Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p11b1348b&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P11B-1348
Physics
[5475] Planetary Sciences: Solid Surface Planets / Tectonics, [5480] Planetary Sciences: Solid Surface Planets / Volcanism, [8149] Tectonophysics / Planetary Tectonics, [8450] Volcanology / Planetary Volcanism
Scientific paper
Volcanism occurs on many planets, from large strato- and shield volcanoes on Earth, Mars, and Venus, to smaller shields and plains volcanism on Io, Mercury, and the Moon. Two important processes that affect the structure of large volcanoes are spreading and sagging. Volcano spreading occurs when a conical edifice is free to gravitationally equilibrate along a basal décollement. This results in radial and concentric extension of the volcano’s upper flanks, accommodated by radially-orientated "leaf grabens", and in radial shortening of its lower flanks, accommodated by concentric thrusts. Notable examples of such structures are found on Maderas volcano, Nicaragua. Volcano sagging results when an edifice’s weight down-flexes the underlying basement or even the entire supporting lithosphere. This leads to radial and concentric shortening of the volcano’s upper and lower flanks, accommodated by a system of outward-verging, convex-upward "terraces". Superb examples of flank terraces occur on Ascraeus Mons, Mars. Yet neither spreading nor sagging alone accounts for combinations of extensional and compressional structures on many volcanoes. For example, Olympus Mons on Mars, the largest volcano in the Solar System, has a large basal scarp and mid-flank terraces, while Concepción volcano in Nicaragua has basal concentric thrusts and lower-flank radial folds. We suggest that an interplay between spreading and sagging may be responsible for such enigmatic cases. To test this hypothesis, we conducted a suite of scaled physical experiments using sand to simulate brittle conical volcanic edifices and upper crust, and silicone putty as an analogue for basal décollements and the viscoelastic lithosphere. Our results show that two factors control the degree to which a volcano will undergo spreading or sagging: 1.) the thickness or rigidity of the supporting brittle crust, and 2.) the presence (or absence) of a basal décollement. A cone welded to a rigid basement experienced little or no deformation, and may be analogous to Kilimanjaro in Tanzania, which shows limited spreading or sagging effects. Models welded to a flexible basement mainly sagged, producing terrace-like structural features similar to those observed on Ascraeus Mons. Conversely, a basally-detached cone upon a rigid basement mainly spread, producing a major concentric basal thrust system and leaf grabens similar to that seen on Maderas. As the flexural rigidity of the basement beneath a detached model was decreased, however, the spreading behavior was increasingly accompanied by some amount of sagging, producing variable combinations of structures characteristic of both end-member processes, e.g. a basal scarp and terraces like those on Olympus Mons. This combination of two discrete processes operating in parallel suggests therefore that a continuum of structural outcomes exists, within which a wide range of large volcanoes on Earth and other planets can be placed.
Byrne Paul K.
Holohan Eoghan P.
Kervyn M.
Murray John B.
Troll Valentin R.
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