3777901 Plate Boundary Changes Following Collision Updated May 2013

7/27/2019 3777901 Plate Boundary Changes Following Collision Updated May 2013

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FACULTY OF GEOSCIENCESUTRECHT UNIVERSITYTHE NETHERLANDS

TECTONOPHYSICS GEO4-1409

FINAL PAPER:PLATE BOUNDARY CHANGES FOLLOWING COLLISION. OBSERVATIONS AND

MODELS.

STUDENT:RAFAEL FERNANDO DIAZ GAZTELU 3777901

LECTURER & SUPERVISOR:ROB GOVERS

DATE:MAY 2013


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Plate boundary changes following collision Rafael F. Daz Gaztelu 3777901

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PLATE BOUNDARY CHANGES FOLLOWING

COLLISION: MODELS & OBSERVATIONS.

ABSTRACT

The collision that usually takes place between two plates may derive into different kindsof events depending on which sites these collisions take part. More specifically,depending on the kind of weak zone of the plate (Mantle Wedge, Plate Interface, Lower

Continental Crust, etc.), there will be some diverse features like Subduction PolarityReversal, Delamination or simply continuation of the subduction process. Comparison

between observations based on both seismicity and Global Positioning System andnumerical models may give an insight on the subject and maybe give directions for

further investigation in order to improve our knowledge of collision tectonics.

INTRODUCTION

Plate Tectonics is a unique characteristic in the known universe, no other planet in theSolar System displays evidence for the existence of plate dynamics. Subduction happens

when two plates converge and one of them overrides the other, which sinks into theEarths mantle.Although the mechanisms that initiate the formation of new subduction

zones are not fully understood even today, there is a consensus among geodynamiciststhat states that sinking of cold, gravitationally unstable lithosphere drives the plates and

indirectly causes mantle to well up beneath mid-ocean ridges, therefore, the driving forceof plate movement is sinking of the lithosphere, and weakening of it is a required factor

for subduction nucleation. Numerical models concerning subduction initiation suggestthat the subducting plate must be forced down at a rate of at least 1cm/yr, otherwise there

is dissipation of thermally induced density effects and the subduction process wont be

self-sustained (Toth & Gurnis, 1998). Once initiated, the temporal evolution of thisscenario is one of the biggest challenges in geodynamics. After the subduction process isinitiated, it undergoes a series of changes, of which this paper covers two of them,

Subduction Polarity Reversal and Delamination.

Subduction Polarity Reversal.

Subduction Polarity Reversal consists on an interruption of the process of subduction dueto a failure in the overriding plate (see frame 3 in Fig. 1) followed by rupture or bending

of the subducting slab and resulting of the overriding plate subducting the formersubducting plate, changing the roles of both plates (McKenzie, 1969).


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Figure 1.Subduction Polarity Reversal depicted in the case of an arc/continent-continent setting, the

numbered frames indicate the temporal evolution of the system. P1 and P2stand for plates 1 and 2,MWindicates the location of the mantle wedge and Ais the Arc. It has been assumed a detachment of

the subducting slab prior to the reversal, but it may also happen with no detachment whatsoever. The

arrows show the main direction of movements of the plates and the slab, however, the mantle flow isassumed to be going upwards, along the slope of the subducting slab.

Subduction Polarity Reversal is observed in the Wetar Thrust, in the Algerian Margin, in

the San Cristobal trench in the Solomon Islands and in the New Hebrides, but in thispaper only the case for the Solomon Islands will be covered and compared with models.

Delamination

As a result of the weakest zone in the setting being the crust itself another possible

scenario following continental collision arises, and that is delamination. This mechanismdisplays a part or the whole buoyant continental plate breaking apart from the lithosphere

in a sort of planar geometry. Resistance to subduction of continental crust causesdelamination of subducted continental crust from the rest of the subducting lithosphere

and formation of a new plate boundary near the former one.

Figure 2.A depiction of delamination in the case of an arc/continent-continent setting. The numbered

frames indicate the temporal evolution of the system. P1, and P2 stand for plates 1 and 2, MWindicates the location of the mantle wedge and Ais the Arc. The arrows show the main direction ofmovement of the plates, however, the mantle flow is assumed to be going upwards, along the slope of

the subducting slab.


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As seen in Fig. 2, the pushing ofP2overP1turns out into the breaking of P1, peeling thecrust flake and stacking it under the arc. Delamination has been observed to happen in the

Himalayas, in the Aegean Region, at the North American Cordillera and in the collisionzone in N-S China. In this paper, only the Himalayas case will be covered.

