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| Titel |
Flexural modelling of circum-Pacific trench - outer-rise systems and its implications for mantle rheology |
| VerfasserIn |
Johnny Hunter, Tony Watts, Dan Bassett |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250093997
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| Publikation (Nr.) |
 EGU/EGU2014-9265.pdf |
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| Zusammenfassung |
| The strength of the lithosphere is determined by its flexural rigidity, which is commonly
expressed through the effective elastic thickness, Te. In oceanic regions, it is widely accepted
that Te increases as a function of age at the time of loading due to thermal cooling of the
lithosphere. A recent trench - outer-rise study, however, has questioned whether such a simple
relationship exists. In order to reassess the relationship between strength and age, we use
trench-normal, ensemble-averaged profiles of satellite-derived free-air gravity anomalies to
model the trench - outer-rise of circum-Pacific subduction zones. A broken elastic plate
model is used, with a finite difference solution that allows Te to vary as a function of distance
from the trench. We use an inverse approach, iterating Te values and solving for a vertical
shear force and a bending moment.
We first model the profiles using a plate of constant Te. Results show that lithosphere
younger than 100 Ma clearly strengthens with age. For example, the Middle America trench
(6 - 29 Ma) has a mean Te of 14.7 ± 2.0 km, the Aleutian trench (42 - 63 Ma) has a mean
Te of 29.8 ± 3.3 km, and the Kuril trench (97 - 129 Ma) has a mean Te of 41.0
± 3.8 km. For lithosphere older than 100 Ma, however, the relationship is not as
clear.
For many subduction zones, a plate of constant Te cannot fit the wavelength of the bulge and
the high curvature of the seaward wall of the trench, suggesting localized weakening. We
therefore model the profiles with a plate that is allowed to weaken trenchward of the
outer-rise. This provides significantly improved fits to observations. We attribute
this apparent weakening primarily to inelastic yielding - a combination of brittle
fracture of the upper lithosphere and ductile flow of the lower lithosphere - due to
high curvatures. Evidence for this can be seen in swath bathymetry and seismicity
data, which reflect zones of pervasive extensional faulting in the trench outer-rise
region.
Curvatures and strain rates derived from best-fit models are used, together with
brittle-elastic-plastic yield strength envelopes, to calculate Te. The calculated and inverted Te
values are then compared in order to constrain experimentally-derived low-temperature
plasticity laws. We find that when inelastic yielding is taken into account, the flow law of
Mei et al. (2010) accounts well for the inverted values. This result contrasts with
that of Zhong and Watts (2013) who used a non-linear viscoelastic model to fit the
seismically-constrained flexure beneath the Hawaiian Islands, finding that the Mei et al
(2010) flow law produced a lithosphere that was too strong to fit observations. We discuss
here the possible causes of the discrepancy between the trench – outer rise and
Hawaiian Islands results and examine their implications for lithosphere rheology. |
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