New paper on subduction initiation

Can subduction be initiated at a transform fault? The short answer is probably not. The not so short answers are below and in the linked paper.

 

Initiation of Subduction Along Oceanic Transform Faults: Insights From Three-Dimensional Analog Modeling Experiments

David Boutelier and David Beckett
School of Environmental and Life sciences, University of Newcastle, Newcastle, NSW, Australia

Subduction initiation is a fundamental component of the plate tectonic theory, yet how subduction starts remains controversial. Oceanic transform faults and fracture zones have been proposed as sites of subduction nucleation because they are thought to be mechanically weak and the large buoyancy gradient across these faults because of the difference in the age of the lithosphere, was thought to facilitate foundering. Self-sustaining subduction, defined as subduction driven by the negative buoyancy of the sinking lithosphere might be achieved if at least ~100 to 150 km of convergence can be imposed on an oceanic fracture zone with sufficient buoyancy gradient across the fault.

Previous modelling however did not take into account the fact that the age of the lithosphere and therefore its strength and buoyancy not only varies across the oceanic fault zone, but also along the strike of the fault since many oceanic transform fault link segments of spreading ridges. Here we investigate using three-dimensional analog models how the spatial distribution of strength and buoyancy along and across an oceanic transform fault zone affects the polarity of subduction and whether self-sustaining subduction can be obtained. We designed three-dimensional analog experiments in which two oceanic lithospheric plates are separated by a weak transform fault and convergence is imposed in the horizontal direction perpendicular to the strike of the fault. The spatial distribution of plate thickness and buoyancy are varied along and across the strike of the transform fault, and whether self-sustaining subduction is obtained is assessed using a force sensor.

Cylindrical experiments reveal that subduction polarity is controlled by the buoyancy gradient and the strengths of the plates. With no inclined weak zones, imposed orthogonal compression results in the nucleation of a new fault in the weakest plate leading to the young and positively buoyant plate subducting. However, with an inclined weak zone, the buoyancy contrast controls subduction polarity with the most negatively buoyant plate subducting and a self-sustaining subduction regime obtained after ~300 km of imposed shortening.

Interestingly, this situation is also obtained when including an inverted triangular weak zone on top of the transform fault associated with the serpentinization of the crust and mantle.

In non-cylindrical experiments, taking into account the change along strike of plate strength and buoyancy, the capacity of the transform fault to generate a self-sustaining subduction regime is greatly reduced. Subduction initiates simultaneously with opposite polarity at the two extremities of the transform segment and, at depth, a lithospheric tear is produced that separates the two subducting slabs. In the center of the transform fault, the lack of buoyancy or strength contrast between the two plates leads to multiple thrusts with variable polarities, overlapping each other, and each accommodating too little shortening to become the new plate boundary. This indicates that additional mechanical work is required in the center of the transform fault which prevents the establishment of a self-sustaining subduction regime.

The paper is open access and can be obtained here.

 

Figure 1. Maps of seafloor age (Müller et al., 1997, 2008), plate thickness and buoyancy relative to underlying mantle for fast spreading ridge separating the Pacific and Antarctic plates (A–C), or slow spreading ridge separating the Africa and Antarctic plates (C,E,F). Half spreading rates from GSRM (Kreemer et al., 2003, 2014). Points labeled 1–6 refer to profiles in Figure 2. Plate thickness and relative buoyancy are calculated using the half-space cooling model and the simple plate structure discussed in the text.

 

Figure 8. Successive stages of Exp. 21. Panels (A–E) show the surface view of the deformed model with PIV vectors, and convergence-parallel horizontal shortening rate. Panel (F) shows the evolution of the force measured at the trailing edge of the plate on the left-hand side. The variation of plate thickness and buoyancy along the strike of the transform fault resulted in generation of multiple faults.


Drawing 3D geological structures with sketchup

I discovered this week-end that it is possible to script sketchup, an application to draw objects in 3D. Here is a first test with two semi-transparent inclined planes, and their intersection lineation. The possibility to use scripts instead of the mouse to draw opens up the possibility of multiple 3D drawings of geological structures.

 

 


Another way to look at strain

Here is another way to look at strain from PIV analysis of analogue models. After computing all the spatial derivatives of the velocity or incremental displacements field, the 2D small strain tensor can be assembled.

Below I calculated the principal strain direction from the small strain tensor. This is the direction of one of the principal strain, measured from the x-axis.

PrincipalSmallStrainsThetap

And then I calculated the values of the maximum principal strain and minimum principal strain. Two horizontal bands appear in the plot. The white cells in the top right and bottom left corners show that this experiment is a sinistral horizontal shear.

The data presented here is not interpolated. Each cell is a data point obtained from image cross-correlation. The maps could be interpolated and be made smoother.

