Transcript ppt - Margins

Evolution & Dynamics of the
Western Pacific, IBM Subduction
Initiation, and IBM-1
Mike Gurnis
Caltech
IBM-Margins Workshop, Nov, 2007
Outline
• Brief review of formation of arcs in the
Western Pacific, especially IBM
• Regional geodynamic ideas & models for
initiation of subduction
• Large-scale geodynamic ideas & models
for the western Pacific
• Relevance of the proposed IBM-1 hole
Brief review of formation of arcs in the
Western Pacific, especially IBM
Gurnis et al. 2004
An Earlier Evolutionary Model for the
formation of the IBM
Originally
from
Hilde et al. [1977]
as modified by
Stern & Bloomer [ 1992].
Stern [2004]
Taylor & Goodliffe [2004] emphasize that :
The bulk of the fabric making up the WPB
formed after the formation of the new arc
at ~49 Ma. The approximate orthogonal
orientation of the KPR cannot be used to
argue that the subduction formed at an old
transform fault.
The new SZ formed at a high angle to
existing structures.
They suggest that the IBM did not
nucleate on a pre-existing weak zone.
Regional geodynamic ideas &
models for initiation of subduction
Stern [2004]
Subduction Dynamics:
Driving & Resisting Forces
fault
friction, Ff
tectonic force, Ft
Fel
viscous resistance, Fv
buoyancy, Fb
subduction occurs if
Fb + Ft > Fel + Ff + Fv
(modified from
McKenzie, 1977)
Use an explicit finite difference method to
solve the force balance equation
Brittle crust (Mohr-Coulomb)
Non-linear, temperature
dependent viscosity in
crust, lithosphere
and mantle
C,
f
Plastic strain
A. Poliakov, Y. Podladchikov & Talbot [ 1993]
Benchmarked method against Rayleigh-Taylor
problem
Method akin to
Fast Lagrangian Analysis of
Continua (FLAC) [Poliakov and
Buck, 1998; Lavier et al., 2000].
•Explict method
•Visco elasto-plastic material
•Track plastic strain
•Frequent regridding
Homogeneous 30 Myr Plate
10 Ma – 40 Ma Fracture Zone
surface velocity (cm/yr)
35
30
25
20
15
10
5
0
-5
0.0 Ma
6.0 Ma
6.8 Ma
-1
topo (km)
0
1
2
3
4
depth (km)
0
-50
-100
-150
-200
0
200
x (km)
400
600
Hall et al., 2003
Evolution of topography
for 10 Ma – 40 Ma
Fracture Zone Model
Large-scale geodynamic ideas & models for the
western Pacific
• Subduction initiation on the boundary of long-lived zone
of downwelling
• Inability of models with radial variations in viscosity to
generate rapid changes in plate motion
• Plate motions could be resisted significantly by ‘plate
bending’
• Idealized geodynamic models show that evolving deep
buoyancy can be efficiently coupled to the surface and
can lead to plate motions changes on ~10 Myr timescales
• Incorporating these critical processes in numerical
models of mantle convection in a spherical geometry is
facilitated by the linkage with a new paleogeographic
system
Philippine Sea Plate Formed within the ‘Indian Domain’
“…It is plausible that there is a relationship between the position of
[the Izu, Mariana Trough, and Lau Basin]…which formed
concurrently at 45-50 Ma [at] the boundary of the asthenosheric domains…
If the boundary is a major zone of downflow in the upper mantle….,one
would predict a propensity for subduction zones to form there or to
become stationary over time.” Hickey-Vargas et al. [1995]
Simple viscous models with only radial variations in viscosity and long
wavelength driving forces cannot predict rapid changes in plate motions
(and presumably rapid changes in in-plane-stress) [Richards and LithgowBertelloni, 1996]
Conrad and Lithgow-Bertelloni [2004]
Plate motions could be resisted
significantly by ‘plate bending’
A simple theory of a viscous plate bending
atasubduction zone
uo subduction velocity
 plate viscosity
H plate thickness
3
2 H 
Fpb     uo
3  Ro 
Buffett [2006]
H  H o sin(f )
Plate bending slows the plate down and leads to a greater propensity for
thinner lithosphere to subduction [Buffett, 2006]
QuickTime™ and a
Animation decompressor
are needed to see this picture.
Zhong and Gurnis [1995]
Limitations of plate motion models can be
overcome with enhanced paleogeographic
systems that are interfaced with dynamic models
• Continuously closed plate polygons
• Geometry of tectonic elements followed (such as
the subduction polarity)
• Interfaced with paleo-age grids
• Ability of incorporate and explore alternative
reconstructions
• Data passed to/from geodynamic models
• A geodynamists view of GPlates
GPlates: A collaborative effort of Sydney Univ., Caltech & Norwegain Geological Survey
GPlates Reconstruction 0.8.6 [Gurnis, et al. 2007]
Müller, Sdrolias, Gaina, Age grids
Relevance of the IBM-1 hole
(Proposal pending within IODP)
Higuchi et al. [2007]
Summary Points
• Periods of rapid back-arc extension follow subduction
initiation. Numerical models and evolution of Western
Pacific broadly consistent.
• Important regional factors for IBM still need to be
considered: Initiation adjacent to Mesozoic ridges,
without existing, coherent weak zones, and above
coherent, long-lived mantle downwelling.
• Models of plate motions at large-scales must consider
the strength of slabs as both a resisting force (plate
bending) and an efficient coupling mechanisms of deep
slabs with the plates.
• The proposed IBM-1 hole will facilitate discrimination
between models of subduction initiation and if there was
a period of uplift and potentially compression before the
onset of boninitic magmatism.