spreading ridge velocities

[MFraters]

Currently it is implicitly assumed that the spreading ridge has not moved over the subduction history when the temperature of the slab is computed. This is not always true, for example with the Cascadia subduction zone, where the ridge has moved closer to the trench. From the perspective of the mass conserving slab temperature plugin, this could be easy to resolve.

The mass conserving algorithm just needs to know what the age of the oceanic plate at the time it subducts and the distance to the ridge. Currently it just computes the distance from the closest trench point to the closest ridge point. Since we know the subducting plate velocity (constant for now, that could be a different issue/project) and the length along the slab to the closest point on the slab, we can compute how long ago this part of the slab was at the surface. We can then then also compute the new location of the ridge and compute a new distance. This new distance is the only thing which the mass conserving plugin needs.

Currently the code to compute the distance from the ridge is all contained within the plugin itself. It might be useful to move it to a utilities function.

The input could just be adding an extra input parameter besides "ridge coordinates": "ridge velocity":[1,2,3], similair to #528. Another option would be to allow to set different velocities for different times like this: "ridge velocity":[[50e3,[1,2,3]],[100e3,[-1,-2,-3]]]. It would be a bit more involved to implement, but it would be more future proof.

[MFraters]

@mibillen Do you think this is a good description of the problem and how to solve it?

[mibillen]

@MFraters. Yes, this is a good summary. Can you just fixed the first sentence in the second paragraph: It just needs to know what the age of the oceanic plate is the distance to the ridge. NEED TO BE: The mass conserving algorithm just needs to know what the age of the oceanic plate at the time it subducts and the distance to the ridge.

[MFraters]

Thanks for the quick reply! I have updated the first comment.