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When θT is 85° to 95° (i.e. Stephen Zatman, Richard G. Gordon, Kartik Mutnuri, Dynamics of diffuse oceanic plate boundaries: insensitivity to rheology, Geophysical Journal International, Volume 162, Issue 1, July 2005, Pages 239–248, https://doi.org/10.1111/j.1365-246X.2005.02622.x. 4). The lithospheric plates ride on the asthenosphere. Over many millions of years, the continents have traveled everywhere on the globe. Secondly, it shows that the the pole of rotation is more tightly confined to the boundary in the along-strike direction for torques along the central strike line than off the central strike line.

Our results imply that the distribution of stresses can be inferred from simply knowing the location of the boundary and the relative plate velocity across it. This rigid, brittle layer is ~100 km thick and is made of the Earth’s crust and the uppermost mantle.

Solid contours: ∂uy/∂y in arbitrary units. Thus, if the torque components are roughly the same size, the pole of rotation will tend to be oriented close to the z-axis, i.e. Fig. The RRD geometry is stable through most of this region, but is unstable near 26 degrees east. Lower: contours of components of the 2-D velocity gradient tensor. This can be estimated, however, from the shape of deformation profiles (England et.
al 2001). al 1985; DeMets et. To expand our analysis to intermediate power laws of geophysical interest (i.e. The main effect is to redistribute the force per unit length from regions of high longitudinal strain to adjacent regions of low longitudinal strain, making it even more difficult to balance a torque about the middle of a diffuse plate boundary. ), then the deformation function d is constant and this case becomes identical to constant Wand the limit n→∞. Results for intermediate power laws resemble the yield-stress rheology much more than they resemble the Newtonian rheology and depend only weakly on the width of the deforming zone.

Houseman G.A. It reinforces the conclusions that the results are insensitive to the power-law exponent for n≥ 3 and that the pole is more strongly confined to the boundary for short boundaries than for long boundaries. Observed values of L for diffuse oceanic plate boundaries range from approximately 0.25 to 0.5 rad (about 15° to 30°), giving a scale factor of approximately 50 to 200. (see. Zhou S. Fourthly, the figure shows that the dependence on the power-law exponent is weak for n≥ 3 and very weak for n≥ 10. We seek a simple velocity field consistent with the boundary conditions. We approximated the vertically averaged rheology of deforming oceanic lithosphere as a power-law fluid (DeBremaecker 1977), for which . They move across the Earth's surface in response to the different motions of the three plates.

Relative velocities are slow across these regions because the pole of rotation for the plate pair is located within or near the diffuse plate boundary. Destructive plate boundary. Astrophysical Observatory. It does, however, lay the framework for attempting to find such magnitudes by balancing torques across DOPBs with torques along other plate boundaries for which the magnitudes can be estimated.

It averages about 100 kilometers in thickness, but that varies greatly from place to place.

7 illustrates how unlikely it is that the pole of rotation lies outside the boundary for . These models show that the pole of relative rotation between component plates is unlikely to lie outside their mutual diffuse boundary irrespective of rheology. Richardson R.M. Here, we examine how rheology alters the relationship between rotation and torque. Convergent (colliding) boundaries are shown as a black line with teeth, divergent (spreading) boundaries as solid red lines, and transform (sliding alongside) boundaries as solid black lines. For this example, a plate boundary 30° in arc length was used. (see. Benjamin Horner-Johnson helped with preparing some of the figures. In the first case (dashed curve), for which the pole of rotation lies along the central strike line and inside the boundary, there is only a modest decrease, from 1.00 to 0.84 in normalized Tz/Tx between n= 1 and n→∞. al 1998) and strongly influence where diffuse oceanic plate boundaries occur in the Indian ocean (Gordon 2000). Thus, the ratio of ωz to ωx or of ωz to ωy must be even larger for a power-law fluid, than for a Newtonian fluid, to balance comparable-sized components of T. For the rest of our analysis, we use the coordinate system elaborated in Fig. Diffuse boundaries, which are broad zones of deformation, are highlighted in pink.

