Reason for the relatively isolated triangle of oldest crust in the Pacific?

Ostsol

Looking at maps of the age of the seafloor, one can see a rough triangle in the Pacific near the Marianas Trench that represents the oldest area of the Pacific plate. Some maps mark the very oldest area as being roughly at the centre of this triangle. Upon seeing this I found myself asking how a point on a plate can be surrounded by younger material.

Looking at seafloor topography maps I also noticed the Hawaiian-Emperor seamount chain. I long known that the Hawaiian Islands are the result of the Pacific plate moving over a mantle hotspot, but the Emperor seamount chain is angle more northerly than the Hawaiian chain. To me this indicates that the Pacific plate's movement changed direction at some point. Some reading about the full seamount chain verifies this.

Going back to the Pacific "triangle", I concluded that the it is due to the changing directions of the Pacific plate. However, it seems to require that the plate's movement completely reversed at some point.

Is this even vaguely an accurate assessment of what happened? Or is there something significant that I am missing?

Thanks.

billiards

You might find you get a better response if you post a picture showing what you mean.

Ostsol

Sorry. The map I was considering is on page 3 of "Age, spreading rates, and spreading asymmetry of the world’s ocean crust", published in "Geochemistry, Geophysics, Geosystems", 3 April, 2008. A pdf can be found at the following link:

http://www.earthbyte.org/people/diet...ym_G3_2008.pdf

davenn

Interesting ... something I hadnt noticed before
One thought I had was maybe that area was that that area was a pole of rotation (what is called a Stage Pole) at some time in the distant past, around which the crust rotated around during its march westwards.
You have to remember when that area seafloor crust formed, it was probably located many 1000's of km east or SE of where it currently is

Yes that's correct, the oldest recognised seamount age is ~ 81 million years ( thats a way up near the Aleutian Islands. The change in direction occurred ~ 47 million years ago.

So you can see that the seamount chain is substantially younger than the oldest Pacific seafloor which is ~ 280 million years in that area east of the Marianas.

Tis times like this I wish I was still at university so i could query my tectonics professors :)

cheers
Dave

billiards

I don't know the answer of hand, but it occurs to me that looking at a reconstruction might shed some light on it. One could download the data quite easily and use a tool such as gplates from the earthbyte website: http://www.earthbyte.org/ to look at the area in detail.

billiards

Whoa! I just watched the reconstruction using Gplates (I highly recommend downloading and playing around -- jump straight to the reconstructions tutorial!). Make sure to load the isochrons feature when you play the reconstruction.

You will see the triangle you were speaking of form as a point source, it evolves into a system of spreading ridges which more-or-less give birth to the whole Pacific ocean! It's pretty impressive.

Ostsol

Wow, that's a great program! It does indeed show that triangle as the progenitor of the entire Pacific plate. It still seems odd to me that it started by spreading out in all directions, though... It looks like a few older and long since subducted plates spread away from a single point and the Pacific plate grew out of the space left behind. Very interesting and thanks again!

Ostsol

Ok, looks like I was right. I managed to find a page that cites a paper ("Evolution of the western Pacific and its margin", Hilde et. al, 1976, published in Tectonophysics) stating that the plate formed at the junction between three older plates.