 |
| Titel |
Local and regional slope instability inferred from sea-floor morphology at accretive and erosive convergent margins: case studies of the offshore Hikurangi and Peru fore-arcs |
| VerfasserIn |
N. Kukowski, J. Greinert, S. Hoth, S. Henrys |
| Konferenz |
EGU General Assembly 2009
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250022628
|
|
|
|
|
|
| Zusammenfassung |
| The mechanics of a forearc, a wedge-shaped part of the overriding plate between the trench
and the volcanic arc, are elegantly and in a straightforward way described in terms of the
critical taper concept. Based on the Mohr-Coulomb failure criterion and applying an
elasto-plastic rheology, it describes the state (sub-critical, stable, super-critical) of any point
of the wedge as a function of its geometry (slope and dip), basal and internal friction as well
as basal and internal fluid pressure parameter. Subduction erosion or the subduction of
seamounts and other lower plate topographic features such as basement ridges lead
to temporarily increasing surface slope and therefore may facilitate mechanical
instability.
Here we study the causes of local and regional failure at the central Hikurangi
wedge offshore New Zealand’s North Island and along the Peruvian margin. The
geometry of both margins is well known from seismic studies and swath bathymetry
coverage and therefore allows to quantify local slope gradients and other curvature
attributes.
New high-resolution swath bathymetry data show a complex seafloor morphology from
the Rock Garden area, offshore Hikurangi Margin, that coincides with the subduction of a
seamount presently located beneath the summit of Rock Garden. Another ridge-shaped
lower plate feature is initially colliding with Rock Garden, forming a re-entrant at is
seaward flank. The slopes of the accretionary ridges are steeper than 10∘ and often
more than 20∘ regionally. Slumping mostly occurs on the trench-ward slopes, with
individual failures up to several km2. Critical taper analysis shows that much of
the seaward slopes probably are outside the stability field and therefore subject
to failure. The most prominent feature of seafloor maps is the trench-ward flank
of Rock Garden with a height of 1800 to 2000 m and an average slope of more
than 10∘. Extensional faults arranged in two sub-circular arcs indicate that Rock
Garden may be on the verge of failure. Critical taper analysis also supports this claim
and shows that if basal fluid pressure approach lithostatic, during a large Mw>8
earthquakes, then a complete failure of the entire trench-ward flank of Rock Garden
would potentially affect an area as large as 150 km2 and a rock volume of 150 to
170 km3. This worst case scenario would generate a tsunami wave some 10s of
meters high. Therefore, the observation that numerous seamounts are entering the
Hikurangi wedge and identified beneath Rock Garden other accretionary ridges
along the margin, suggests a thorough assessment of these features needs to be
undertaken and incorporated into tsunami hazard models along the East Coast of North
Island.
A transtensional fault system, identified from swath bathymetry data acquired during the
GEOPECO campaign, is extending for more than 200 km forming the boundary between the
lower and middle submarine continental slope off Peru Normal faults have been identified in
the entire submarine forearc up to the shelf and also onshore raising the question of
their tectonic significance and their role as a potential stress regime indicator in a
convergent margin setting. Swath bathymetry reveals that the lower slope is locally
very rough and suffering from numerous local scale slumping. This observation
suggests that the lower slope is at the verge of failure throughout the entire Peruvian
margin. The middle and upper slopes have a relatively smooth topography, but still
comprise locally steep slopes. To analyse the mechanics of the Peruvian offshore
wedge, we performed a classical critical taper analysis along several transects across
the margin for which we have the precise geometry from swath bathymetry, wide
angle seismic, and seismology. We identified wedge segments according to the
morphological segmentation of the continental slope. Using realistic estimates for the
basal and internal friction as well as the basal and internal fluid pressure ratios we
find that the lower slope and some unusually steep portions of the upper slope are
close to extensional failure whereas the shelf seems to be unconditionally stable.
Employing the dynamical wedge concept we suggest that fluid pressure built up during a
seismic cycle is the most likely means to weaken the basal interface beneath the
lower slope and cause failure, such that it would be in a conditionally unstable state.
This may also be locally the case for the middle and upper slope. However, this
still leaves it difficult to explain the large number of normal faults. Therefore we
include subduction erosion and gravitational collapse of the Peruvian fore-arc,
which has the largest topographic gradient between the trench and the top of the
juxtaposed Andes ( > 12 km) worldwide, in our discussion of potential causes of normal
faulting.
The strength of the plate interface as well as the amount of over-pressuring play a crucial
role for the mechanical stability of both margins. Fluid pressure fluctuations within the
seismic cycle are well capable to bring large parts of the lower and middle slopes
outside the taper stability field. Our comparison highlights that, while the causes
of individual small and very large slumps are different at both types of margins,
seismic behavior, and local as well as regional mechanical stability are intimately
linked. |
|
|
|
|
|
|