PrincipalSmallStrainsEmax

PrincipalSmallStrainsEmin

and then we can claculate the maximum shear. We can see where the shear is occuring, where the difference between the maximum and minimum principal strains is largest.

PrincipalSmallStrainsGmax

Finally, since we have the directions and values of the principal small strains, we can plot them as crosses. The length of the line is linearly proportional to the magnitude of the principal strain, the orientation of the line is the orientation of the principal strain, and red indicates contraction while blue indicates extension.

PrincipalSmallStrainsCrossesCrop

A zoom-in shows the directions of S1 and S2 around the developping plastic shear zone fitting what we expect for a sinistral horizontal shear.

PrincipalSmallStrainsCrossesZoomCrop

I will integrate this into TecPIV rapidly.


Exhumation of (U)HP/LT rocks

Crustal rocks metamorphosed at ultra-high pressure (UHP) record burial to 100–150 km depths and subsequent return to the surface. Although it is well accepted that UHP rocks are formed by deep subduction of continental passive margin rocks, the mechanisms by which these rocks are exhumed remain debated.

Here, three-dimensional thermo-mechanical analogue models investigate how diachronous slab breakoff may lead to the exhumation of subducted continental crust. Slab breakoff initiates spontaneously in one location and migrates laterally along the plate boundary, causing a transient excess downward pull force on the plate boundary in front of the propagating slab tear. This pull force locally reduces the pressure between the plates, which promotes buoyancy-driven exhumation of subducted crust.

However, both the surface area undergoing the pressure reduction and its duration are limited. Our experiments show that the rate of slab breakoff propagation controls both the duration of the pull force and the magnitude of pressure reduction. Our results further demonstrate that exhumation occurs where the slab breakoff propagation rate is lowest, rather than where the pull force is strongest, corresponding to where the slab tear initiates or terminates.

Here is the link to the JSG paper.

Analogue modelling of diachronous slab break-off causing exhumation of subucted crust

 

Illustration of proposed dual-mechanism exhumation of (U)HP rocks associated with propagating breakoff. 1: Horizontal propagation of detachment in the subducted lithosphere; 2: Excess slab pull generated ahead of the propagating tear; 3: Normal pull is produced on interplate zone causing reduction of pressure; 4: Pressure reduction allows buoyancy-driven exhumation of subducted crust; 5: After passage of tear, the lower plate bounces upward causing normal push on plate boundary and increase in interplate pressure; 6: Increased pressure terminates and crustal units are squeezed further upward.


Bending orogens into oroclines

Structural geologists have long debated the tectonic significance of the sinuous map patterns of mountain belt trend lines. The term orocline was originally defined by Carey (1955) to denote map-view curves that developed by bending of an existing linear orogenic belt about a vertical axis of rotation. Although considerable evidence has thus been reported for oroclines, the mechanisms by which these belts acquired their arcuate shape remains disputed.

An arcuate shape can be produced by bending or buckling of a linear object. Bending (flexure) characterises the deformation of a linear object subjected to an external load applied perpendicularly to its long axis, while buckling is the deflection caused by an external load applied parallel to the long axis. A common view is that oroclines develop in response to an along-strike gradient of tectonic forces oriented at a high-angle to the long axis of the orogen. Such bending about a vertical axis can be generated in response to a horizontal pull produced by a sinking, negatively buoyant, lithosphere, or in response to a horizontal push (compression) due to the arrival of an obstacle or indenter in a subduction zone, or a combination of pushing and pulling (e.g., Rosenbaum and Lister, 2004).

An alternative proposition is that oroclines develop by horizontal buckling in response to a tectonic force oriented parallel or sub-parallel to the long axis of an orogen. Suggested tectonic scenarios for such buckling about a vertical axis include escape or extrusion out of a collisional orogen, attempted subduction of a
continental ribbon or orogen oriented at a high angle to the subduction zone, or by margin-parallel drag (e.g., Johnston, 2001; Offler and Foster, 2008; Cawood et al., 2011).

Here we employ three-dimensional analogue laboratory experiments to explore how such buckling may produce an orocline and the geodynamic conditions required for it to occur.

A first series of experiments demonstrates that a crustal ribbon carried by a subducting plate cannot buckle and detach from its mantle root because it weakens and deforms when entering the subduction zone, such that little compressive stress is transferred through the ribbon.

A second series of experiments shows that the aspect ratio of the ribbon impacts the wavelength of buckling and that the experimental tank employed is too small (maximum equivalent length is < 1500 km) to generate multiple buckles.

Finally, a third series of experiments shows that if the plate boundaries surrounding the ribbon resist its horizontal lateral motion, thrusts or strike-slip fault systems may be generated in the ribbon thereby preventing buckling.

