#Subduction & Ocean Basin
8 messages in this thread
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
Doug
>>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.<<
Bad wording… This is a biggie for me…
The meeting of the three plates have places of converging forces..
Looking at the point at the top of the Antarctic plate it has a spreading rate
along each side..picture the triangle …it would really move sort of straight
down towards the pole here as a result of the two motions leaving each of the
sides motion in reference to the adjoining plates north of it.. would need to
slide along the mid ocean ridge. try it with your hands on a globe…there
would have to be a relative motion along the interface of the other two plates.
There are two problems with this…while the eastern interface with the
african plate "looks" like it has been sliding there is no evidence of sliding
along the other interface with the australian ridge. Its shape would prevent
that. Also the shift faults would have been sliding and would not line up. Face
this one down and you will see the start of the Global motion problem.
>>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.<<
The shift faults would not line up like they do think about it they would be
sliding on one side or the other. Simulate the motion..take a small piece of
paper and mark the shift faults. Now move the paper in the direction you think
the plate went and notice the faults would be curved( the easter antarctic&
australian plate connection works in one direction sort of..don't let that one
give you hope..the rest of the motions would all cause slanted shift faults in
every direction. Worse look at the forces of the motion.. there are three
plates each one has motions along this interface that show converging
forces…the australian arc converges at borneo, the antarctic triangle
converges at a 45 degree angle and the african plate converges just above South
Africa. This area just doesn't work on paper or in theory if you plot the
motion. Its worse on a globe than on a flat map.
>>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.<<
Not at the north end it is moving west on the east side and east on the west
side. You can't reconcile all these motions and keep the shift faults and
transform faults aligned across the Mid Ocean Ridges. I can see africa as the
place where the whole world is spreading away from…but not antarctica also as
well as not enough subduction..There is no direct evidence of subduction on the
northern part of India either, looks like its folding or bunching to me.
>>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.<<
There is a big shift at Owen fracture Zone that breaks and shifts the
carlsberg ridge as you say. The Carlsberg Ridge is part of the three leg split
we discussed before, but it is spreading at a tapered rate terminating in the
crotch of the rifts. Why would the Indian side of the Owen shift zone be
further south where the India smashed up the Himalayas that where the African
side has a split between it and the arabian plate?
>>Here you are really getting into flat map distortion. On a map that
attempts to avoid distortion, it looks reasonable to me.<<< Look at it on a
globe and you will think again.
>> 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<<
I don't see a SE Indian Ridge but go ahead..It looks like India is part of
Australia on my materials.
>> (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.<<
It has taken the longest for me to work out the original position of
antarctica but I believe the Niney East ridge that 2500 mile straight long
ridge South of calcutta and the Chagos Laccadive Ridge show us the path of
Antarctica. Down then swing west with S. America one smooth three or four day
event.
>>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).<<
Not true the shape of the Mid Ocean Ridge and the mid point is approximately
the location of the connect the dots locations of the continents drawn along
the shift fault lines more or less.
>>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.<< Nice description
Here you can still connect the dots along the shift faults from antarctica
more or less up toward India.
>>I gave a reason before which you ignored;<< Not ignored…the shift faults
as I explained would not stay aligned when you plug in the motions of the
Plates.
See next post for balance.
Kamron
Hi, Kamron–
>> Looking at the point at the top of the Antarctic plate it has a spreading
rate along each side..picture the triangle …it would really move sort of
straight down towards the pole here as a result of the two motions leaving each
of the sides motion in reference to the adjoining plates north of it.. would
need to slide along the mid ocean ridge.
That is what I said, there may be sliding along the southwest Indian ridge,
which in any case is not spreading fast, and/or sliding along those angled
transform faults across that ridge. The Antarctic plate appears to me to be
moving mostly parallel to the southwest Indian ridge, which leaves me seeing no
problem.
>> there is no evidence of sliding along the other interface with the
australian ridge.
I do not need or want such sliding; I expect Antarctica is moving pretty close
to directly away from Australia. With no sliding along ridges except in the
southwest Indian, I expect the others to have perpendicular transform faults,
which they more or less do.
