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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