[GPlates-discuss] Flow lines and ridge jumps

[Mark Brandon]

Nicky and others, 
Given this discussion about flow lines, it may be useful to ask how one might construct "synthetic isochrons" in GPlates. 
The review paper linked below has a discussion about this idea. 

https://protect-au.mimecast.com/s/AMWFC91ZkQtjNYV0UoJMX7?domain=cambridge.org

The objective is nearly identical to producing an grid for plate age. To ask more directly: Is there a simple way to create synthetic isochrons in GPlates?
Best,
Mark 

> On Dec 17, 2018, at 8:13 PM, Nicky Wright <Nicky.Wright at anu.edu.au> wrote:
> 
> Hi Mark,
>  
> At what time are you looking for plate age around the spreading centre? I will admit I haven’t thought about this at all lately.
>  
> As to why the flowline calculation doesn’t work between 40-29 Ma for your second case (Pacific-JdF):
> The JdF rotations (plate 910 in the rotation file) are relative to the Pacific plate (plate 901), and have no relative motion with the Pacific plate before 28.3 Ma.
>  
> To get it to work for the entire time period, you can:
> 	• Create the flowlines for your second case (present-day JdF ridge) using plate 903 (Vancouver) instead - the flowlines will work for the entire time period and will actually use the 910-901 rotations (JdF-PAC) between 0 and 28.3 Ma (this is the easiest option)
> 	• If you’re comfortable editing the rotation file itself and you’d rather stick to plate 910 for your present-day geometry (I know the colour difference in the flowline points might help a little here), you can open the rotation file in a text editor and replace the rotations for 910 with…  (I’ve copied from the existing JdF 28.3 Ma rotation from Matthews et al 2016 here):
> 910 28.3   81.35 -117.91  -30.67  901 !JFC-PAC @REF Wright_++_2016 @DOI"10.1016/j.earscirev.2015.11.015" @CHRONID"c10n1ny"
> 910 28.3    0.0    0.0    0.0  903 ! JFC-VAN
> 910 250.0    0.0    0.0    0.0  903 ! JFC-VAN 
>  
> Cheers,
> Nicky
>  
>  
> From: GPlates-discuss <gplates-discuss-bounces at mailman.sydney.edu.au> on behalf of Mark Brandon <mark.brandon at yale.edu>
> Reply-To: GPlates general discussion mailing list <gplates-discuss at mailman.sydney.edu.au>
> Date: Monday, 17 December 2018 at 11:44 am
> To: GPlates general discussion mailing list <gplates-discuss at mailman.sydney.edu.au>
> Subject: [GPlates-discuss] Flow lines and ridge jumps
>  
> 
> I have been trying out GPlates' flow-line calculation to represent the evolution of the Juan de Fuca spreading center and adjacent plates over the last 40 Ma. As you might know, the JdF spreading center has had some ridge-jump events, one at 20 Ma and another at 5 Ma. My issue here concerns how to represent a change in the spreading center with respect to the flow-line calculation. 
> 
> I document this situation here in two ways, as shown in the two attached movies. The data sources for the reconstructions are: Mueller et al 2016 AREPS Isochrons (Reconstructed Geometries), Matthews et al 2016 GPC, Mesozoic-Cenozoic Plate Topologies (Reconstructed Geometries). The reference frame for the reconstructions is North America fixed, and the time interval is from 40 Ma to present.
> 
> Note that the Yale email server strips out movie attachments, so the attachments here have extensions of .4pm to avoid this problem. Save the files and then convert the extensions to .mp4, and all will be good. 
> 
> The two reconstructions are:
> 
> 1)  Movie: "flowlines Pacific-Vancouver 40-0 Ma"
> The 40.1 Ma anomaly on the Pacific plate is set as the left-side endpoint for all flow lines, with the Vancouver plate on the "right side". 
> 
> 2) Movie: "flowlines Pacific-JdF 40-0 Ma.mp4"
> The modern Pacific/Juan de Fuca boundary is set as a "spreading center", with the flow lines calculated symmetric relative to that reference. (As an aside, I don't know why the flow-line calculation does not work in this case for the time interval between 40 and 29 Ma.) 
> 
> The first reconstruction provides a clear correspondence between the seed points and the coeval spreading center during the early part of the reconstruction. This relationship breaks down after the ridge jumps at 20 and 5 Ma. 
> 
> The second reconstruction shows a poor correspondence between seed point and the coeval spreading center during the early part of the reconstruction. That situation switches to a good correspondence during the later part of the reconstruction. 
> 
> For course, this is all expected given that the seed points for the first case are based on the initial configuration of the spreading center, and the second case, on the final configuration of the spreading center. 
> 
> The first case does a good job representing the motion of the bounding 40.1 Ma isochrons move away from the spreading center. The second case does a good job representing the age distribution near the final spreading center. Both calculations seem to give the same estimate of the displacement field relative to the coeval plate boundary (aka active spreading center). This last point is important, but difficult to precisely verify from these examples. 
> 
> >> It would be useful to hear from the GPlates programmers about this last statement. 
> 
> My objective is not the displacement field, but rather the plate age around the spreading center. My thought at this point is that the plate age might be represented by the minimum of the two age fields. In other words, a ridge jump operates to introducing younger lithosphere in the area around the new ridge. That said, I think there may be some ridge-jump situations that can defeat this simple view.
> 
> I am hoping to get feedback from others, especially those who have already thought through this situation...
> 
> Best,
> Mark 
> 
> 
> 
> 
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