>> The shift faults would not line up like they do
I assume the northward movement of Africa since the S. Atlantic opened has been
modest compared to the rate of east-west spreading, hence the predominantly
east-west transform faults on the east-west ridges. Africa may have stopped
all northward motion for all I know.
>> the australian arc converges at borneo
Toward Borneo maybe, but all the action is in the subduction zones surrounding
Indonesia + Phillipines. The north-moving Australian plate hits the Asian
plate and subducts at the generally east-west Java trench; the generally
east-west motions of the Asian and Pacific plates meet at generally north-south
trenches east of Indonesia+Phillipines.
For all I know, there may be weaknesses in our understanding of some of the
details at many points; maybe 10-20% of the puzzle pieces have not been fit
into plate tectonics yet, and they may hold a few surprises. But with 80-90%
of the pieces in place, fitting together and forming a sensible big picture, I
am confident the missing pieces will be found and placed.
The theory may take some revisions; Newton's physics was overthrown by both
relativity and quantum mechanics in turn, but it works so well in normal realms
that it is still used, and the revisions continue to use large parts of
Newton's infrastructure. Given the degree of coordination in independent lines
of evidence (radioactive datings, paleomagnetism, deep quake mappings, seamount
chain orientations, etc.), and its ability to predict, plate tectonics has
surely achieved that Newtonian status.
Concerning the predictions of plate tectonics: the Scientific American of Nov.
1986 has an article about how plate motions had already been measured directly
before the advent of GPS, by long-baseline interferometric measurements using
radio telescopes keying on quasars. A decade ago they had already measured
long term movements between stations in Germany, Sweden Connecticut, Texas,
Hawaii, Alaska, Kwajalein and Japan, and found them almost entirely in accord
with predictions based on plate tectonics (all right, so they found CT and TX
moving together a bit, but 8 out of 9 is not at all bad – what other theory
predicted half as many of the observations?). After another decade of
measurements, if they had found any great problem for plate tectonics we would
have heard the great hullabaloo by now.
>>There is no direct evidence of subduction on the northern part of India
either, looks like its folding or bunching to me.
Correct, as far as I know. It was mentioned in a hedging way, as if not enough
seismic evidence was available to know whether the lithosphere underneath the
Himalayas is still subducting.
>> Why would the Indian side of the Owen shift zone be further south where the
India smashed up the Himalayas that where the African side has a split between
it and the arabian plate?
I see no Owen shift zone on my maps, thus no comment.
>> I don't see a SE Indian Ridge but go ahead..It looks like India is part of
Australia on my materials.
The southeast Indian ridge is the third leg of the junction in the mid-Indian
Ocean – it passes between Antarctica and Australia.
India was wedged between S. Africa and Antarctica in Pangaea. It took off in a
sprint (geologically speaking) towards Asia in the late Triassic, before
Australia and Antarctica split. Between it and Australia there must have been
a fast-moving transform fault; I have assumed without evidence that the 90-East
ridge is associated with that fault. The fault must be mostly inactive since
India hit Asia, thus India would seem to be part of the Australian plate. But
I expect it is a rather loose association; since India has this problem with
Asia that the rest of the Australian plate does not, it would not be surprising
to see them go their separate ways. If subduction has stopped or slowed under
the Himalayas, this might be happening already.
I do not see what the 90-East ridge would have to do with Antarctica, which was
a neighbor of S. Africa in Pangaea, nowhere near Asia. Are you claiming
otherwise? If so, we are going to get you because I am sure we can find geo
formations that correlate Antarctica with S. Africa, not Asia, from the
Triassic.
>> Here you can still connect the dots along the shift faults from antarctica
more or less up toward India.
What I expect to work in that case is a connection of India's margins with
points on the southwest Indian ridge corresponding to where India was connected
to Africa and Antarctica before that earliest split of Gondwana.
–Doug
Hello Doug
Oh I feel like I may be close here..
>> That is what I said, there may be sliding along the southwest Indian ridge,
which in any case is not spreading fast, and/or sliding along those angled
transform faults across that ridge. The Antarctic plate appears to me to be
moving mostly parallel to the southwest Indian ridge, which leaves me seeing
no problem.<<
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?
