| Spatial
and Temporal Variations Along the New Zealand Plate Boundary:
Decoupling, Delamination, and Localization |
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| Kevin P. Furlong | ||
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  Transition from subduction to transpression at the north end of the south island - the Pacific Chisel Model |
New
Zealand sits astride the Pacific-Australia plate boundary and hosts two
fundamental transitions in plate interactions.
Subduction of the Pacific plate beneath the North Island along the
Hikurangi margin ends and the plate motion is taken up
along the Alpine Fault translational plate boundary in the
Marlborough-Kaikoura regions of the South Island. This
subduction-translation transition has migrated southward since 20-25
Ma. In the south, a small sliver of the Australia plate
subducts beneath the Fiordland region, accommodating an ~100km
transpressional left-step in the Australia-Pacific
plate boundary south of the South Island. At the northern termination
of this small subduction zone the Alpine Fault
initiates. This subduction-translation- subduction plate boundary
structure has undergone significant evolution over the past
15-20 million years, driven by changes in relative plate motion and
significant lithospheric deformation focused in the
transition zones along the plate boundary. At the northern transition,
we argue that the encroaching subducted Pacific slab
acts as a chisel on the lower lithosphere of the overriding Australian
plate driving the active delamination of much of the
mantle lithosphere. This mass removal makes the necessary space to
accommodate the slab. Additionally it drives substantial
vertical tectonics of the Australia plate producing rapid and localized
uplift in the zone of the active delamination. Also a series of
ephemeral sedimentary basins have developed and subsequently been
exhumed in the wake of the advancing slab edge.
At the southern transition, we argue that the localized subduction of
Australia beneath Fiordland is enabled by the
progressive tearing of a sliver from the Australian plate. This leaves
a newly formed edge to the Australian plate that
translates northward along the plate margin becoming the western side
of the Alpine Fault plate boundary. It is useful to
distinguish between the well-described near-surface Alpine Fault (AF)
and the less well understood deeper plate boundary
shear zone, which we term the Southern Alps plate boundary (SAPB). As a
result of the southward migration of the Hikurangi
subduction, the SAPB has been shortening in time. Concurrent with the
shortening of the SAPB (since ~15 Ma) plate
motions between the Pacific and Australia plate have changed, driving a
clockwise rotation in the azimuth of motion along the plate boundary
through New Zealand. This rotation produces a mismatch between the
sense of shear in the ductile lower
crust/upper mantle of the plate boundary and the orientation and
location of the upper crustal AF. Localization of
deformation along the SAPB shear zone can lead to a significant
decoupling between the crust and mantle lithosphere. Evidence from
upper mantle shear-wave anisotropy (SKS splitting) and deformational
modeling suggest that such a decoupling has
occurred, and the resulting spatial and temporal variability in
crust-mantle coupling across South Island, New Zealand may
lead to variability in deformational style along the Southern Alps
orogen. The offset between the upper crustal plate boundary - Alpine Fault System - and the lower crust/lithospheric mantle plate boundary shear zone implies that the central South Island crust is decoupled from the mantle below it. This could result in a zone of 'thin-skinned' tectonics in the region. |
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 Focal mechanisms at the south end of the South Island, showing two distinct slip characteristics. We interpret those with plate-motion perpendicular slip vectors as earthquakes associated with a tear in the subducting slab. |
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© 2003 Pennsylvania State University
