CompuServe Messages

Hi, Kamron– >> Take for example the area to the west of Madagascar where the weir Ridge south of Madagascar (weird because its shift faults line up with the general direction of the ridge).. >> All three plates are converging in on them selves. That is if you take the lines of direction along each of the plates and they will converge..meaning the plate must get smaller as it moves away from the ridge. You cannot mean the 3 plates meeting at the mid-oceanic ridges in the Indian Ocean since they are diverging, not converging; I figure you are worried about how Africa is almost surrounded by spreading ridges. Imagine two plates are being pulled apart, but there is a third plate between them with a spreading ridge on either side of the middle plate. The two outer plates will move away and the ridges will follow them at half speed as I described before (the plate pulls away, both sides of the separation are added to equally, so the center of the seam has moved halfway with the plate; repeat indefinitely). The middle plate just sits there, no pressure, no movement, except it grows. This is how I see Africa when looking at the east-north-east to west-south-west components of forces upon it – they mostly cancel. Looking at the north-south stresses, the big picture is spreading south of Africa and subduction/compression to the north (at the Alps – Italy was part of the African plate). Africa is moving generally northward. The southwestern Indian ridge is indeed a confuser. One map I see appears to suggest is that it is changing character from a spreading ridge to a transform fault. It may be simply a retired spreading ridge. The angling of the transform faults would seem to be a response to the Carlsberg ridge – transforms from both seem to line up at first glance. The Carlsberg is more clearly active, as evidenced by its carving of the Gulf of Aden; perhaps it is "taking over" all the action. >> Looking at the Antarctic plate…as the plate forms near the tri plate conjunction it must shift along the ridge for motion to occur. There should be some off sets in the shift faults on each side of the Ridges, but they are still all lined up across the Mid Ocean Ridges. Here you are really getting into flat map distortion. On a map that attempts to avoid distortion, it looks reasonable to me. Since the southwest Indian ridge was the first to split Gondwana, it is not difficult to imagine it is mostly dormant; taking the Carlsberg and Southeast Indian ridges (both of which got going later) as the main event for the Cenozoic, the angles of the transforms appear sensible to me. In particular, the last separation between Antarctica and Australia appears to be a simple case of the connect-the-dots principle. >> I use the same approximations to establish the Ridge in the middle of the current continental positions. It is derived from the simple basic principles of plate tectonics. I see no way to derive the connect-the-dots principle from your theory (the dots you should be connecting should have nothing to do with original juxtapositions, only with the shape of the continents and equal-speed meeting points). >> The Northernmost interface of the Antarctic and African plates is very strange. Its shift faults running roughly parallel to its partner Mid Ocean Ridges yet this Ridge runs perpendicular… I expect transform faults to be generally perpendicular to spreading ridges when I am assuming there is no lateral slip along the direction of the ridge (as if the spreading zone were a transform fault itself). But as noted above, the Southwest Indian ridge may be undergoing such slipping as the younger upstart ridges fail to respect their feeble elder. >> Why would the shift faults match so well across the Mid OCean Ridge… I gave a reason before which you ignored; here is another: let differential pull on one the northern segment ("hotshot") of a north-south spreading ridge cause it to migrate eastward relative to an adjacent ridge segment ("slowpoke") at its south. There must be a transform fault connecting the separated ends of hotshot and slowpoke, since the plates are moving in opposite directions in between. Now let us travel eastward from the south end of hotshot, extending the transform fault we just "discovered" — on our left the seafloor created by hotshot is closer to hotshot, and therefore higher in altitude (shallower in depth) than the part on our right created by slowpoke. They meet each other where an extension of the transform fault would be, with this sharp altitude difference where they meet. Let us move west of slowpoke's north end – on our right, plate made by hotshot is now the lower one, being further from hotshot than slowpoke's plate on the left is from slowpoke. Could that altitude difference be all that there is to the extensions of the transform faults? This would explain why they "disappear" farther out without even needing to hide them under sediments – the altitude difference becomes trivial. >> The matching shift faults would form across plate boundaries only if the plates were comming together. They would only stay parallel if there has been no motion in the plate along the connection (which is necessary in some plate connections) since their formation. The mechanism I described produces them in plates moving apart. The ridge will tend to cut directly across the pull, for the same reason tearing cloth is perpendicular to the tension, I assume. The transforms will therefore tend to parallel the tension. Does "Motion in the plate along the connection" mean motion parallel to the ridge (making it act like its own transform fault) such as I described for the Southwest Indian ridge? It would tend to counteract, not reinforce, perpendicularity of ridge and transforms, and it is not the normal case (as with cloth). –Doug