Extended Abstract -
Calderas form by subsidence of the roof of a magma reservoir. In addition to vertical displacement, a fundamental feature of this subsidence is the horizontal movement of roof material. As has been observed during deflation episodes at restless calderas and during collapse events, such horizontal motion is typically directed radially and inward toward the roof centre. This inward horizontal motion has been long known to lead to radial extension around the roof’s periphery and radial shortening above the roof’s centre. If roof material moves horizontally along all radial directions toward a central point, however, it must also undergo some shortening in the concentric direction, the structural resolution of which has generally received scant attention in the literature. Here, we report the results of analogue subsidence experiments in which a variety of structures accommodating concentric shortening were documented.
Our models comprise a sand pack of sand and plaster (mixed 4:1 by volume) containing a sill-like reservoir of creamed honey. The reservoir could be depleted by opening a honey conduit at its base, which led to subsidence of the overlying reservoir roof. The geometric and dynamic attributes of the crustal and magma chamber analogues are scaled to those in nature. Sequential photographs of the surface expressions of structures produced during subsidence were analysed in detail by eye and by means of Particle Imaging Velocimetry (PIV) software in order to constrain the structural kinematics. Cross sections of the models were also made to constrain the nature of the structures at depth. We investigated the effects upon the structures formed of the reservoir roof’s thickness/diameter ratio (T/D), which we increased from 0.2 to 1.
For very thin roofs (T/D = 0.2), the central area sags and radial wrinkle ridges form. The wrinkles are radially-orientated, although a slight asymmetry to the subsidence has caused amplification of those in a north-south orientation. The peripheral area extends along a complex network of intersecting normal faults and tensile fractures. Between the centre and periphery areas, there is a zone with several obliquely-trending faults with a subtle strike-slip component. These faults down-throw inward or outward; PIV shows that their mode is oblique-normal. With a slightly thicker roof (T/D = 0.33), a similar intermediate zone of strike-slip component faults is also seen. Radial wrinkles are not produced however. Instead, a central ring fault becomes well-developed. Several faults splay from this central ring fault and down-throw inward with reverse sense of motion. PIV shows that these obliquely- trending reverse faults also have a strike-slip component, i.e. they are oblique-reverse mode. With a very thick roof (T/D = 0.8), these oblique reverse faults are very well developed and may link for the central area right out to the main normal fault in the caldera periphery. Very subtle oblique motions on many of the intersecting normal faults in the periphery can also be seen by direct observation. Some can be seen with PIV, but frequent scarp failures inhibit systematic definition.
The results of this study highlight several structures for accommodating horizontal concentric shortening during caldera subsidence. Moreover, the experiments that the flexural rigidity of the reservoir roof, as determined by its thickness/diameter ratio, decisively influences the nature and development of these structures. The structures include radial wrinkle ridges, obliquely to sub-concentrically trending strike-slip faults (sometimes conjugate) and obliquely-trending oblique-reverse faults. Kinematically, the strike-slip faults and oblique-reverse faults act like the boundaries between the plates of camera lens shutter. Structures inferred to have formed in response to concentric shortening have only been reported from a handful of calderas, one of which is that of Olympus Mons, Mars. Here, predominantly radially-trending ‘wrinkle ridges’ occur on the caldera’s floor. Other structures present include a set of concentrically-trending ridges, which were thought to have relieved radial compression. These ridges seem to locally offset the wrinkle ridges in a strike-slip fashion, however, and they have scarps indicating that they also have a dip-slip component of displacement (down-throwing centrally). Such characteristics, including these fractures’ position within the caldera, are very similar to the oblique-reverse and oblique-normal faults produced in our experiments. This indicates that structures accommodating concentric shortening may be more widespread in natural subsidence processes than previously recognised.
Abstract of talk given to:
Collapse Caldera Workshop 2010, Reunion Island, October 2010.