Page 386 - Caldera Volcanism Analysis, Modelling and Response

Facilitating Dike Intrusions into Ring-Faults 361

Deflating spherical magma reservoir, opening at ring-fault

A) Map view, model setup B) Map view, displacement vectors

Magma chamber
(10 MPa 20
pressure drop)
10

0 Subsidence
Subsidence
Subsidence
5 km
y
Opening -10
ring-fault
x -20
-20 -10 0 10 20 -20 -10 0 10 20

C) Side views on opening ring-dike [in m]
z z 5
x y 0
0

Dike Dike

Opening -10 Opening

-4 0 4 -4 0 4
Magma chamber
Figure 5 De£ation of a spherical magma chambercauses opening at a circumferential ring-
fault. (A) Model setup (x--y plane).The magma chamber (5 km radius) is emplaced at (x, y)
coordinates (0,0) at10 km depth, and is subject to apressure dropof10 MPa. A ring-fault is de¢ned
surrounding the magma chamber, with a radius of 5 km at 0--20 km depth. Boundaryconditions
are set so that the ring-fault is allowed to open. (B) A mapviewof horizontal displacement vec-
tors at the surface (x--yplane) shows convergence toward the de£ating source.Vector magnitudes
are largest at the periphery and tend tobecome zero inside the caldera (piston subsidence).
(C) Side views on opening ring-fault (left showing x--z plane, right y--z plane).The projection
of the spherical magma chamberenclosed by the ring-fault in (C) is indicated as a dashed ring
centered at10 km depth. Maximum opening occurs near the magma chamber.The pattern is
radially symmetric, so dike intrusion (shown byblack arrow) mayoccur as a complete ring.

implying that a dike intrusion could occur anywhere along the ring-fault and in a
complete circle around the magma chamber. Separate tests were done to study
the effect of a ring-fault dipping in or out (not shown in figures), which affects
the amount of opening in the sense that a larger amount of opening is found