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| Titel |
Limits to large-scale fault strength: going directly from geometry to strength using wedge mechanics |
| VerfasserIn |
John Suppe  , En-Chao Yeh |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250054353
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| Zusammenfassung |
| It is of course difficult to obtain direct in-situ constraints on present-day fault strength and
crustal stresses at great depth. However in the special case of critical failure represented by
active accretionary wedges and fold-and-thrust belts we can directly constrain the
large-scale limiting strength and critical far-field stress of the basal sliding surface
and overlying deforming wedge directly from the wedge geometry observed in
seismic images. These results indicate that the basal detachments are very weak
with effective friction of 0.1-0.02, whereas the overlying deforming crust is much
stronger.
Recent advances in wedge mechanics provide remarkably simple relationships between
the surface slopes α and detachment dips β of accretionary wedges and fold-and-thrust belts
and their limiting basal and internal strengths F and W , without strong assumptions about
deformation mechanisms. For example, in the case of homogeneous compressive
wedges F = Cα + (α + β)W , where F = ÏăÏ-¯Ï gH, which can be considered
an effective coefficient of friction, W = (Ïă1 - Ïă3)-ϯ gH and the buoyancy term
C = [1 - (Ïf-¯Ï )] is 1 for subareal wedges and  0.6 for submarine wedges, with Ïf
being the density of the overlying air or sea water. Thus if we have independent
constraints on W we can immediately constrain F . For example, borehole and other
constraints suggest that W is typically in the range 0.5-1. Furthermore, in the case of
wedges with low taper α + β, the range of possible detachment strengths F depends
largely on the surface slope α and is only weakly dependent on wedge strength W .
Analysis of a set of active wedges world-wide, including, including Nankai, Barbados,
Cascadia, New Zealand, Gulf of Alaska, western Taiwan, Niger delta, and Mexican
Ridges, indicate that they have exceedingly weak fault strengths F in the range
0.1-0.02.
Furthermore, many accretionary wedges can be shown to be mechanically heterogeneous
based on their variable taper, typically with steeper surface slope near the toe. Application of
heterogeneous wedge theory to the Barbados accretionary wedge shows that lateral variation
in basal fault strength dominates the variation in wedge taper, with an increasing basal fault
strength towards the toe.
Finally, in situ observations in the Chelungpu scientific borehole after the Chi-Chi
earthquake indicate the Chelungpu thrust ramp is also exceedingly weak, similar to the basal
detachment of the western Taiwan thrust belt F = 0.09 whereas the wedge is strong
W = 0.6, implying that off-fault strength dominates the crustal wedge strength. These results
from critical-taper wedge mechanics are consistent with the weak-fault–strong crust
perspective, with a variety of dynamical and quasi-static mechanisms contributing to
large-scale critical fault weakness. |
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