#Fold mountains
26 messages in this thread
Kamron
>> The deposits having slopped over the edge of the shelf would have seams that
tilted upward. The general direction of the force pushing on them if it came
from an ocean basin plate would be working the section in an upward angle. How
would this transfer back hundreds of miles behind the coast leaving the coast
in many areas relatively flat? <<
The original angle of the rock layers would only be a couple degrees (or less)
from horizontal–we're talking about an area hundreds of miles wide, with very
gentle slopes. Most of the compression would be in the horizontal direction.
The flat coastal plains you see today are the result of weathering of the fold
mountains, and deposition of sediments from rivers running from the mountains
to the sea. The coastal plains are relatively recent features.
>> Looking at the eastern US mountains…there has been a general spreading
since it separated from the african coast. When did the pushing occur there to
form the mountains? <<
The Atlantic closed (with continental collision) first, then opened, as it
continues to do today. The collision occurred during Devonian time, creating
the mountains of the eastern US. The Atlantic started opening up again in
Triassic time.
Howard
<<The Atlantic closed (with continental collision) first, then opened, as it
continues to do today. The collision occurred during Devonian time, creating
the mountains of the eastern US. The Atlantic started opening up again in
Triassic time.>>
Agreed with most of your post, but I understood the collision BEGAN at this
time or a little earlier and progressed southward. Or else a brief mountain
building event in the early Devonian in what is presently New England was
followed by a longer, more involved event sometime later in the areas to the
south.
Most of the Devonian layers in PA and NY are sediments from an earlier uplift.
These Devonian beds, and subsequent Mississippian and Pennsylvanian deposits
are involved in the folds of the Appalachian Range from NE PA southward,
indicating that at least this portion of the Appalachians went up no earlier
than the late Pennsylvanian. Also I think I recall that the shallow Permian
basin in SW PA lies conformably on top of the Pennsylvanian, which would mean
the bulk of the continental collision occurred even later, in the early
Permian. Am I way off base here?
ed–
Ed–
>> Agreed with most of your post, but I understood the collision BEGAN at this
time or a little earlier and progressed southward…the bulk of the continental
collision occurred even later, in the early Permian. Am I way off base here?
<<
Sounds like you know more about it than I do, Ed. I was just briefly trying to
simplify things for Kamron, and generalized (always dangerous!), using some
diagrams in _Plate Tectonics and Crustal Evolution_ by Kent C. Condie (1976),
which show North America and NW Africa scrunched together in Devonian, and
spreading apart in Triassic. Thanks for the clarification (I'm a Rocky Mountain
boy, and treading on dangerously thin ice when I talk about eastern NA!).
Howard
<<I'm a Rocky Mountain boy, and treading on dangerously thin ice when I talk
about eastern NA!>>
How is life in paradise <g>? I only got to play geologist in the Rockies once,
in 1978, and had no idea what I was looking at … but it was fascinating as
heck anyway!
Hi, Ed–
<<How is life in paradise <g>? >>
Here in paradise, there's 8 inches of snow on the ground, and it's been about
-20 deg. for the last week or more, with a cold east wind! Oh, well, at least
the skiing's good. 😉
–Howard (Calgary, Canada)
Howard
>> The Atlantic closed (with continental collision) first, then opened, as it
continues to do today. The collision occurred during Devonian time, creating
the mountains of the eastern US. The Atlantic started opening up again in
Triassic time. <<
If this all happened…why do the continents fit so well together right now.
I have done clay modeling on a globe and the continents fit almost perfectly
together in their current shape. The edge of the continents are virtually a
vertical break and they fit together now. This cannot be just a coincidence.
It appears that the margins have not changed since the separation. How could
all this change and additional coast be added and still they fit together?
Kamron
Kamron,
>> I have done clay modeling on a globe and the continents fit almost
perfectly together in their current shape. The edge of the continents are
virtually a vertical break and they fit together now. This cannot be just a
coincidence. It appears that the margins have not changed since the separation.
How could all this change and additional coast be added and still they fit
together? <<
You're right, it isn't just a coincidence <g>. The jigsaw-puzzle-like fit of
the eastern coast of South America with the western coast of Africa is one of
the first pieces of evidence that was advanced for continental drift,
eighty-odd years ago.
I suspect your globe was on too small a scale for you to see how and why the
continents aren't "perfect" fits anymore. The eastern margins of NA and SA and
the western margins of Europe and Africa haven't changed greatly since the
Atlantic formed because there wasn't much happening there to change them.
Seafloor spreading is a slow, steady, not-very-disruptive process. The
continents are carried along by plate motion. They don't collide with
anything, so there's no tectonic forces causing orogenies, volcanic activity,
or subduction.
To see where things have been _happening_, you have to look to the west of the
Americas, and to the east coasts of Asia and Africa, and the northern and
eastern coasts of Australia. IOW, places where plates are moving along each
other, or are being subducted. The Rocky Mountains formed after the Atlantic
opened; I think the Andes did too, as did Japan and all those Pacific islands
that have volcanic origins. The rifting in East Africa is so recent that I
think there's a chance it's still going on.