>>I do not need or want such sliding; I expect Antarctica is moving pretty
close to directly away from Australia. With no sliding along ridges except in
the southwest Indian, I expect the others to have perpendicular transform
faults, which they more or less do.<< I agree that that is how it moved. Look
now at the big shift in the Ent Nin Fracture Zone in the lower Pacific Rise. It
has shifted towards S. America or on the Australian towards Australia this is
opposite from what you would expect here isn't it?
>>I assume the northward movement of Africa since the S. Atlantic opened has
been modest compared to the rate of east-west spreading, hence the
predominantly east-west transform faults on the east-west ridges. Africa may
have stopped all northward motion for all I know.<< Ok I agree
but did you try the line up of the shift faults using a piece of paper? If the
mid ocean Ridge is forming it would slide easily and the shift faults would all
be shifted instead of perfectly aligned with their counter parts across the
Ridge.
>>> the australian arc converges at borneo<<<
>>Toward Borneo maybe, but all the action is in the subduction zones
surrounding Indonesia + Phillipines. The north-moving Australian plate hits
the Asian plate and subducts at the generally east-west Java trench; the
generally east-west motions of the Asian and Pacific plates meet at generally
north-south trenches east of Indonesia+Phillipines.<<
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. It also doesn't eliminate the need
for the shift faults to slide and become un aligned across the Mid Ridge.
>>Given the degree of coordination in independent lines of evidence
(radioactive dating, paleomagnetism, deep quake mappings, seamount chain
orientations, etc.), and its ability to predict, plate tectonics has surely
achieved that Newtonian status.<< It is an awesome body of evidence..but the
shift faults would not remain aligned if all the motions are occuring
>>Concerning the predictions of plate tectonics: the Scientific American of
Nov. 1986 has an article about how plate motions had already been measured
directly before the advent of GPS, by long-baseline interferometric
measurements using radio telescopes keying on quasars. A decade ago they had
already measured long term movements between stations in Germany, Sweden
Connecticut, Texas, Hawaii, Alaska, Kwajalein and Japan, and found them almost
entirely in accord with predictions based on plate tectonics (all right, so
they found CT and TX moving together a bit, but 8 out of 9 is not at all bad –
what other theory predicted half as many of the observations?). After another
decade of measurements, if they had found any great problem for plate tectonics
we would have heard the great hullabaloo by now.<<
It looks like only I am concerned.<g>
>>I see no Owen shift zone on my maps, thus no comment.<<
It 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. But we have the rifts in the
Mediterranean.
>>The southeast Indian ridge is the third leg of the junction in the
mid-Indian Ocean – it passes between Antarctica and Australia.<< OK
>>India was wedged between S. Africa and Antarctica in Pangaea. It took off
in a sprint (geologically speaking) towards Asia in the late Triassic, before
Australia and Antarctica split. Between it and Australia there must have been
a fast-moving transform fault; I have assumed without evidence that the 90-East
ridge is associated with that fault.<<
Thats the best explanation for that I see.
>> The fault must be mostly inactive since India hit Asia, thus India would
seem to be part of the Australian plate. << Thats how it is shown on my maps
>> But I expect it is a rather loose association; since India has this problem
with Asia that the rest of the Australian plate does not, it would not be
surprising to see them go their separate ways. If subduction has stopped or
slowed under the Himalayas, this might be happening already.<<
Why is there no fault associated with the 90 ridge? It is the longest
straightest ridge on earth and lies there without notice. I associate it with
the motion of Antarctica but I like the idea of India plowing the ridge up as
it passes. I don't have an explanation for all that motion though.
>>I do not see what the 90-East ridge would have to do with Antarctica, which
was a neighbor of S. Africa in Pangaea, nowhere near Asia. Are you claiming
otherwise? If so, we are going to get you because I am sure we can find geo
formations that correlate Antarctica with S. Africa, not Asia, from the
Triassic.<< Get me then because this area is the least obvious part of the
earth motions for me. You have added something for me to think about on the
india and 90 ridge deal.
>>What I expect to work in that case is a connection of India's margins with
points on the southwest Indian ridge corresponding to where India was connected
to Africa and Antarctica before that earliest split of Gondwana.<<
There are some interesting curves in the Mid Ridge below India showing a
sweeping motion but which way?
