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Titel |
Clarifying the role of rigidity contrasts and rock interface strength in sill formation processes |
VerfasserIn |
J. Kavanagh, M. Pavier, T. Menand, R. S. J. Sparks |
Konferenz |
EGU General Assembly 2012
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 14 (2012) |
Datensatznummer |
250059670
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Zusammenfassung |
Field observations, numerical and analogue models suggest rigidity contrasts may play an
important role in sill formation processes. We present results from analogue experiments and
rock strength tests which explore the additional role that rock interface strength could have on
the geometry and propagation dynamics of magmatic intrusions. Dyed water (a magma
analogue) was injected into layers of solidified gelatine (a crustal analogue) to form
experimental dykes and sills. The intrusions were pressure-driven and injected under
initially hydrostatic conditions. From 4 to ~15 °C gelatine deforms elastically,
and under these conditions the tensile strength of the gelatine (Youngs Modulus)
evolves following a power-law relationship that plateaus with time. Varying the
concentration of the gelatine allows layer strength contrasts as low as 1% to be created.
Our results show that in a two-layered system the upper layer needs to be at least
10% more rigid than the lower layer to cause dyke arrest. An experimental sill
then forms if the interface between the layers is weak; otherwise a lateral dyke
forms.
To test rock interface strength variation in nature we used a 1 kN servo hydraulic test
machine to test 5 mm thick cuboidal specimens of sandstone-siltstone rock core where the
interface between the strata is preserved. By measuring the load required to grow a starter
crack running along the lithological contact between the layers we can calculate its
fracture toughness (a measure of the material resistance to the growth of a crack). The
results show the rock interfaces have intermediate fracture toughness to their parent
units.
These results bring into question the relative roles of magma viscosity versus
rock fracturing in controlling the nature and propagation dynamics of magmatic
intrusions. |
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