OBSERVATIONS

Solomon Islands

This arc-trench system is only a part of the convergent boundary between the westwardmoving Pacific Plate and the northward moving Indo-Australian Plate. Clear evidence of

this happening is the presence of volcanic arcs since the early Eocene.

Figure 3.Spatial seismicity study of the Solomon Islands (ISC less than 4.7 Mb, from the 1st of

January 1964 to the 30th of June 1984). The dashed-dotted line marks the North Solomon trench. This

map also displays the four cross sections A-D useful in the next figure. (Cooper & Taylor 1985).

In the event of a convergent plate boundary, the downgoing slab is spatially mappedthanks to the deep seismicity that it induces. The hypocenter map that shapes the slab is

known as the Wadati-Benioff Zone. Regional seismicity studies reveal the existence oftwo juxtaposed Wadati-Benioff zones of opposite polarity. This is evidence for a reversal

in the polarity of the arc region (Fig. 4) as a result of the convergence between theOntong Java plateau ad the Solomon arc.


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Figure 4. Projections on vertical planes (located in Fig. 3) of ISC seismicity. NBT stands for New

Britain Trench, SCT is San Cristobal Trench and NST stands for North Solomon Trench. (Cooper &

Taylor 1985).

Himalayas

About 60 million years ago, the Indian Plate moved northward carrying the Indian

subcontinent, closing the Neo-Tethys ocean at about 40-50 Ma (Mattauer 1986), and as itsubducted under the Eurasian Plate, an accretionary wedge accumulated from the

sediments and oceanic crust scraped off the descending plate. Rising magma from thedescending plate thickened the Eurasian Plate crust. Approximately 30 to 50 million years

ago, the Indian subcontinent collided with Tibet, but India was too buoyant to besubducted into the mantle, so India broke along the Main Central Thrust fault (Molnar &

Lyon-Caen 1988). As the collision continued, the motion was taken up along the thrust

fault, and a slice of Indian crust and shelf sediments was stacked onto the oncomingsubcontinent. From 10 to 20 million years ago, the Main Boundary Fault developed,stacking a second slice of crust onto India and lifting the first slice. Therefore, it was

proposed that the continued subduction was possible due to the peeling away of thesubducting lithospheric mantle from the corresponding continental crust, what is known

as delamination (Bird 1978).


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Figure 5. Stacking of tectonic units in the Himalayas. MCT- Main Central Thrust. MBT - Main

Boundary thrust. MFT - Main frontal thrust. (Johnson, 2002)

Also, it was proposed that the deformation in the Himalayas is driven by shear

delamination of the continental crust along the crust-mantle below the crust along theMoho (Mattauer 1986). Moreover, the stacking of thrust sheets in the central part of the

Himalayas is indicative of shear delamination and continued subduction of thelithospheric mantle (Johnson, 2002).

MODELS

There are numerous models describing the various effects taking place in a subductionscenario. Chemenda et al. (2001) constructed a both physical and numerical model inwhich the forces associated with the asthenosphere and the subducting plate were the

boundary conditions themselves. On the other hand, Baes, Govers & Wortel (2011)covers a wide area of study as they proposed a series of models deployed with respect a

reference model in which they changed some properties in order to depict the threeprincipal outcomes described at the beginning of this paper. Also, Baes et al (2011) cover

both continent-continent and arc-continent collision. Using the GTECTON (Govers &Wortel 1993) finite element code, they studied the deformation patterns during early

stages of continental collision and solved the momentum equation to obtain stresses andvelocities. Lastly, Midtkandal et al. (2013) built a series of analogue experiments,

building a lithosphere model made of sand and silicone putty, simulating a plateconvergence, and although the model is based on the Iberian-Eurasian plate convergence

and oriented to the understanding of deformation patterns and the local orogen, itprovides a clear visualization of the process.


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Analogue modeling

Analogue models are the simplest way to carry out a simulation. The method consists onsimulate simplified stress profiles, which incorporate brittle (simulated by several kinds

of sand) and ductile (simulated by silicone putty) rheologies with gravity forces.

Figure 6. Experimental setup by Midtkandal et al 2013, performed in the Experimental Tectonic

Laboratory, Universit Rennes I, France, showing the model geometry. The cross-sections 1 and 2 are

detailed on the right. The numbers accompanying each layer are the densities of each layer.(Midtkandal et al. 2013).

