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| Titel |
The Effect of Horizontal Thermal Contraction on Oceanic Intraplate Deformation: Examples for the Pacific |
| VerfasserIn |
Corné Kreemer, Richard G. Gordon, Jay K. Mishra |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043964
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| Zusammenfassung |
| Young oceanic lithosphere is not rigid. Instead it contracts horizontally owing to thermal
cooling. Because depth-averaged cooling rate, and thus depth-averaged linear contraction
rate, is inversely proportional to the age of the lithosphere, contraction rates range from 0.01
to 1 nanostrain/yr for 170 and 1.7 Myr-old lithosphere, respectively. These rates are small, but
are one to three orders of magnitude greater than the average oceanic intraplate
strain rate inferred from seismic moment release. Predicted displacement rates are
different for directions parallel and normal to mid-ocean ridges, depend strongly
on lithospheric age, and are dependent on the actual ridge-transform geometry.
Ridge-parallel velocities can be substantial (several mm/yr) when integrated over long
distances along young lithosphere for hypothetical plate geometries having no offsets of
isochrons.
We will use the method of Haines and Holt to model the self-consistent horizontal strain
rate field that best fits the expected strain rates, and calculate the corresponding velocity field
relative to the oldest part of the considered plate. For a simple case of a large rectangular
plate bounded by a 1300-km-long ridge on one side, we find ridge-parallel velocities
for 1.7 Myr-old lithosphere to be 1.3 mm/yr over the length of ridge, as expected.
For more realistic cases with stepped and crenellated transform offsets, velocities
could locally near the ridge reach several mm/yr, but the total ridge-parallel rate
is significantly less. When we model the velocity field for the Pacific plate using
observed ridge-transform geometries and ages from Müller et al. (2008), we find
northwest-directed velocities of up to 0.8 mm/yr near the southern East Pacific rise and
southwest-directed velocities of similar magnitudes near Baja California. Through
additional analysis of GPS data on the Pacific plate, we will show whether this
deformation can be observed and how it would alter the definition of Pacific plate
motion.
Although small, the effect of horizontal thermal contraction needs to be quantified and
considered when estimating plate motions from sea floor data. In particular, the contraction
will cause systematic biases in transform fault orientations. Furthermore, the effect can add
up to a few mm/yr when considering certain plate circuits, and could partly, if not entirely,
explain the non-closure of some circuits in global plate motion models such as NUVEL-1A
and MORVEL. |
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