The, The points where the edges of three plates meet are called triple junctions. As boundary length decreases, the fraction of possible torque orientations that gives rise to a pole of rotation inside the DOPB increases. ω is the angular velocity of component plate B relative to component plate A and T the associated torque that component plate B applies to plate A via the boundary. The left column (panels a to c) shows how ωθ varies with Tθ when Tψ=ωψ= 0, the middle column (panels d to f) shows how ωθ varies with Tθ when Tψ= 30° and the right column (panels g to i) shows how ωψ varies with Tθ when Tψ= 30°.
Variations of the angular coordinates of the pole of relative motion (ωθ, ωψ) with the location of the boundary torque (TθTψ), the boundary length L and the power-law exponent n. Each graph contains curves for n= 1, 3, 6, 10 and n large (to approximate n→∞). A reference axis runs through the middle of the diffuse boundary, which is assumed to lie astride a great circle, GC. the equator of the coordinate system. It is not necessary to know the material properties or width of the zone to infer much of the dynamics. One of these two gradients, ∂uθ/∂θ, can give rise to no force on a vertical plane parallel to the central strike line. 6). The magnitude of this component of the velocity gradient tensor increases monotonically with increasing x. Mackwell S.J. The main implication of these graphs is that the vertically averaged rheology makes little difference: the curves for n= 3, 6, 10 and n→∞ lie nearly on top of each other, and are similar to the curve for n= 1 in each graph. Thus, if the pole of rotation lies inside the boundary, the relation between the torque and pole of rotation is insensitive to the rheology. al (2001) showed from analytical models that the pole of rotation in the diffuse boundary is better confined in the along-strike direction for n→∞ than for n= 1 when Tψ= 0. Plate boundaries.

In contrast, for the Newtonian case, a pole of rotation outside the boundary can generally produce a torque about the centre of the boundary, but not as large a torque as when the pole is in the boundary (for the same rate of rotation). If the pole of rotation lies outside the boundary along the central strike line, the force across the boundary nowhere changes sign and the distribution of forces is capable of making only a small contribution to the torque about the middle of the boundary except in the Newtonian case and perhaps in the n= 3 case (lower half of Fig.

(2) shows that variations in W are unimportant, so one ne not know the across-strike profile of the deformation. For example, if the boundary material is Newtonian and W is proportional to sine of the distance from the pole of rotation (i.e. 9 as a function of the power-law exponent. Cowles S.M. It is widely assumed that oceanic tectonics is well described by a model of rigid plates separated by narrow plate boundaries such as transform faults, ridges and trenches, across which all deformation occurs. The results reinforce the conclusion that most of the sensitivity to the power-law exponent is in the smallest exponents (n < 10 and especially n < 3). The figure illustrates several key results.

Using analytical models, we previously addressed the question of why the pole tends to lie in the boundary (Zatman et.

We then turn to the numerical models and results.

We can build maps of world geography in the geologic past—paleogeographic maps—and model ancient climates. Diffuse plate boundaries extend over broad deformation zones hundreds to thousands of kilometers wide. 9 shows is much more likely to occur than a pole lying outside the boundary in the along-strike direction. These results reinforce the prior conclusion that the pole is more strongly locked into the boundary if a DOPB is short than if it is long. 9 displays maps of the critical orientation of torque that cause angular velocities precisely parallel to the edge of the diffuse plate boundary. (see, Most of the world's volcanoes are related to subduction. Hence, in the Newtonian case, the balancing torque (which is proportional to the strain rate) is expected to be proportional to the displacement rate. Thus, this gradient can be ignored in the calculation of the force and torque that one component plate exerts on the other if they are calculated along the central strike line.

10, which shows the percentage of solid angle enclosed by the curves of critical torque orientation for three different boundary lengths as a function of the power-law exponent. If so, it would provide a means of further constraining how strength varies with depth in deforming oceanic lithosphere (Gordon 2000). Each component of torque external to the plate boundary is balanced by a torque across the plate boundary as a result of deformation in the boundary, which is in turn a result of rotation between component plate A and component plate B. Tz induces a component of angular velocity ωz. Similarly, the shaded bands in panels (g) to (i) are reference values for ωψ. Because the stability zone encompasses the intersecting lines representing the other two plate boundaries, this trench-trench-diffuse (TTD) triple junction is stable. Plates move with respect to each other in three ways: they move together (converge), they move apart (diverge) or they move past each other. On a velocity vector diagram, the diffuse plate boundary is represented as a broad "stability zone" extending between the northern and southern edges of the diffuse deformation. These relationships are further quantified in Fig. The location of poles of rotation of component plates are known observationally only to within a few degrees at best (in the case of the Indian and Capricorn component plates) and to within tens of degrees at worst (Capricorn—Australia and Macquarie—Australia; Royer & Gordon 1997; Conder & Forsyth 2001; Cande & Stock 2004).

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