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

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