What happens after a supervolcano erupts and a

caldera collapses ?

1. Resurgent doming

2. Styles of volcanic activity

3. Magma compositions

Resurgence, T / D = 1

Acocella et al 2001

Principal high-angle reverse fault

Normal fault

Intrusion of silicone analogue magma here, due to (a) decrease in shear stength in this weak zone (see also Mathieu et al 2008) and (b) dilatation during uplift

Resurgence, T / D = 0.4

Acocella et al 2001

Extrusion and “eruption” of silicone magma

Development of radial fractures at these shallow-level, near-surface conditions

Central crestal depression due to apical tensile stresses

Collapse followed by resurgence

Acocella et al 2001

Normal faults developed during subsidence re-used as reverse faults during resurgence

Reverse faults developed during subsidence re-used as normal faults during resurgence

IN ALL CASES, FAULTS NUCLEATE FROM THE BOTTOM OF THE SANDPACK

ASYMMETRY: MOST UPLIFTED PART COINCIDESWITH MOST SUBSIDED PART OF CALDERA

Faulting during collapse and resurgence

Acocella et al 2001

Acocella 2007, Earth-Sci Rev

Rabaul

PinatuboMt St Helens

Interesting seismicity patterns from a caldera viewpoint

Saunders 2001

Rabaul calderaMaximum uplift

Saunders 2001; seismicity from Jones and Stewart 1997

Saunders’ model of ring fracture intrusion to cause central uplift

Saunders 2001

Saunders’ model for Rabaul

Saunders 2001

Dykes, cups, saucers, and sills: some ideas from Mathieu et al 2008•Nature of host material is key: is it strong, highly lithified material (e.g., granite, gneiss), or is it weak granular-like material (e.g., breccias, fault zones, pyroclastics, etc.)

•Does magma intrusion induce (a) hydraulic fracturing in strong country rock, (b) self-induced shear faulting in weak country rock, or (c) both processes?

•Should country rock be modeled using gelatin (strong) or granular material (weak) ?

•In Mathieu et al’s experiments, note the highly irregular and lobate nature of the intrusions, as well as the preponderance of dyke branching

Mathieu et al 2008

Intrusion into developing reverse faults

Mathieu et al 2008

Low-viscosity analogue magma

High-viscosityanalogue magma

Branching dyke

Apical graben

Developing reverse fault

Dome diameter > sill diameter…and note that highest viscosities produced deepest cup-shaped intrusions in the experiments

Magma intrusion into weak zone

1. Resurgent doming

2. Styles of volcanic activity

3. Magma compositions

Valles – circular moat rhyolite pattern

Yellowstone geology

Yellowstone – vents of post-caldera lavas follow regional structural grain, not a ring fault

Christiansen 2001

Long Valley resurgent dome

•Note the strong regional structural grain running through the caldera

•Note this strong grain particularly for vents on the resurgent dome

•Also evident for “moat” rhyolites which may not be moat rhyolites at all

•Note the strong N-S vent alignment from Mammoth Mountain north out the caldera

•And note the unusual arcuate pattern of Mono Craters vents

Hildreth 2004

Hildreth 2004

1. Resurgent doming

2. Styles of volcanic activity

3. Magma compositions

Some important observations and comments

• Lavas and pyroclastics erupted immediately after collapse tend to be among the most “primitive” silicic products of the entire caldera cycle.

• What is the significance of this observation ? Are the eruptions simply tapping the bottom-most dregs of the chamber ?

• Or does the primitive material signify influx of new magma from deeper crustal levels ?

• If so, when does this influx occur? Before, during, or after the supervolcanic eruption? All of the above ?

• And what can we say about eruption triggers in this regard ?

Valles caldera – Redondo Rhyolite is the most primitive rhyolite

Yellowstone – Upper Basin Member rhyolites most primitive

Long Valley – Dome 7403 may be immediately post-caldera and is a hornblende-bearing dacite erupted subaqueously

Hildreth 2004

Long Valley geochemistry

Hildreth 2004

Ossipee ring complex

Textural variation from inclusion mingling in the syenite ring dyke

Anorthoclase

Diffuse inclusion

Crystal transfer

Discreteinclusions Orthoclase

Anorthoclase-rich patches,evidence of old inclusion

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