We conclude that oroclinal buckling is favoured when a crustal ribbon is pulled by subduction, causing backarc extension. Hence, buckling and bending models for orocline formation are not mutually exclusive but reinforce each other.

 

You can find the paper here

 

Experimental results of crustal buckling (top), and lithospheric buckling (bottom) with (bottom right) or without (bottom left) side plates

 

 

Continuum between bending and buckling and associated sense of movement on strike-slip faults through the orogen.


Geometry of the Gilmore Fault Zone

Deepika Venkataramani successfully completed her M.Phil and her first publication has been out for a while now.

Deepika used a joint inversion of geophysical potential fields to assess the geometry of the Gilmore Fault Zone, a key fault to unlock the story of the Lachlan Fold Belt and the assembly of eastern Australia. Here are the key findings:

  • In this new work we interpret the GFZ to be a west-dipping, crustal penetrating thrust fault that is distinct from the shallow, east- dipping fault that should be separately classified as the Barmedman Fault.
  • The models presented herein show that the Macquarie Arc is thrust under the WMB along a separate major thrust which does not reach the surface.
  • The GFZ separates the ~20km deep Siluro-Devonian Tumut Trough to the east from the Ordovician–Early Silurian Macquarie Arc (thus the GFZ may have initiated as a normal fault!)
  • however, it is not a crustal suture (as defined by Scheibner and Basden, 1998) as there are slices of the same aged rocks (Ordovician–Early Silurian and Siluro-Devonian) bound by the Barmedman Fault further west.

The paper can be obtained here.



Viscoelasticity at large strains

The Maxwell body of a linear-elastic Hookean spring in series with a Newtonian dashpot is the simplest rheological model for geological deformation. It has been employed to describe many deformation processes.

However current Maxwell models display well-known errors when the associated strains and, importantly, rotations are large. Such conditions are often met in Earth sciences. Large rotations pose a mathematical challenge when elasticity is considered in the rheology of highly transformed materials as one requires an objective formulation of the stress rate (time derivative of stress).

In a new publication, Schrank et al. introduce a new large-strain model for Maxwell viscoelasticity with a logarithmic co-rotational stress rate (the ‘FT model’). An analysis of homogeneous isothermal simple shear with the FT model compared
to a classic small-strain formulation (the ‘SS model’) and a model using the classic Jaumann stress rate (the ‘MJ model’) leads to the following key conclusions:

  • At W  ≤ 0.1, all models yield essentially identical results.
  • At larger W, the models show increasing differences for γ > 0.5. The SS model overestimates shear stresses compared to the FT model while the MJ model exhibits an oscillatory response underestimating the FT model.
  • The MJ model violates the self-consistency condition resulting in stress oscillations and should be disregarded. It does not deliver truly elastic behaviour.
  • In the intermediate-W regime, the shear-stress overestimates of the SS model may constitute acceptable errors if energy consistency is not important. If energy consistency is desirable, the SS model should not be used at W ≥ 0.3.
  • In the high-W regime, stresses in the SS model become unacceptably
    large. The FT model should be used in this domain.

The FT model constitutes a physically consistent Maxwell model for large non-coaxial deformations, even at high Weissenberg numbers (W). It overcomes the conceptual limitations of the SS model, which is limited to small transformations, not objective and not self-consistent. It also solves the problem of the energetically aberrant oscillations of the MJ model.

 


Slab breakoff: insights from 3D thermo-mechanical analogue modelling experiments

tectonophysics-10-2016

 

http://www.sciencedirect.com/science/article/pii/S0040195116304735

The detachment or breakoff of subducted lithosphere is investigated using scaled three-dimensional thermo-mechanical analogue experiments in which forces are measured and deformation is monitored using high-speed particle imaging velocimetry (PIV). The experiments demonstrate that the convergence rate in a subduction zone determine if and when slab detachment occurs. Slow subduction experiments (with scaled convergence rates ∼1 cm yr −1) have lower Peclet numbers and are characterised by lower tensile strength subducted lithosphere, causing detachment to occur when the downward pull force exerted by a relatively short subducted slab is relatively low. When continental collision is preceded by slow oceanic subduction, the subducted lithosphere therefore need not be very long or extremely negatively buoyant to cause detachment because the subducted oceanic lithosphere is hot and weak. Under such conditions detachment may occur sooner after the onset of continental subduction than previously predicted. In contrast, if a collision is preceded by rapid subduction (∼10 cm yr −1), breakoff will be delayed and occur only when the convergence rate slowed sufficiently to thermally weaken the slab and cause its eventual failure. The analogue experiments further confirm that slab detachment occurs diachronously as it propagates along the plate boundary. Stereoscopic PIV reveals a characteristic strain pattern that accompanies the detachment. Horizontal contraction and subsidence (with scaled values up to 1200 m) in the trench and forearc area preceeds the passage of the detachment, which is followed by horizontal extension and uplift (up to 900 m). High-frequency monitoring captures rapid propagation of the detachment along the plate boundary at rates of up to 100 cm yr −1. However rate is not constant and interaction between the slab and lower mantle or opening of a backarc basin in the upper plate can reduce or stop slab breakoff propagation altogether.

