The geological and tectonic implications of variations in plate
vector magnitude and orientation along the boundary between the
Indo-Australia and Pacific Plates By Aaron Chia BSc Applied Geology
Curtin University
Introduction:
Plate tectonics though a relatively young science impacts every
area of our planet from the creation of microscopic minerals to the
formation of entire mountain belts and whole oceans. To better
understand past geological conditions and predict future
environments on Earth we need to further study the motion of these
tectonic plates (McCue, 1999). The plate margin along the
Indo-Australian and Pacific plates is a key natural laboratory
where we can better understand the nature of convergent margins and
how these tectonic regimes evolve over time (Schellart and
Rawlinson, 2010).
Convergent margins occur when two plates advance towards one
another. These form collisional zones with either two continental
plates or one continental and one magmatic arc. The complexity that
exists within these tectonic settings is arguably the most
complicated and has been a focus of study since the inception of
the plate tectonic theory (Schellart and Rawlinson, 2010).
This essay will discuss the transition in tectonic regimes and
geological and tectonic implications along the plate boundary at
four points in the study area (A, B, C and D)(Appendix 1). This
covers the plate boundary from the Toga-Kermadec Trench (A) north
of New Zealand, Alpine Fault (B), Puysegur Trench (C) and the Hjort
Trench (D) near the southern Macquarie Ridge Complex to the south
of New Zealand. As a reference point to determine plate vectors the
Australian plate was assumed to be fixed in relation to the Pacific
plate this was used to calculate relative linear velocities and
direction from a common Euler pole. The relative angular velocity
between the two plates was assumed to be 1.18/Ma (Appendix 3).
I will first discuss the geotectonic background of the boundary
an explain the change in tectonic regimes and relative plate
motions at the boundaries through Euler pole calculations and
graphical representations. Lastly I will report the geological and
geophysical expressions from the tectonic regimes and discuss why
these occur.
Geotectonic Background:
The entire plate boundary from the Tonga-Kermadec Trench in the
north to the Macquarie Ridge Complex below the Puysegur Trench in
the south show an evolution from an originally transpressive margin
into a subduction zone (Furlong and Kamp, 2009). Subduction changes
direction with the Pacific plate in the north towards the west
under North Island and subduction of the Australian plate in the
south towards the east under South Island (Delteil et al., 1996).
This displays a unique model for subduction initiation that differs
from the standard Wilson cycle model (Furlong and Kamp, 2009). This
regional tectonic study starts with the Kermadec Trench at location
A where evidence of subduction can be seen.
The Tonga-Kermadec Trench in the north at location A (Appendix
1) is a fast moving subduction zone with the oceanic Pacific plate
subducting under the oceanic Australian plate at 64.30mm/year
(Appendix 3). Progressing further south the Kermadec Trench
transitions into more oblique subduction until reaching the leading
edge of the subducting slab near Hikurangi. The subducting slab
acts as a chisel to drive delamination of the lower Australian
lithosphere as it gradually shifts into a transpressive regime
(Furlong and Kamp, 2009).
The transpressive regime at location B further south is
identified as the Alpine Fault South Island New Zealand which links
directly to the subduction zone in the north through the splaying
of multiple fault traces in the Marlborough Fault System (Delteil
et al., 1996). The overall movement along this transpressive regime
is around 27 5mm/yr parallel to the plate margin (only a proportion
of total plate motion due to compressive characteristics of
transpression) however plate velocities fluctuate upon closer
investigation along this fault. The maximum dip-slip plate
velocities exist towards the centre of the fault around 10mm/yr
with a tapering off in velocity to either end. The northern extent
drops down to around 4mm/yr and the southern drops to zero (Norris
and Copper, 2001). The heterogeneity that exists along the Alpine
Fault is overlooked on small-scale maps, which display the fault as
a straight line suggesting simple strike-slip regimes. This is not
the case in reality as on larger scale maps the fault is more
complex with thrusts in a NNE-SSW direction connecting to ENW-WSW
transfer faults (Appendix 2). South of the Alpine Fault marks a
shift in tectonic regime towards subduction this subduction setting
is the Puysegur Trench located at point C.