Jon W.
Jon
>> I suspect your globe was on too small a scale for you to see how and why
the continents aren't "perfect" fits anymore. <<
I first modeled on a small globe using the continents themselves and of course
not the coastlines. I also added in the the fragments that have drifted and
then transferd the shapes to Cad modeling program. The entire globe of
continents fits together remarkably well if you use the real continental shapes
and not the coastline shapes.
They all fit together Remarkably well. The pacific rim marks the area where
the continents are missing the rest of the crust.
Kamron
Jon
>> To see where things have been _happening_, you have to look to the west of
the Americas, and to the east coasts of Asia and Africa, and the northern and
eastern coasts of Australia. IOW, places where plates are moving along each
other, or are being subducted. The Rocky Mountains formed after the Atlantic
opened; I think the Andes did too, as did Japan and all those Pacific islands
that have volcanic origins. The rifting in East Africa is so recent that I
think there's a chance it's still going on. <<
The most interesting and difficult crustal formations for me are those around
north and east of Australia. The crust is in thin ridges..I bet they are a lot
more massive under the visible areas. There are places like both sides of New
Guinea and Fiji where the formation curls up.
I am wondering about the volcanic Island arcs being tied to large curved
fractures..for instance looking at the Elt NINN Fracture Zone system that arcs
between The gap between S. America and Antarctic and goes west curving north
there is a line of sea mounts starting with Louisville Ridge …that goes
through the Tonga Trench and on to the Gilbert and Marshal Islands. What about
this fracture – Island arc connection?
Kamron
>>>The Rocky Mountains formed after the Atlantic opened; I think the Andes did
too, as did Japan and all those Pacific islands that have volcanic origins.<<<
Although a lot of Japan is Volcanic, there are also a diverse collection of
sediments — some as old as Silurian. Don't know whether these are exotic
terrannes or what, but Japan is definitely not a simple volcanic entity like
Iceland or Hawaii.
Same thing for New Zealand incidentally.
Greetings Don
I am wondering about some of the other Plateau's out in the pacific that have
larger areas and vertical sides, like the Manthiki, Ongtong Java, Solomon and
others. Has there been drilling in these to determine if there is a geological
crust there also like in the New Zealand Plateau?
If so why has the moving pacific basin not swept these into the all consuming
subduction zone?<g>
Kamron
Sorry — don't know a thing about the Pacific plateaus.
As I understand it, continental rock is very light and doesn't get sucked into
subduction zones. That's how all those exotic terrannes John McPhee
)"Assembling California") writes about got tacked onto the West side of
Northern California.
Don
>> As I understand it, continental rock is very light and doesn't get sucked
into subduction zones. That's how all those exotic terrannes John McPhee
)"Assembling California") writes about got tacked onto the West side of
Northern California. <<
So we should expect the same over at the other side of the pacific basin…but
there have been some crustal sections left stranded way out in the pacific.
If the ocean basin slowly inches towards the subduction zone the crustal
sections should be riding in the same direction. If the basin is only 200
million years old how can there be crustal sections out in the Pacific that
have lower Paleozoic layers in them?
Kamron
Hi, Kamron–
>> If the basin is only 200 million years old how can there be crustal
sections out in the Pacific that have lower Paleozoic layers in them?
Note in my reply to Don in this thread the discussion on the separations of
Japan and New Zealand from their mainlands. Let one of these get swept away by
a full-blown spreading, and you have your Pacific crustal sections. I have
already told you many of the buildups atop the Pacific seafloor are
non-continental eruptions from mantle plumes, especially Cretaceous ones; which
ones does that not cover?
Another mechanism is described in the Scientific American of July 1989. After
India rifted away from Africa and Antarctica, at the end of the Cretaceous it
was blown apart by the arrival of a hotspot that announced itself by erupting
the tremendous Deccan traps. A rift formed there and split off a piece that
contains the Seychelles islands NE of Madagascar; the rift is now the Carlsberg
ridge, and seems to have been a relocation of an earlier rift that propelled
India and carved off Madagascar. It seems reasonable to assume that the
hotspot eruption caused the rift, as the combination appears in other cases.
The hotspot is now near Reunion island E of Madagascar.
The Seychelles are in the wrong ocean, but could be a model for similar
findings in the Pacific.
–Doug
Doug
>>Another mechanism is described in the Scientific American of July 1989.
After India rifted away from Africa and Antarctica, at the end of the
Cretaceous it was blown apart by the arrival of a hotspot that announced itself
by erupting the tremendous Deccan traps. A rift formed there and split off a
piece that contains the Seychelles islands NE of Madagascar; the rift is now
the Carlsberg ridge, and seems to have been a relocation of an earlier rift
that propelled India and carved off Madagascar. <<
That figures well for the large crescent that the Seychelles are part of with
the spreading ridge between them and India….well balanced to boot. The
pacific is different with the main motion in one direction. I am just
speculating that the plateaus are crustal sections with a geological table. Is
there any drilling core data available from these mesa like plateau in the mid
western pacific?