The experimental setup (Fig. 6) was built in a 42x44cm container, partially filled with awater solution of sodium polytungstate, representing a low-viscosity asthenosphere. The

southern edge consists on a vertical wall attached to a pair of sidewalls reaching halfwayalong the length of the model and it pushed at a constant rate. The eastern part consists of

two plates with continental-type strength profiles separated by a narrow weak zonewhereas the western part the two continental plates are separated in map view by a larger

wedge shape plate with a brittle, oceanic-type strength profile.The experience consisted on a series of 24 experiments, among which 19 of them were

considered successful. The variables of the experiment were the thickness of the highstrength lithosphere mantle (from now on HSLM) and the shortening velocity.

Three different typical experiments were selected, corresponding to three bulk

deformation patterns; subduction polarity reversal, uniform subduction polarity andtransition from subduction to folding.


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Figure 7.The plot shows the distribution of experimental results as a function of both the shortening

velocity and the HSLM. CC stands for Continental Crust whereas OC stands for Oceanic Crust.(Midtkandal et al. 2013).

The first deformation pattern, which corresponds to subduction polarity reversal ispredominant when HSLM >5 mm and velocity of shortening >5mm/h and is the most

prevalent scenario (see Fig. 7), covering 12 of the 19 successful experiments. As seen inFig. 8a, there is a switch of subduction polarity between the segments CC and OC.

Figure 8. Selected profiles for the three patterns, from West to East of the experimental model.

(Midtkandal et al. 2013).

Fig. 8b corresponds to the second pattern, which displays no subduction polarity reversal

whatsoever, obtained with lower convergence rate and thicker HSLM, indicating that astrong decoupling between upper brittle crust and HSLM can prevent switch of polarity.


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Another interesting model is the one done by Chemenda et al. (2001). Using a 2Dphysical and finite-element numerical modeling technique, they studied the evolution and

failure of the overriding lithosphere during subduction, which turned out to be plausibleand in some occasions, even inevitable.

The key to these changes is weakening of some parts of the setup. This weakening is due

to the interaction between the subducting lithosphere and the asthenosphere in the mantlewedge between the two plates, and due to back-arc spreading.In the case of oceanic subduction, the weakest part is the volcanic arc area, and when it

becomes weak enough, the lithosphere fails there, occurring along a fault dipping underthe arc in either of two possible directions and results either in subduction polarity

reversal or subduction of the fore arc due to fragmentation of the overriding plate(delamination).

Figure 9. Experimental model. 1= Oceanic overriding lithosphere; 2= Oceanic segment of thesubducting lithosphere; 3= Plastic upper continental crust with strong strain weakening; 4= Ductile,very weak lower crust; 5= Plastic continental lithospeheric mantle; 6= Piston; 7= Liquid low-viscosity

asthenosphere; Lb= Back-arc spreading centre/trench distance; La= arc/trench distance. The table

shows an outline of the model parameters throughout the series of experiments. (Chemenda et al.

2001).

The model setup can be seen schematised in Fig. 9; it includes a one-layer overridingoceanic lithosphere and a three layer continental lithosphere. All the lithosphere layers

possess plastic properties and the upper continental crust, the continental lithosphericmantle and the oceanic lithosphere have the same yield limit and are characterized by a

strong strain weakening. The lower continental crust is considerably weaker and moreductile (see the table in Fig. 9 for more details). The lithosphere is underlain by a

low-viscosity asthenosphere, which in the experiment is just pure water. Lastly, theconvergence is driven by a piston moving at a constant rate throughout the experiment.


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Figure 10.Time evolution of the 4th

experiment.(Chemenda et al. 2001).

After a series of experiences with rather different features, experiment 4 (this one havingan increased Lb by 1.4 cm, which in would be about 40 km in real life), there was a

failure (Fig. 10f) in the overriding plate in the opposite direction, followed by subductionpolarity reversal (Figs. 10f and g).


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Numerical Modelling

Baes et al (2011) constructed a series of numerical models of a subduction scenario,changing properties among them in order to show the different responses. The reference

model is a box 1200 km wide and 660 km deep in which two convergent plates moving

with a velocity of 1cm/yr, which is consistent with the conditions for self-sustainability ofa subduction process (Toth & Gurnis 1998). The continental crust lies below the arc, witha channel (8km wide) in between. The Slab broke off and is 30 km deep below the

subducting continental crust edge, which is enough to assume a decoupling between thetwo (See Fig. 11).