North of Puysegur Trench an inverse in subduction polarity
exists as the oceanic Australian plate subducts underneath the
continental Pacific plate. A transition from a strike-slip
(transpressive) regime to subduction occurs forming a tear in the
Australian plate at the southern end of the Alpine Fault system
(Hayes, Furlong and Ammon, 2009). This marks another transition
with the strike-slip motion being transferred between the offshore
Alpine Fault and the Puysegur Fault before shifting into oblique
subduction at Puysegur Trench at point C towards the southern
boundary near the Macquarie Ridge Complex (Lamarche et al., 1997).
Subduction ceases below the Puysegur Trench with no convincing
geophysical evidence for subduction at point D.
The southern most location under investigation is point D along
the Hjort Trench where the Australian plate underthrusts the
Pacific with little to no subduction occurring with plate motion in
a NNE/SSW direction respectively. This differs from the northern
Subduction zones as convergence occurs between two oceanic crusts.
One theory as why Subduction has not initiated is due to the
rheological differences between the two oceanic crusts. This
similar density postpones possible subduction and encourages
further underthrusting (Meckel, 2003).
Relative Plate Motion at Four Localities:
Location A
The relative linear velocity calculated at point A on the
Pacific plate along the Kermadec Trench is the highest among all
four points from the north of the study area to the south with a
velocity of 64.30mm/yr (Appendix 3). The direction of motion is
almost orthogonal towards the Australian with an azimuth of 269.17
(Appendix 1). The above calculated values of relative velocity and
azimuth can be attributed to both the distance from the Euler pole
and the orientation of the plate boundaries in this region. As
distance increases away from the Euler pole the relative plate
velocity increases up to the maximum of 90 from the rotational
axis, after which plate velocity will decrease back down. The
azimuth of linear velocity being almost orthogonal to the
Australian plate at 269.17 will create a purely convergent margin
consisting of dip-slip with rapid subduction of the denser Pacific
plate under the Australian. This tectonic regime changes to an
increasingly strike-slip component towards the South Island of New
Zealand.
Location B
The Pacific plate on the South Island North of Fiordland
contains point B with a calculated relative velocity of 39.97mm/yr
towards the Australian plate at 240.27 (Appendix 3). This smaller
linear velocity in relation to point A is due to the closer
proximity of point B to the Euler pole. The azimuth has moved
anti-clockwise down to 240.27 as the location of point B has
shifted southwest closer towards the Euler pole. A line taken from
the Euler pole to point B can be drawn from which the tangent
towards the direction of rotation will show the azimuth of linear
velocity. The transpressional regime of this region is attributed
to the orientation of the Alpine Fault boundary roughly parallel to
the azimuth of Pacific plate movement. This creates a dominantly
strike-slip regime however some dip-slip is accommodated due to the
small deviation of Pacific plate azimuth from the strike of the
Alpine Fault (Pacific plate boundary). South of location B the
strike-slip component of plate movement reduces until it reaches
the Puysegur Trench where a convergent regime is seen with the
Australian plate subducting under the Pacific.
Location C
Puysegur Trench at point C on the Pacific plate south of South
Island New Zealand has a calculated relative plate velocity of
36.61mm/yr towards azimuth 231.90 (Appendix 3). This reduced linear
velocity in comparison to point B is a result of the closer
distance of point C to the Euler pole. A tangent to the line from
Euler pole to point C in the direction of rotation shows the
direction of linear velocity with azimuth 231.90. This Velocity
vector intersects the Australian-Pacific plate boundary at a high
angle (Fig 1) that produces a convergent margin of the denser
Australian oceanic crust subducting under the Pacific crust (Collot
et al., 1996). Below location C the Australian-Pacific boundary
shifts from a subduction zone back into a strike-slip regime, as
the velocity vector is almost parallel to the plate boundary.
Further south at location D towards the
Australian-Pacific-Antarctic triple junction the Hjort Trench
displays a shift into a convergent margin.
Location D
The Hjort Trench at point D on the Pacific plate has a
calculated relative velocity of 29.17mm/yr towards the Australian
plate with azimuth 191.40 (Appendix 3). This linear velocity is the
lowest among all four points as it is located closest to the Euler
pole. The tangent taken to the line in the direction of rotation
from the Euler pole to point D shows an azimuth of 191.40. The
orientation of the Australian-Pacific plate boundary is oblique to
the plate movement vector on the Pacific plate. This geometry
creates a convergent regime with the oceanic Australian crust
underthrusting the oceanic Pacific crust (Meckel, 2003).