The fact that the shift faults stay aligned is the big stick in the spokes for
plate motion. If the motions exist as the theory states the shift faults will
shift along the ridge as new ridge is formed during plate motion.
Kamron
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
Doug
>>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.<<
This would only be true in certain conditions but for most of the plates
there is an impossible situation present. The condition is exaggerated where
the ridges come together especially a three axis connection.
If the motion of antarctica plate is expressed by the shift faults in the
Southwest Indian Ridge making this Ridge is mostly a transform fault.
It is moving more or less in the direction of S. America and some pinching is
seen in the area between them which is all logical.
Then the motion would slide the plate along the Southeast Indian Ridge a small
amount in response to the growth of the plate.
Moving around the plate further the motion would increase along the ridge.
On the opposite side of the Antarctic plate from the South East Indian
Ridge…this is the area under New Zealand the motion of the plate would be
perpendicular to the Mid Ocean Ridge causing a lot of sliding along the Mid
Ridge which has not occurred. The proof that it has not occurred is the shape
of the ridge and the total alignment of the shift and transform faults.
(Having a globe with the basins on it helps see this clearly.)
The shape of the ridge and the configuration of the shift faults proves the
plate is not moving.
Kamron
Doug
>> 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.<<
I agree that a unified motion is what is stated but you cannot have a unified
motion into and out of a large arc. Motion to the inside necessitates reduction
of the size and motion to the outside of the arc needs thinning expansion or
fracturing radially.
>>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.<<
This is what I was pointing out. Another of earths head scratchers.
Madagascar was north of its present position (relative to Africa), in what is
now the Somali basin.<< I put it there at first but now have it south at the
next inset in one simulation and north in the gap at the top of the horn of
africa. I hope to settle this within a few more weeks.
>>As near as I can tell, Africa hit Europe and raised the Alps, Carpathians
and Caucasus Mts. before India was halfway to Asia.<<
I agree…I see the mediterranean as a rift that split europe from asia and
Greenland from North America before the Atlantic split apart. Then after the
atlantic split africa was forced Northward and jammed Europe.
>> 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.<<
Yes I agree..if I understand the point. The width of the South West Indian
Ridge is measured in a SW to NE direction. It is similar in age as the South
East Ridge where it connects to but it has spread along the transform fault at
a much more rapid rate showing the sprint of India as you say.
>>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.<< Only where the transform faults are parallel.
Again when you take any plate and using the shift or transform faults as the
key to motion which we have both agreed to you come up against the big problem
for plate motion.
The shift faults point in multiple directions all on the same plate with the
same unified motions. Look at Antarctica for a clear example of what my
observation states. The motions in some places are 90 degrees to the observed
shift faults. This can't be so if the basins are forming over time.
Kamron
Hi, Kamron–
>> I agree that a unified motion is what is stated but you cannot have a
unified motion into and out of a large arc.
Says who? All it needs is some subduction where the plate is not moving quite
perpendicular to the subduction boundary.
>> This is what I was pointing out. Another of earths head scratchers.
If the Owens fracture zone parallels India's movement, why is that any big
surprise? It was not on the direct path, but I see no reason it should not
remain parallel to the nearby action.
>> I put it [Madagascar] there at first but now have it south at the next inset
in one simulation and north in the gap at the top of the horn of africa.
Keep an eye on the geological formations. Madagascar fits the Mozambiquan
mountain province, but I am not aware that province extends as far north as
that (besides, that leaves you scrambling for a home for Arabia.
>> I see the mediterranean as a rift that split europe from asia and Greenland
from North America
What does the Mediterranean have to do with Greenland?!? There was once true
ocean (the Tethys Sea) between Europe and Africa (where the Alps are now), but
that has since closed. The Mediterannean floor is all African continental rock
(maybe some European – I do not know where the boundary is outside the Alps),
not oceanic crust, as far as I know – it is just a low part of Africa that got
flooded. It was more or less dry land 6 million years ago.
>> Look at Antarctica for a clear example of what my observation states. The
motions in some places are 90 degrees to the observed shift faults.
Where?
–Doug