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Titel |
Inferring surface deformation from the form and slope of large-scale river basins : examples from Mars and the Earth. |
VerfasserIn |
S. Castelltort, S. Pochat |
Konferenz |
EGU General Assembly 2009
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 11 (2009) |
Datensatznummer |
250021460
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Zusammenfassung |
By studying nearly un-dissected surfaces on Earth it has been shown that there exists a
relationship between the slope of a given surface prior to drainage initiation and the
form of the river basin that such a surface develops. Essentially, steeper slopes
develop narrower basins. Thus, in a dissected river basin, if remains of the initial
surface are preserved and one can estimate their current slope, it is theoretically
possible to tell whether the river basin has been tilted, or deformed, since its initial
development.
For example, if a river basin acquires its form on a very gentle slope and is subsequently
tilted, its form will not be in equilibrium with its new steeper slope.
The shape of a basin can be measured by its width to length aspect ratio. However this
requires enclosing a basin having a possibly complex shape within a simple rectangle and
there is no unique way of doing so.
Instead, we employ the convergence angle of river basins, defined in Castelltort et al.
(submitted) as the angle α that determines the width-to-length (respectively W and L) aspect
ratio of the basin following tanα = W∕2L. To measure the convergence angle α we use a
method based on calculating source-outlet vectors. For every point in a drainage basin, the
source-outlet vector is defined as the vector between the current pixel and the global basin
outlet. In a rectangular basin, the convergence angle is such that half of the source-outlet
vectors possess an azimuth given between +α and –α of the mean basin azimuth. The
convergence angle is thus given by a fraction of 0.674 of the standard deviation σ in order to
represent half of the data (equivalent to a 50% confidence interval). We compute the standard
deviation of the azimuths of all the source-outlet vectors for each basin to obtain
the convergence angle according to α = 0.674σ. The convergence angle is thus a
characteristic measure of how wide or narrow a basin is. A large and a small convergence
angle correspond respectively to a wide and a narrow basin. The advantage of this
method is that it gives a one-number measure of basin form for basins having very
different and complex boundaries, and that it can be easily implemented with digital
datasets.
The regional slope is computed as the slope of a plane fitted (least squares) to the
topography. Here the slope of a best fit plane through the topography is considered the
simplest way to represent the steepness of a weakly dissected river basin. Excessively
dissected basins (poor correlation between best fit plane and actual surface) are excluded
from the analysis.
By comparing surfaces taken from deformed areas in Earth orogenic belts (SRTM1
and SRTM3) with tilted surfaces on Mars (MOLA) we show that the surface of
Mars has probably been tectonically quiet since the development of its river basins. |
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