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| Titel |
Flexural modelling of gravity anomalies seaward of Pacific subduction zones |
| VerfasserIn |
Johnny Hunter, Anthony Watts, Daniel Bassett |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250080918
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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 cooling and thickening of
the lithosphere. Evidence for this comes from studies of free-air gravity anomalies and
bathymetry at seamounts, fracture zones and subduction zone outer-rises. 75% of
Te estimates from seamounts lie between the 300 Ë C and 600 Ë C isotherms as
predicted by a cooling plate model, whilst the majority of subduction zone and
fracture zone estimates lie between the 500 Ë C and 800 Ë C isotherms. Recent
outer-rise investigations, however, have questioned whether such a simple relationship
exists and suggested that either the strength of the lithosphere is independent of
plate age, or that Te cannot be measured with sufficient accuracy to reveal such a
relationship.
In order to reassess the relationship between lithospheric strength and age, we
use trench-normal, ensemble-averaged profiles of satellite-derived free-air gravity
anomalies to model the outer-rise of all the major Pacific subduction zones. Profiles are
corrected for sediment loading, as well as for thermal cooling effects. A broken
elastic plate model is used, with a finite difference solution that allows Teto vary as
a function of distance from the trench. We use an inverse approach, iterating Te
values and inverting for end-conditioning forces (a vertical shear force and a bending
moment).
Results show that oceanic lithosphere younger than 90 Ma clearly strengthens with age,
with Te roughly following the 550 Ë C isotherm. For example, the Middle America trench (4
- 25 Ma) has a mean Te of 14.1 ± 2.8 km, whilst the Alaska-Aleutian trench (43 - 60 Ma) has
a mean Te of 34.3 ± 8.0 km. For older lithosphere, the pattern is not as clear. We suggest that
this is due to thermal rejuvenation, which has two effects: it weakens the lithosphere,
lowering Te estimates; the associated magmatism masks the flexural signal, producing
scatter. For many of the subduction zones, gravity profiles require that the lithosphere has
been weakened in the region of the seaward wall of the trench. We attribute this to
inelastic yielding - a combination of brittle fracture of the upper lithosphere and
ductile flow of the lower lithosphere - due to high bending moments. Evidence
for this can be seen in swath bathymetry data, which reveals zones of pervasive
extensional faulting. Hydrothermal alteration of the lithosphere might also contribute to
weakening, if bend faulting allows hydration and serpentinization of the upper mantle. |
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