Geological and Geophysical Expressions:
The Kermadec Trench at location A can be identified through the
bathymetry of the seafloor showing a deep trench (up to 10,047m) to
the north of North Island New Zealand. The deep earthquake depth of
100-250km in alignment with a 2000m bathymetric contour that
straddles the continental lithosphere of the Australian plate and
oceanic plate suggest Subduction (Walcott, 1998). Further south
under North Island shows earthquake epicentres adjacent to the
trench with shallow Benioff zones (10-20) extending 200km west. The
slab continues to dip steeper as it descends shifting to 60-70 dip
around 300km west until reaching Wellington and South Island where
the subducting slab steepens to an almost vertical orientation
(Collot et al., 1996). South of Wellington near Hikurangi in the
North Island unique geological and geophysical characteristics are
found representing the transition from subduction into
transpression at the Alpine Fault (Norris and Copper, 2007).
As the Australian lithosphere is delaminated near Hikurangi
towards the south of North Island the Motueka sliver is created
which due to a higher density sinks vertically pulling down the
above continental crust and creating ephemeral sedimentary basins.
These basins are created from the delamination and initiation of a
slab window. As the subduction progresses south, the falling sliver
pulls the upper crust down creating subsidence from the fusion of
hot asthenosphere to the crust. Once the junction of the sliver to
the lithosphere becomes unstable it periodically breaks off
unloading the tension on the upper crust and initiates basin
inversion. These separate ephemeral basins such as the Otunui and
Wanganui are examples of this transition from transpression
movement to a subduction zone setting (Furlong and Kamp, 2009).
South of this transition zone exists the Alpine Fault that has a
general orientation 055/50E. A variety of lithological markers can
be found ranging from displaced metamorphic zones of olioclase to
pegmatite swarms and channel deposits (Norris and Copper, 2007).
Displacement totals 100km to the southwest on the opposite side of
the Alpine Fault that indicates an estimated dextral strike-slip
motion of around 355mm/yr (Walcott, 1998).
Location B along the Alpine fault displays specific geological
evidence in support of the dextral sense of movement along and
adjacent to the fault zone (Baldock and Stern, 2005). Closest to
the fault cataclastites are located which extend up to 50m from the
fault trace. Further out up to 1km from the fault trace mylonites
are exhumed from depths of 25-30km towards the surface. Towards the
periphery of the main fault trace pysudotachyltes can be seen
resulting from the frictional melting of phyllosilicates (Norris
and Copper, 2007). This geological evidence of a strike-slip regime
gradually transitions into oblique subduction under Fiordland in
the South Island of New Zealand. The Puysegur Trench at location C
marks a definitive shift from strike-slip at location B to
subduction at C (Meckel, 2003).
Puysegur Trench subduction is identified at location C with
geophysical surveys showing a Benioff zone extending to around
150km depth and seismic derived bathymetry displaying a maximum
depth of 6300m shallowing to the north towards Fiordland (Meckel,
2003). Gravity surveys over Fiordland show some of the largest
anomalies in the world with -150Mgal free air to the west over the
ocean and +150Mgal bouger to the east over central Fiordland
(Walcott, 1998). Geological evidence can also be seen with a series
of orthogonal fracture zones on the Australian plate displaying a
ridge and trough fabric creating a saw-tooth type pattern
indicating the bending of the plate into the subduction zone.
Additionally small seamounts are located adjacent and parallel to
the trench of which appear to be active (Delteil et al., 1996).
South of Puysegur Trench transitions back to a more strike-slip
regime until reaching location D at Hjort Trench where a
convergence regime exists (Hayes, Furlong and Ammon, 2009).
Location D towards the southernmost extent of the
Australian-Pacific plate boundary at the Hjort Trench involves the
convergence of oceanic crust from the Australian plate seen a
trench to the west. The Pacific plate is located on a ridge and
plateau to the east away from the trench. An identification of a
thrust fault is found within the trench with gravity and seismic
surveys of the Australian plate showing underthrusting into the
pacific plate to around 50km east. Furthermore seismic surveys can
only detect the Australian plate dipping east up to 20km depth. To
the east above the central underthrusting zone from Hjort Trench an
abnormality in gravity surveys has detected up to 250mGal where a
number of underwater volcanoes exist (Meckel, 2003).