 

fig4

Successive side views of the models in Experiment 1 and 2. Experiment 1 (a-e), the subducting lithosphere is pushed by the piston at the constant velocity of 2.5 × 10 −4 m s −1 (equivalent to ∼10 cm yr −1 in nature). The slab becomes vertical due to the negative buoyancy but does not break. It folds when hitting the rigid plate that models the impenetrable lower mantle. Experiment 2 (f–j), The model is identical to Experiment 1 but it is the upper plate that is pushed instead of the lower plate. The model evolution is similar to Experiment 1 until the slab touches the lower mantle. The slab angle reduces in the late stages (dashed line in panel j).

 

fig5

Successive side views of the models in Experiment 3. The model is identical to that employed in Experiment 1 (Fig. 4), but the imposed rate is one order of magnitude lower, 2.5 × 10 −5 m s −1 (equivalent to ∼1 cm yr −1 in nature). Very slow subduction leads to multiple slab detachments at 2283, 4266 and 6420 s. We note that the repeated detachment caused extension in the trailing edge of the upper plate, and a slab graveyard sitting on top of the rigid upper mantle.

 

fig-9

Sketch of propagating slab detachment with distribution of surface deformation and uplift. Horizontal contraction and surface subsidence is generated ahead of the breakoff tip, while horizontal extension and uplift follow.

 

fig10

Maps of earthquakes hypocenters along the Aleutian subduction zone (a), and Java-Sumatra-Andaman subduction zone (b), with profiles showing the Wadati-Benioff zone. Earthquakes hypocenters are represented by circles with diameter proportional to magnitude, and color indicating depth (see profiles for color scale). Hypocenters are from EHB catalogue (Engdahl et al., 1998). White arrows represent the convergence vectors calculated using the MORVEL global kinematic model (DeMets et al., 2010). V is the convergence rate (in mm yr −1), θ is the obliquity (angle between the normal to the trench and convergence vector), and Vn is the convergence in the direction of the profile (i.e. convergence corrected from obliquity, in mm yr −1). Topography/bathymetry from Smith and Sandwell (1997).

 

 

fig11

Slow oblique subduction along the northern branch of the Caribbean subduction zone. Map shows the topography of the trench characterized by a deep through between 65 and 67°W. Convergence from MORVEL global kinematic model (DeMets et al., 2010), is only 19 mm/yr at 64°W (white arrow) and the obliquity is approximately 67°, which yields about 7 mm/yr of normal convergence in the subduction west of 64°W. 3 North-South profiles are plotted showing that this area is also characterized by a deep negative free air anomaly (Sandwell et al., 2014), the peak of which is located the forearc. Based on our experimental results we propose that both the topography and gravity anomalies are caused by an excess downward pull in the subducted lithosphere due to ongoing slab detachment. Topography/bathymetry from Smith and Sandwell (1997), gravity from Sandwell et al. (2014).

 


GEOS2190 – Potato Point – South Coast NSW

GEOS2190 – Structural and Field Geology changed its location in 2016!

Instead a mapping on km-scale in the Hill-End trough, we travelled to Potato Point and Mystery Bay in South Coast NSW. The objective was the mapping of bedding and younging in simply folded Ordovician turbidites of Potato Point to build a cross-section, and stereonet analysis. This exercise really pushes the idea that structure and stratigraphy must be worked out together in folded areas.

Then a short day at Mystery Bay introduced the notion that many areas have been folded several times. Next step, the multiply folded and metamorphosed meta-turbidites of Broken Hill.

#UonGeos #GEOS2190

My friend Stefan Vollgger from Monash Uni made the basemap used by the students with his UAV. It worked great. A 3D model is also available here: https://skfb.ly/ToHY Check it out!

 

14659461_1710770505910383_119594064436789248_n

Rock plateform Potato Point

 

14718452_198906417187055_702751168970883072_n

Kink fold – Mystery bay

 

14607054_1323734267661023_6180817629570138112_n

Microtectonics – Mystery Bay

 

14714606_1290928914283163_8278040664034246656_n

View from the cabin at the Beachcomber caravan park

 

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Students setting up their tents at night because our bus broke down and we arrived late

 

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Locals at Beachcomber caravan park


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