CompuServe Messages

Hi, Kamron– >> The is no way for Antarctica to move in any direction without sliding the shift faults into an non aligned condition along some of the Mid Ocean Ridges. What specifically are referring to when you say the SW Indian ridge? The southwest Indian ridge is the spreading ridge passing between Africa and Australia and on to the 3-way junction of ridges in the center of the Indian ocean. We have already discussed the transform faults which uncharacteristically slant across it instead of crossing perpendicular to it as is more normal (sorry). I am interpreting that angling as a sign of slippage along the length of the ridge (which seems near dead as a spreading center, and may be changing into a transform fault, as near as I can tell). Thus I expect Antarctica is moving directly away from Australia these days. >> If the mid ocean Ridge is forming it would slide easily and the shift faults would all be shifted If a ridge is active, I presume it has oriented itself to cut across the tension (if I pull cloth apart, the rip tends to cut across the pull), for the most part. That means there is little sideways tension to make the plates slide along the length of the ridge, or tilt transform faults away from the ridge-perpendicular direction. Once a ridge is well-established, I am not clear on how well it can reorient itself if the direction of tension changes – but that is not going to be commonplace and if the change of direction is large it may encourage a new ridge to take over the action, which might be what happened to the southwest Indian ridge. >> I see that but that doesn't lessen the requirement that the plate shrink as it moves away from the arc where it is forming. As stated before, I am aware of no motions that require a dramatic amount of plate compression to fit through a narrow "exit". In the Indonesian case we are talking about different subducting plates approaching from different directions – each plate is traveling in a mostly unified direction as far as I can tell. >> [The Owen fracture zone] is the transform fault parallel to the Arabian and African coasts about 500 miles east, starting at the India River and moves south. The Mid Ocean Ridge makes a 3 or 400 mile jog the wrong way for Indias motion but correct for what formed the Caucasus Mountains and Alps. Those faults are lined up well for India's sprint, though it is farther out to sea at the Carlsberg ridge where this is really expected. Madagascar was north of its present position (relative to Africa), in what is now the Somali basin. India started not far from that former position of Madagascar (some maps have India a little farther south against Africa in Pangaea, as I recall, but the one I am looking at has the southern tip of India next door to Madagascar and northern India near the horn of Africa). As near as I can tell, Africa hit Europe and raised the Alps, Carpathians and Caucasus Mts. before India was halfway to Asia. It was well after that when the Carlsberg ridge extended into the Gulf of Aden. I presume it was the southwest Indian ridge (not the Carlsberg) that supplied the material for Africa's northward move, with the transform faults since realigned by the more recent action that moved India. >> But we have the rifts in the Mediterranean. You mean the Red Sea? That is a much younger rift just getting started. I see no significant features in the Mediterranean except some striations that probably come from the same collision that raised the Alps. >> Why is there no fault associated with the 90 ridge? I am betting there is a more-or-less dormant long transform fault in that area, perhaps buried under the ridge (say, if it formed because the Indian plate was jamming against the Australian plate until they welded together, or some similar reason). We do not get to see all the faults in the seafloor, especially if they have ceased activity. Alternative: the ridge may be the trail from a hotspot, in which case the fault would be nearby but not at the ridge. It seems a little straighter than I would have guessed in either case, and not angled precisely right, but not horrendously so. >> There are some interesting curves in the Mid Ridge below India showing a sweeping motion but which way Be careful not to confuse the seeming direction of a ridge with the direction of the individual spreading segments of the ridge. When the transform faults are relatively long and the spreading ridge segments are short, the feature as a multi-segment whole seems to trend more in the direction of the faults. But the spreading is along the faults, generally perpendicular to the individual spreading segments. This becomes important to remember in the Indian Ocean – I was forgetting it when I spoke of tracing India back to the ridge. The proto-Carlsberg that separated India from Africa was actually spreading more in the northwest-southeast direction, nearly India's net travel direction, thus it would not have biased India's direction relative to Africa more than a little bit. This also means if you look again, you may figure as I do that the Carlsberg, central, and southeast Indian ridges are all spreading in nearly the same direction. >> The fact that the shift faults stay aligned is the big stick in the spokes for plate motion. See above. –Doug