Conclusion:
The Indo-Australia and Pacific plate boundary that extends from
the Tonga-Kermadec Trench at point A, south to location D near the
Australia-Pacific-Antarctic triple junction shows a variety of
tectonic regimes. Point A at the Kermadec Trench is a rapidly
moving subduction zone with relative velocity of 64.30mm/yr towards
269.17. A deep trench up to 10,047m is seen which shows Benioff
zones with 60-70 dip 300km west.
Point B along the Alpine Fault is a transpressive regime with
dominantly strike-slip and minor dip-slip movement. The relative
velocity is 39.97mm/yr towards 240.27. Geological units such as
cataclastites, mylonites and pysudotachyltes are evident along the
Fault that displays a dextral strike-slip movement.
The Puysegur Trench at point C has been identified as a
convergent margin with a polarity reversal from point A showing the
Australian plate subducting beneath the Pacific. The relative
velocity is 36.61mm/yr towards azimuth 231.90. Geophysical surveys
have detected a Benioff zone down to 150km with large gravity
anomalies either side of the trench with -150mGal to the west and
+150mGal on the east. The trench that reaches a maximum depth of
6300m has active small seamounts to the east as a result of the
subducting Australian plate.
Hjort Trench at the southernmost point D also reflects a
convergent margin with the Pacific plate moving with a relative
velocity of 29.17mm/yr towards the Australian plate with azimuth
191.40. Geophysics has only detected the Australian plate dipping
east from the trench for 20km. However a major thrust fault along
the trench is identified which has produced underthrusting of the
Pacific plate up to 50km east.
The different tectonic regimes can all be attributed to the
location of each point to the Euler pole and orientation of the
plate boundaries. This in turn affects the relative velocity and
azimuth of plate movement as each point is located on a small
circle with the Euler pole as the common axis of rotation. Points
located on a small circle further away from the Euler pole (axis of
rotation) will show higher linear velocities while those closer
will be lower. The principles used in this essay to calculate plate
movement vectors can be applied worldwide to any type of plate
boundary to determine the past or present tectonic regime.
References:
Baldock, G. and Stern, T., 2005. Width of mantle deformation
across a continental transform: Evidence from upper mantle (Pn)
seismic anisotropy measurements. Geology, 33(9): 741-744.
Collot, J.-Y., Delteil, J., Lewis, K.B., Davy, B., Lamarche, G.,
Audru, J.-C., Barnes, P., Chanier, F., Chaumillon, E. and
Lallemand, S., 1996. From oblique subduction to intra-continental
transpression: structures of the southern Kermadec-Hikurangi margin
from multibeam bathymetry, side-scan sonar and seismic reflection.
Marine Geophysical Researches, 18(2-4): 357-381.
Delteil, J., Collot, J.-Y., Wood, R., Herzer, R., Calmant, S.,
Christoffel, D., Coffin, M., Ferrire, J., Lamarche, G. and Lebrun,
J.-F., 1996. From strike-slip faulting to oblique subduction: a
survey of the Alpine Fault-Puysegur Trench transition, New Zealand,
results of cruise Geodynz-sud leg 2. Marine geophysical researches,
18(2-4): 383-399.
Furlong, K.P. and Kamp, P.J., 2009. The lithospheric geodynamics
of plate boundary transpression in New Zealand: Initiating and
emplacing subduction along the Hikurangi margin, and the tectonic
evolution of the Alpine Fault system. Tectonophysics, 474(3):
449-462.
Hayes, G.P., Furlong, K.P. and Ammon, C.J., 2009. Intraplate
deformation adjacent to the Macquarie Ridge south of New ZealandThe
tectonic evolution of a complex plate boundary. Tectonophysics,
463(1): 1-14.
Lamarche, G., Collot, J.-Y., Wood, R.A., Sosson, M., Sutherland,
R. and Delteil, J., 1997. The Oligocene-Miocene Pacific-Australia
plate boundary, south of New Zealand: Evolution from oceanic
spreading to strike-slip faulting. Earth and Planetary Science
Letters, 148(1): 129-139.