Kamron
Hi, Kamron–
The article on large igneous provinces implied that they had tentatively
identified many (all?) of these plateaus as basaltic masses based on seismic
data; then expeditions went out to check, and verified it at Ontong-Java,
Vorland (N. Atlantic), and Kerguelen. Cores or surface samples? I do not
recall.
–Doug
If current trends continue, in a few tens of millions of years there will be a
2000 km long Island Arc in the North Pacific — the present California Coast
from Drake's Bay South plus Baja California. It will contain some Paleozoic
rocks — much metamorphised and hard to date from fossils as it happens — but
that's just the luck of the draw, not a necessary consequence of the plate
motion phenomena that will put it there. Presumably other Landmasses are
getting rafted into unlikely positions by much the same sort of phenomena.
Don
If all the sections of crust in the pacific are from separations of crust due
to rifting then each in sequence was cut away by a different older rift from an
earlier time. Any part carved off would be equidistant from the parent
continent like the Cook Islands Manihiki Plateau is roughly the same distance
from the Mid Ridge as S. America….so this fits well.
We have a lot of other similar features to explain as we move further west.
There doesn't appear to be enough time in the basin to account for all the
other formations. What explains this?
Kamron
Hi, Kamron–
I do not easily buy the Cook Islands Manihiki Plateau as something carved of
S. America by the E. Pacific spreading ridge, because I prefer to guess S.
America has been chasing that ridge since Pangaea. With subduction creating
the Andes, that ridge could have moved away from S. America only by spreading
faster than the subduction. It is the fastest spreading ridge we know, but
that is presumably because the subduction is fast also. Note that N. America
has apparently been catching up with and "eating" the northern continuation of
that ridge, though S. America is the faster moving continent.
Do the Cook Islands have continental rock? If none has been found, I will
simply assume that plateau is a basaltic eruption from the mid-Cretaceous
superplume episode or some such.
–Doug
–Doug
The island(s) that will eventually be created from the California(s) coast(s)
isn't necessarily going to be in the middle of anything. It's caused
(apparently) by a continent enroaching on a plate that is rotating with respect
to another. So what seems to be happening is that a sliver of land (3000 km
long by 200 wide) is being ripped from North america and swung off Counter
clockwise into the Pacific. This same phenomenon probably could (and probably
has) occur(ed) elsewhere and can put continental chunks just about anywhere.
Hi, Don–
The Scientific American of Sept. 1983 has an article that says "Behind the
volcanic arc the overriding plate may be extended or compressed. … If the
extension of continental crust behind the arc proceeds until new oceanic crust
is formed, a marginal sea will result. …just as the marginal sea called the
Sea of Japan intervenes between Asia and the islands of Japan."
Again, it says "Occasionally, the divergence of two continental bodies occurs
near an older continental margin, and fragments of continent are rafted away to
form small plateaus of continental crust partially or entirely submerged in the
oceans and surrounded by oceanic crust. Examples include the Lord Howe Rise
(with its highest part, New Zealand)…". It goes on to say New Zealand split
off about 100 million years ago.
Thus both New Zealand and Japan are fragments separated from Australia and
Asia, respectively.
–Doug
Kamron–
>> If this all happened…why do the continents fit so well together right
now.<<
No reason why they shouldn't fit together…all they've been doing is moving
apart since the split, with a certain amount of erosion. Actually, the edges of
the continental shelves (under water) of the continents are a better fit than
the shorelines. The "additional coast" I was talking about was added mostly
onto the surface of the continental shelf, and on the coastal plains (inland of
the shoreface).
>> No reason why they shouldn't fit together…all they've been doing is
moving apart since the split, with a certain amount of erosion. Actually, the
edges of the continental shelves (under water) of the continents are a better
fit than the shorelines.<<
I am modeling on a globe that shows the basins….and it all fits not just
along the atlantic..the antarctica and australian pieces fit great also.
>> The "additional coast" I was talking about was added mostly onto the
surface of the continental shelf, and on the coastal plains (inland of the
shoreface). << Yea I got that…the coastal areas with deep slough deposits on
top.
Kamron
Hi, Howard–
You are making me nervous about my understanding that the Appalachians formed
between the coasts where Laurentia met Africa, and that the modern coast east
of the Appalachians was part of Africa before the N. Atlantic opened up.
I expect that stolen bit of Africa was later covered by detritus from the
Appalachians as you describe, but is this African connection not the main
reason the Appalachians are so far from the modern continental margin?
–Doug
Hi, Doug–
Please don't be nervous on my account! You are probably right about an old
slice of Africa being welded onto NA, east of the Appalachians–I don't
know–my knowledge of the various Eastern NA orogenic events is very
cursory…I'm afraid I've become a victim of my own rash generalizations. My
earlier remarks were an attempt to explain in simple terms the general effects
of continental collision and the origin of fold mountains. I just threw in the
the Appalachians as an example, without intending to get into details of timing
of local events or complications of re-rifting. Sorry if I started a ruckus!
–Howard
Hi, Howard–
No ruckus, just an amateur (me) wondering if his understanding was outdated
(it has been known to happen).
–Doug