Well, drat! I just spent an hour drafting a little essay on the expectable shape of oceanic plateaus/large igneous provinces. Then I went to see how the Seahawks were doing (not so hot) and when I came back I somehow screwed up and lost the whole blinking thing! I have a sister-in-law who is a computer whiz; she might have pulled my bacon out of the fire, but apparently she is out somewhere, enjoying life – so I will just summarize my thoughts and spit them out without any attempt at literary embellishment. Here goes.
But first:
remember that I have been out of the tectonics game for nearly twenty years,
during which time great strides have been made (and many new conundrums
discovered). Anyway, don’t take my
speculations to the bank
The question
is: what is the shape of the typical oceanic plateau at the moment it
docks? Is it more or less circular
(equi-dimensional, in the horizontal plane. equant)? Or is it an elongate “gummy
bear”, to use Nick Zentner’s flavorful terminology? Well, here goes:
Large
igneous provinces erupted on land seem to be roughly equant; think CPB, Deccan
traps and others. If LIP are the tops of
pipe-like mantle plumes one would expect them to be roughly circular,
everything else equal. But if the plume,
assumed to be stationary or nearly so, lies beneath a moving tectonic plate
then the expectation is that they would be elongate in the direction of
plate motion. How much? Well, that of course depends on both plate
velocity and how long it took for the oceanic plateau to accumulate. Assuming reasonable numbers – a velocity of five
cm/Ma and an accumulation time of five Ma – an undistorted circle of 1000 km.
diameter would be transformed into a 5X4
ellipse; still substantially equi-dimensional. To
derive a worm-like shape would require an improbably rapid rate of plate
motion, a very long eruption interval, and/or a very small LIP accumulation, and probably some (unlikely) combination of all
three. Voila: our LIP probably arrived as roughly equi-dimensional
cow pies, not as string beans
Hence, I
continue to suspect that our Cordilleran terranes arrived as blobs, and then then were sliced and diced to their present elongate appearance by action of a persistent dextral shear zone.
Pretty
simplistic, yeah. What have I
overlooked?
Seahawks - it was a tough game to watch, especially in-person...
ReplyDeleteOn Nick's Eocene C show, he wonders (or was it Jerome L.) whether rifts could also be a source for LIPs (producing gummy worm shapes).
It seems to me that the geochemist have established trace element differences between basalt formed at different tectonic settings. Am I right? If so, can you tell plateau basalt from rift basalt that way? I honestly don’t know. If so, it would be very important evidence
DeleteHi Myrl,
ReplyDeleteI will use the Gorda Sub-plate as an example because it has the highest resolution of any sea floor formation on Google Earth; it is vividly shown with sequential advancement, and appearing like sliced bread. Since the other plateaus probably are similarly constructed with their weakest points at their shared sequential margins, could the Gorda explain how an equi-dimensional "Sponge Cake" plateau (10-100 times larger) can be sliced and diced and accreted as "String Beans"?
Sincerely,
Delete-John Nash
Geology Green-belt
Dangled if I Know. I have a hard time conceiving of a mechanism for “slice and dice” actually within a plate. Maybe Nick will enlighten us.
Delete