This is Geology Bites with Oliver Strimpel.
If you were to name the world's greatest mountain range, it
would probably be the Himalaya. It has the world's highest peaks
and stretches uninterruptedly for over 2500 kilometres from
Afghanistan in the West to Burma and even Indonesia in the east.
And it gives its name to the Alpine Himalayan Orogeny, the
giant orogenic belt associated with the gradual closure of the
Tethys Ocean that extends for over 15,000 kilometers from the
Atlas Mountains and the Alps in the West to the Caucasus that
border Europe and Asia to the Himalaya and even further E to
Southeast Asia. But just north of the Himalaya,
like several other great mountain ranges, we hear a lot
less about the Karakoram, the Hindu Kush, the Pamir, the
Gangdizi, the Kunlun Shan and the Tianshan.
How can we explain the existence and geological makeup of these
mountains? And are they part of the same
geological circumstances that created the Himalaya?
Mike Sahl has studied the Himalayan, Karakorum and Pamir
mountain ranges for about 45 years, probably logging more
hours in the field there than any other geologist.
He has published hundreds of papers on the region and is
perhaps best known for suggesting and documenting the
evidence for a process called channel flow in the Himalaya, in
which metamorphose crustal rocks are extruded towards the South,
sandwiched between a thrust fault below and low angle normal
faults above. He is emeritus professor of
Wealth Sciences at the University of Oxford.
Mike has been a keen advocate and advisor for the podcast
since it's launch in 2020. He was my guest in the inaugural
and the 50th episodes. Mike, I'm really happy to
welcome you back to Geology Bites for this, the 100th
episode. Well, thank you very much,
Oliver. It's great to be back on Geology
Bytes and thanks for inviting me again.
Where exactly are these Central Asian mountains?
And let me just point out for listeners that the supporting
web page on geologybytes.com includes maps and the glossary,
which might be especially useful for this episode.
The geography of Central Asia is a little bit complicated, but as
you said, the Himalayas are approximately 2 1/2 thousand
kilometers long, running along the northern borders of
Pakistan, India, Bhutan, Nepal and Tibet.
And we're going to talk about the mountain ranges to the north
of the Himalaya. So this group includes mountain
ranges like the Pamir, which is almost entirely in Tajikistan,
the Hindu Kush, which is the mountain range along the
Pakistan Afghanistan border, the Karakorum, which is in
northernmost Pakistan and Ladakh.
And then there's a bunch of mountain ranges around Tibet.
So the Gangesi range is about 2000 kilometres long, stretching
from Kohistan in the West all the way along the southern
margin of the Asian place in South Tibet.
The Kunlun runs along the northern margin of the Tibetan
Plateau in China, and the mountain ranges to the north of
this are all in Outer Mongolia. So the Tianshan runs from the
Pamirs in the southwest for about 3000 kilometres along the
borders of Xinjiang province in China with Kazakhstan,
Kyrgyzstan and Uzbekistan. By world standards, these are
massive ranges. The Karakoram boasts 4 of the
world's 8000 metre peaks, and the Hindu Kush, the Pamir, the
Kunlunshan and the Tianshan each have many peaks above 7000
meters. No mountain ranges outside this
region have such high mountains. Yet we seldom hear much about
these ranges. Do you think that is because
they're overshadowed by the Himalaya?
Well, quite possibly, yes. The Himalaya are actually
readily accessible from the plains of India and Pakistan and
are inhabited, so there are access roads, lots of villages
up there and lots of trekking routes.
So the access in the Karakoram is almost entirely by trekking
or else mountain climbing at higher elevations.
The Pamir, Tian Shan, and Kunlun are all extremely remote, but
they're high, so they're on the plateau or around the plateau,
and they do have Rd. access into their margins.
Before we delve into the geology of some of these individual
mountain ranges, can you give us a high level overview of the
origin of these mountains? Yes.