Concerning temperature, the initial field in the subducting slab away from the trench isbased on a steady-state geotherm (surface heat flow of 65 mW/m

2). The temperature of

the mantle below the lithosphere (75 km thick) changes adiabatically with a gradient of0.4K/km. The driving forces of the setup are the gravitational forces associated with

density variations and the forces imposed with the convergence rate. Lastly, the modelfollows a viscoelastic-plastic-rheology. The state of stress, strain rate and temperature

drive the domains of elastic, viscous and plastic behaviour. The viscosity associated withthe sinking slab is set to 1023

Pa s whereas the viscosity of the channel and the mantle is

assumed to be 5 1020

Pa s.

Figure 11.General depiction of M1, in which distances, boundary conditions and temperature(colours) are represented. The broken off slab is 70 thick, 600 km long and is 30 km below the

detachment edge. It has a vertical velocity of 1.5 cm/yr. (Baes et al. 2011).

Then M1 was modified by replacing the newly formed shear zone by another narrow

dipping channel (4km thick), with a sediment layer on top of the surface to lubricate thechannel with sedimentary material. This new model evolution is shown in Fig. 12. There

is an uplifting motion on the initial subducting slab. The oceanic lithosphere has been

subducted beneath the arc along the new plate boundary (Baes et al. 2011).


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Figure 12. Model M1ch. The left frame shows the effective strain and the right frame displays theshear strain, both at t = 3.2 Myr for model M1ch. Black arrows represent the velocity field at theindicated times. The vector in the green box on the lower left side of the figure indicates the scale of

velocity vectors. The arrows of the colour bar of total shear strain show the sense of shear as seen in

the vertical model section. (Baes et al 2011).

In modelM2there is no detachment of the sunken slab. There is a leftward shear motionin the channel and the shear motion along the arc/backarc boundary is rightward, which

means subduction of the backarc beneath the arc. Similarly as in M1 (and M1ch), anuplift motion takes place in the subducting slab as well as on the arc. It is greater in M2

though, due to the lower suction force (no detachment). Also, vertical displacementshows a similar pattern as inM1.

Figure 13.Two different modes for M2: (a) to (d) corresponds to M2a: No imposed velocities at thebottom of the slab. Frames (e) to (h) correspond toM2b, which has an imposed velocity at the bottom

of the slab which is the same as in the convergence of the two plates (Baes et al 2011).


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There is sinistral shear motion within the channel, which indicates that the plate interface

is still active. Whereas the shear motion along the arc/backarc boundary is dextral,implying subduction of the backarc beneath the arc. It must be highlighted the increase of

both effective and total shear strains in the fault between the arc and the overriding plate.

In both models,M2aandM2bthe response is a subduction polarity reversal even thoughthe first subducting slab is still attached. Unlike in M1 and M2, which had a weakastenosphere and therefore a weak mantle wedge, in model M3, they assumed a much

stronger mantle wedge, giving the asthenosphere a much higher viscosity, 1023

Pa s.

Figure 14: ModelM3. The left frame shows the effective strain and the right frame displays the shear

strain, both at t = 1.65 Myr. The vector in the green box on the lower left side of the figure indicatesthe scale of velocity vectors. The arrows of the colour bar of total shear strain show the sense of shearas seen in the vertical model section (Baes et al 2011).

As seen in Fig. 14, three deformation zones develop, one along the arc-backarc boundary

and the other two localised in the subduction lithosphere, dipping parallel to the plate

interface. There exists no shear motion whatsoever within the channel (black arrowsvanish at this point), which means that the subduction contact is not operative.Two opposing motions, dextral and sinistral, in the arc-backarc boundary and in the

subducting plate respectively leads to the existence of a shear zone in between that doesnot extend throughout the whole lithosphere beneath the arc, meaning that it is not as

active as the shear zone which develops on the subducting slab.As it turns out, the weakest part of the system is the lower continental crust, leading to the

delamination of the continental crust. This is, breaking apart from the rest of thesubducting slab and the consequent formation of a new plate boundary near the former

trench. This happens when the contrast between the mantle wedge and surroundinglithosphere (MW/ Lith) is less than one order of magnitude.