McCue, K., 1999. Seismic hazard mapping in Australia, the
southwest Pacific and southeast Asia. Annals of Geophysics,
42(6).
Meckel, T., Coffin, M., Mosher, S., Symonds, P., Bernardel, G.
and Mann, P., 2003. Underthrusting at the Hjort Trench,
AustralianPacific plate boundary: Incipient subduction?
Geochemistry, Geophysics, Geosystems, 4(12).
Norris, R.J. and Cooper, A.F., 2001. Late Quaternary slip rates
and slip partitioning on the Alpine Fault, New Zealand. Journal of
Structural Geology, 23(2): 507-520.
Norris, R.J. and Cooper, A.F., 2007. The Alpine Fault, New
Zealand: surface geology and field relationships. A Continental
Plate Boundary: Tectonics at South Island, New Zealand:
157-175.
Schellart, W.P. and Rawlinson, N., 2010. Convergent plate margin
dynamics: New perspectives from structural geology, geophysics and
geodynamic modelling. Tectonophysics, 483(1): 4-19.
Walcott, R., 1998. Modes of oblique compression: Late Cenozoic
tectonics of the South Island of New Zealand. Reviews of
Geophysics, 36(1): 1-26.
Appendix 1:
Map of study area showing relative plate motion vectors at four
localities
(Adapted from Google Earth 2014)
Appendix 2:
Representation of a large scale map of the Alpine Fault showing
heterogeneity along tend of fault
(Adapted from Walcott, 1998).
Appendix 3:
Calculations of relative plate motion
Point A:
= longA longE/P = 176.5 - 175.7 = 0.8
cos a = (cos b cos c )+ (sin b sin c cos )cos a = cos(90-59.4)
cos(90-30) + sin(90-59.4) sin(90-30) cos 0.8cos a = 0.871cos-1
0.871 = 29.40a = 29.40
Linear Velocity
V = 111[km/deg] [deg/Ma]V = 111 1.18 sin 29.40V = 64.30mm/yr
Sin =
= = 0.014
sin-1 0.014 = 0.829 = 0.83
Azimuth
Azimuth = 180 + (90-) = 180 + (90-0.83)= 269.17
Point B:
= longB longE/P = (180 - 167) + (180-175.7) = 17.3
cos a = (cos b cos c )+ (sin b sin c cos )cos a = cos(90-59.4)
cos(90-45) + sin(90-59.4) sin(90-45) cos(17.3)cos a = 0.951cos-1
0.951 = 17.77a = 17.77
Linear Velocity
V = 111[km/deg] [deg/Ma]V = 111 1.18 sin 17.77V = 39.97mm/yr
Sin =
= = 0.496
sin-1 0.496 = 29.73 = 29.73
Azimuth
Azimuth = 180 + (90-) = 180 + (90-29.73)= 240.27
Point C:
= longC longE/P = (180 - 164.5) + (180-175.7) = 19.8
cos a = (cos b cos c )+ (sin b sin c cos )cos a = cos(90-59.4)
cos(90-48) + sin(90-59.4) sin(90-48) cos (19.8)cos a = 0.96cos-1
0.96 = 16.23a = 16.23
Linear Velocity
V = 111[km/deg] [deg/Ma]V = 111 1.18 sin 16.23V = 36.61mm/yr
Sin =
= = 0.617
sin-1 0.617 = 38.09 = 38.09
Azimuth
Azimuth = 180 + (90-) = 180 + (90-38.09)= 231.9
Point D:
= longD longE/P = (180 - 158.9) + (180-175.7) = 25.4
cos a = (cos b cos c )+ (sin b sin c cos )cos a = cos(90-59.4)
cos(90-59.3) + sin(90-59.4) sin(90-59.3) cos (25.4)cos a =
0.975cos-1 0.975 = 12.87a = 12.87
Linear Velocity
V = 111[km/deg] [deg/Ma]V = 111 1.18 sin 12.87V = 29.17mm/yr
Sin =
= = 0.98
sin-1 0.98 = 78.60 = 78.60
Azimuth
Azimuth = 180 + (90-) = 180 + (90-78.60)= 191.40
1