccias intimately
interbedded within and associated with the ignimbrite
sequence
This association is probably diagnostic of active
syn-caldera faulting, i.e., the development of a
caldera
The location of breccia within an ignimbrite sequence
can help constrain the timing and nature of caldera
collapse
Opening of vents and/or
crevasses in peripheral
extensional zone of
caldera?
Early breccias
Moore and Kokelaar 1998
Late breccias
Moore and Kokelaar 1998
I think a very interesting question is how a
caldera subsides, i.e., the timing and nature of
the subsidence:
•When does subsidence occur: early, middle, or late?
•Does it occur incrementally?
•And what is the relationship between subsidence
mechanics and magma chamber dynamics?
A key question in this regard:
•Does a caldera subside passively (response to magma
evacuation from chamber) ?
•Does a caldera subside actively, i.e., does subsidence of
the roof “push” magma out of the reservoir ?
(64 hours)
Katmai 1912,
Pinatubo 1991
Silicic
magma
Stix and Kobayashi 2008, JGR
(8-9 hours)
Basaltic magma
Stix and Kobayashi 2008
Basaltic magma
Stix and Kobayashi 2008
Subsidence experiments of
Kennedy et al. 2008
These experiments examined the
settling of the caldera roof into
the magma reservoir, modelled by
aqueous corn syrup solutions
At high Reynolds numbers:
•Flow mainly vertical
•Eddy formation
•Vorticity at the end…intense stirring
Higher flow velocities and Reynolds
numbers:
•In wider ring dikes
•In magmas with reduced viscosities
Two-block experiments (piecemeal collapse)
Kennedy et al 2008
Ponding of ignimbrite in
tectonically-controlled basins
Perhaps no active faulting during eruptions
Perhaps downsagging (flexure) is important
Lack of megabreccias and mesobreccias ?
Would there be breccias (e.g., fanglomerates)
related to tectonic faults, what would they look
like, and how might they relate to the volcanic
sequence?
Caldera faulting and megabreccias
At Glencoe, it appears that faults
are re-used during different
eruptive and caldera-forming
events
This raises the possibility of
ignimbrite of older calderas
becoming the megabreccia and
mesobreccia of younger
calderas which are nested
Yellowstone may be a good
example
Yellowstone geology, courtesy USGS
Glencoe ring fault
It is not clear – at least to me –
what the role of the ring fault at
Glencoe was
Perhaps it pertains to a later
stage of caldera development
The caldera “space” problem
Roof aspect ratio
Aspect ratio = roof thickness / roof width
width
Low aspect ratio
High aspect ratio
reservoir
reservoir
thickness
Scaling
Cohesion of roof
Cohesion needs to be scaled as * = *g*l*, where * is
the cohesion ratio, * the density ratio, g* the gravity
ratio, and l* the length ratio (most important)
A difficult thing to estimate is the cohesion of natural
materials, and its variability in space and time….people
typically use values of 106 – 107 Pa
Magma viscosity
Viscosity scales as * = *T*, where T* is the time ratio
The use of water and silicone as magma analogues can
result in very different viscosity scaling
aspect ratio = 0.5
Caldera faulting
•Downsagging is observed at early
stages
•A set of main reverse faults which
controls subsidence
Faults propagate upward from
margins of reservoir
Fault dips shallow upward (listric)
•A zone of peripheral extension which
develops as a result of subsidence
this peripheral zone is a region of
breccia production
extensional crevasses and vents
may develop in this region (see
Moore and Kokelaar)
Roche et al 2000
Influence of chamber shape
From Roche et al 2000, Kennedy et al 2004, GSA Bull
Aspect ratio
As the aspect ratio of the roof block increases:
•Area of undeformed piston decreases
AR=0.2
•Area of peripheral extension increases
•Intersection of initial reverse faults at depth
This might promote stoping of roof
blocks into the chamber
AR=1
Caldera asymmetry –
plan view
Note how the circular nature of the
caldera decreases as roof aspect ratio
increases (magma chamber dimensions
are constant)
AR=0.2
•So if the aspect ratio is low, it is
possible to infer the shape of the
reservoir, but this becomes
increasingly difficult to do at higher
aspect ratio
Another influence may simply be the
scale of the experiment compared to the
scale or grainsize of the sand
AR=1
Experiments at larger scale –
greater asymmetry
1 meter
Ossipee ring complex,
New Hampshire
From Kennedy et al 2004, Kennedy and Stix 2007
Caldera asymmetry – cross-section
Roche et al 2000
•Subsidence almost always
asymmetric in crosssection
•Nucleation and
development of first fault –
principal fault with greatest
throw
•A trapdoor-style caldera
results
•Vents concentrated here
•Seen in Roche et al, and
also in strike-slip regimes
in Holohan et al
experiments
•Lateral propagation of
faults (which is stopped by
high-angle regional faults –
see Holohan et al)
Kennedy et al 2004
Holohan et al experiments
•Tangential regional structures important during collapse
Faults within chamber margin: used as caldera reverse
faults
Faults outside chamber margin: develop into peripheral
normal faults
•Non-tangential structures important during tumescence
and resurgence ?
•Strike-slip faults as preferential pathways for magmas
and fluids
Repeated uplift and collapse
experiments
From Troll et al 2002, Geology, 30, 135-138.
The end