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COMPARISON AND DISCUSSION

In this paper, plate boundary changes following collision were studied (and focusing

mostly on Subduction Polarity Reversal) first looking at observations for those changes,then going through some models (both analogous and numerical) for the above mentioned

phenomena. When it comes to compare what was found in the models with theobservations recorded at some local spots, a really straightforward diagram arises.

Figure 15.The horizontal axis indicates the relative strength of the mantle wedge which is expressed

by ratio of the viscosity of the mantle wedge (MW) to the backarc lithosphere's viscosity (Lith),

whereas the vertical axis displays coupling between continental crust and lithospheric mantle (of thesubducting plate) which is defined as ratio of the viscosity of the lower continental crust (LCC) to the

viscosity of the upper lithospheric mantle (ULM). (Baes et al 2011).

There are three possible outcomes as a result of two convergent plates (either

continent-continent or arc-continent), depending on where is located the weakest point ofthe setup. If the weakest point happens to be the mantle wedge, the outcome of the

subducting scenario turns to be subduction polarity reversal (upper left quadrant of Fig.10). This happening only when the viscosity of the mantle is at least one order of

magnitude lower than the average viscosity of the lithosphere. In the case of


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continent-continent collision, the ratio of the viscosity of the lower continental crust (LCC)to the viscosity of the upper lithospheric mantle (ULM) must be equal or greater than

0.006. The weakness of the mantle wedge can be checked by two means, the slab lengthand the subduction rate. Concerning slab length, tomography reveals a 2000km long

flat-lying anomaly below the Solomon Islands (Hall & Spakman 2002) that can be

interpreted as remnant of past subduction zones (Baes et al 2011). Also, the slab, beinglarger than 650 km may allow weakening of the mantle wedge due to slab dehydration.Solomon Islands subducting rate is about 7-8 cm yr

-1 which is fast enough (Baes et al.

2011). If the weak part of the setup happens to be plate interface, the result is plaincontinuation of the subduction. If the weak part of the setup turns out to be the lower

continental crust, the outcome is delamination (lower right quadrant on Fig. 10),happening when the ratio between the viscosity of the mantle wedge and the viscosity of

the surrounding lithosphere are at least of one order of magnitude (i.e. greater than 0.1).Taking the same criteria as in the case for subduction polarity reversal in the previous

paragraph, the Himalayas have a slab length of 6000 km and a convergence rate of 10 cmyr

-1.

Chemeda et al. (2001), on the other hand, concluded that an increase in non-hydrostatic,horizontal tectonic compression of the overriding lithosphere combined with the fact that

the lithosphere is weakened due to interaction between the subducting lithosphere and theasthenosphere (in the mantle corner, between the two plates) and due to back-arc

spreading, it can fail in the arc, triggering either a switch in subduction polarity orsubduction of the fore-arc lithosphere.

Finally, Midtkandal et al. (2013) concluded that Subduction Polarity Reversal occurswhen there is a medium to strong coupling between the brittle and the ductile lithospheric

layers, and for thicknesses of sub-Moho mantle hHSLM < 5 mm, which, in turn, implies amedium to high strength in the upper lithospheric mantle (see Fig. 7).

EFFECT CAUSE MODEL OBSERVATIONS

Delamination Weak lower continental crustBaes et. al 2011

Himalayas

SubductionPolarity

Reversal

Weak Mantle wedge

Solomon Islands

Weak Lithosphere. Increaseof compression

Chemenda et al. 2001

Strong upper lithosphericMantle

Midtkandal et al. 2013

Table 1. Comparison between models and observations concerning Subduction Polarity Reversal and

Delamination.

A general overview of the Models versus Observations concerning changes following

collision can be seen in Table 1. Concerning Subduction Polarity Reversal, theappearance of this phenomenon is characteristic of long-time lasting subduction,

consequently providing weakening of the mantle wedge or lithosphere through hydration,consistent with both Baes et al. (2011) and Chemenda et al. (2001) respectively. However,

the effect that causes the overriding plate to fail varies from model to model; furtherinvestigation and modeling should clarify this point.


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CONCLUSIONS

Two main scenarios resulting from the collision of two plates were studied in this paper,

Subduction Polarity Reversal (or the absence of it) and delamination. A switch insubduction polarity is caused by faulting in the overriding plate whereas delamination is

found to be caused by highly buoyant colliding plates. The variables that trigger thesechanges appear to be related to the strength or weakness of diverse parts of the set-up,

according to the corresponding models, these are summarised in Table 1.

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3777901 Plate Boundary Changes Following Collision Updated May 2013