 |
| Titel |
Magma explains low estimates of lithospheric strength based on flexure of ocean island loads |
| VerfasserIn |
W. Roger Buck, Luc L. Lavier, Eunseo Choi |
| Konferenz |
EGU General Assembly 2015
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250114216
|
| Publikation (Nr.) |
 EGU/EGU2015-14525.pdf |
|
|
|
|
|
| Zusammenfassung |
| One of the best ways to constrain the strength of the Earth’s lithosphere is to measure the
deformation caused by large, well-defined loads. The largest, simple vertical load is that of
the Hawaiian volcanic island chain. An impressively detailed recent analysis of the 3D
response to that load by Zhong and Watts (2013) considers the depth range of seismicity
below Hawaii and the seismically determined geometry of lithospheric deflection. These
authors find that the friction coefficient for the lithosphere must be in the normal
range measured for rocks, but conclude that the ductile flow strength has to be
far weaker than laboratory measurements suggest. Specifically, Zhong and Watts
(2013) find that stress differences in the mantle lithosphere below the island chain
are less than about 200 MPa. Standard rheologic models suggest that for the ~50
km thick lithosphere inferred to exist below Hawaii yielding will occur at stress
differences of about 1 GPa. Here we suggest that magmatic accommodation of flexural
extension may explain Hawaiian lithospheric deflection even with standard mantle flow
laws.
Flexural stresses are extensional in the deeper part of the lithosphere below
a linear island load (i.e. horizontal stresses orthogonal to the line load are lower
than vertical stresses). Magma can accommodate lithospheric extension at smaller
stress differences than brittle and ductile rock yielding. Dikes opening parallel to an
island chain would allow easier downflexing than a continuous plate, but wound
not produce a freely broken plate. The extensional stress needed to open dikes at
depth depends on the density contrast between magma and lithosphere, assuming
magma has an open pathway to the surface. For a uniform lithospheric density
ÏL and magma density ÏM the stress difference to allow dikes to accommodate
extension is: δÏăxx (z) = g z (ÏM - gÏL), where g is the acceleration of gravity and z
is depth below the surface. For reasonable density values (i.e. ÏL = 3300 Kg/m3
and ÏM = 2800 kg/m3) this ‘magmatic yield stress’ is 250 MPa at 50 km depth.
Dikes accommodating flexing below Hawaii would be at most about 2 km wide.
This amount of intrusion would significantly heat the lithosphere, leading to lower
stress differences below the islands. Since Hawaii marks the highest magma flux on
Earth today it seems that ‘magma assisted flexure’ offers a viable alternative to
extremely weak lithospheric rheology as an explanation for low stresses below this load. |
|
|
|
|
|
|