Jumping issue when attaching blocks together

Q&A
1

Hello there,

Sorry in advance if such a kind of topic/question has been already done but I am facing a big issue when using GPlates.

I am currently trying to understand the transition from subduction to collision in SE Asia. To do so, I have defined several blocks based on geological maps and I want to reconstruct their palaogeography through time. However, I am currently experiencing a big issue when I start to attach one block to the other : the attached block jumps to its new position while the motion of the main block remains smooth.

To make it clearer… In the example below, there are two blocks 609005 (Western Philippine Basin) and 609021 (part of the Western Huatung Volcanic Arc).

609005 0.0 90.0000 0.0000 0.0000 301 ! Western Philippine Basin
609005 0.8 48.2300 156.9700 0.8680 301 !
609005 1.0 48.2300 156.9700 1.0850 301 !
609005 2.0 48.2300 156.9700 2.1700 301 !
609005 8.0 48.2300 156.9700 8.6800 301 !
609005 15.0 31.1868 142.6885 21.8410 301 !
609021 0.0 90.0000 0.0000 0.0000 301 ! Inner Western Huatung Volcanic Arc North
609021 2.0 -2.9035 111.0604 -3.9501 301 !
609021 2.0 15.2534 123.5419 -5.2534 609005 !
609021 8.0 15.2534 123.5419 -5.2534 609005 !

609005 has a motion from 0 to 15 Ma defined according to Eurasia. 609021 has a motion from 0 to 2 Ma defined according to Eurasia based on published GPS data and from 2Ma backwards, it should have the same motion as 609021. In other words, the reference plate changes at 2Ma.

To define the 2Ma rotation pole, I have been using the kinematic tools from Utilities > Calculate Finite Rotation > Compute Difference Rotation. When doing so, I observe a “slight jump” when passing from 2.0 Ma to 2.1 Ma, i.e. when the transition is supposed to occur. Similar jumps are observed for many other blocks in the region that experienced a similar tectonic evolution but this seems complitely erratic as the jumps may occur before or after the time of attachment, reproducing sometimes very unrealistic motions of several tenth of kilometers.

Maybe my method is absolutely wrong and completely misunderstood how the kinematic tools in GPlates work. Can someone help me on this, please?

With many thanks in advance.

Cédric

2

Hi Cedric/@bubulle,

this sounds like a cross-over issue. So the finite rotation poles of your 609021 plate at 2 Ma need to be switched over from 301 to 609005. This is what you have already attempted, but there might have been some issues.

One way I go about this, is to do this manually. First, you can

609021  0.0 90.0000 0.0000 0.0000 301 ! Inner Western Huatung Volcanic Arc North
609021  2.0 -2.9035 111.0604 -3.9501 301 !
609021  4.0 -2.9035 111.0604 -3.9501 301 ! # Artificially extending the rotation time span

Then, I fix a plate of reference (commonly the one for which I want to specify a relative motion), and reconstruct to the time of interest (the x-over time) and use Reconstruction → View Total Reconstruction Poles → Equivalent rotations to fixed plate. The row for plate 609021 will contain the information you need for the x-over.

You can edit the rotation file entry directly, leaving the rotation of 609021 relative for 301 as it currently is specified, and then add another entry for 2 Ma with the newly derived poles (that should look hopefully a bit different to the ones you have in there now). Save the file and reload the rotation file in GPlates. (CTRL+M, the search for the right row and press reload).

609021  0.0 90.0000 0.0000 0.0000 301 ! Inner Western Huatung Volcanic Arc North
609021  2.0 -2.9035 111.0604 -3.9501 301 !
609021  2.0  x.xxx    y.yyy   p.ppp  609005 ! @XOVER

Make sure to build all successive plate motions for 609021 relative to 609005 back in time on this new rotation pole (if there are relative plate motions).

One other way is to do this in pyGPlates - here and here might be good entry points.

HTH,
Christian