Overall, these mountain ranges are all part of the present day
Eurasian plate, in contrast to the Himalayas which remember are
all Indian plates. So the suture zone dividing the
Himalayas from Southern Tibet, which runs right the way through
Ladakh and S Tibet, that's the boundary between the collision
zone between India and Central Asia.
So these mountain ranges to the north of the Himalayas, in a
nutshell, are all related to the progressive accretion or the
collision of continental blocks onto the stable pre Cambrian
shield areas of Siberia and Mongolia.
So these continental terrains cause collisions dating way back
into Paleozoic times. So that's more than 304 hundred
million years ago. And the accretions of these
terrains becomes younger as you go towards the South.
So the oldest suture zones like the Mongol Altai suture zones in
Mongolia are Paleozoic. The Kunlun is Triassic getting
on for 300 to 250 million years ago.
Central Tibet is a Jurassic Lower Cretaceous collision zone
along the Bangong Suture. And the Indus Suture is the
youngest of all of them, which is the one that caused the
Himalayan collision. So this accretion of suture zans
onto the main pre Cambrian blocks of Siberia and Mongolia
could be the beginnings of formation of a giant crayon like
we see in many of the old pre Cambrian belts, like the Cap Val
Zimbabwe crayon in South Africa, where you have a bunch of
Proterozoic terrains all accreted around the core of old
Archaean rocks, some of the oldest rocks on the world.
So the southern margin of Eurasia was this active plate
margin right up through the Mesozoic as Tethys was forming
to the South after the break up of Gondwana, you had active
subduction underneath southern Tibet with this 2000 kilometre
long granite baffa lifts of the Gangesi and that was all
associated with this northward dipping subduction zone which
consumed the oceanic lithosphere of Tethys to the South.
So the Indian plate margin, the Himalayas was a long lasting
passive continental margin with over 200 million years of stable
shallow marine sedimentation. But the southern margin of Asia
was an active Andean type margin.
So it could actually have been relatively high and thick crust
prior to the collision of India. Wow, that's fascinating.
So I said in the introduction that you logged more hours in
the area than any other gorges. Have you been to all these
mountain ranges yourself and studied their geology?
Well, yes, I have. I've worked for over 15 years
along the Karakoram Ranges in North Pakistan, mostly in the
Baltoro Glacier region. I published a big geological map
of the central Karakoram in my book The Geology and Tectonics
of the Karakoram Mountains. At least six of these
expeditions to the high peaks were all on mountaineering
expeditions. I soon figured out that the only
way you could get up to this terrain was to have the
logistics of a major Himalayan mountaineering expedition, and I
usually invited 3 or 4 of my climbing mates along so that we
could actually get high. So we had expeditions to K2,
Mashibram, Trango Towers, Bialy and Hushay all during the late
80s. And I've been to the Hindu Kush
several times along the Pakistan Afghanistan border.
It's always a bit dangerous crossing into Afghanistan.
That's where the Panchaya Valley fighting was going on with the
Mujahideen in those days. And I've also worked around
Tibetan, the Tibetan Plateau for six or seven field seasons.
We're going to talk in a moment about 3 of the ranges that you
studied in a little bit more detail than the other ones.
But first, I wonder if you could just briefly put into context
the origin and the nature of the ranges that we will not be
talking about in detail, which are the Tian Shan, the Kunlun
Shan and the Gangdizi ranges. Yes, sure.
Well, the Tian Shan and the Kunlun are older mountain
ranges. They were formed during the
middle or late Paleozoic. So we're looking at somewhere
around 304 hundred million years ago.
But their recent uplift has been attributed to the far field
effects of the India Asia collision within the last 4030
million years. Like all of the mountain ranges
of Asia, the Gangesi range is different.
This is a huge Andean type range that runs right the way across
southern Tibet and it's almost entirely made-up of seduction
related I type granite. So horn blend biotype bearing
granites, granite diorites and andesitic volcanics as well,
which span a long time, over 150 million years and these were
formed above the northward dipping subduction zone that was
consuming Teethis. So it was a geological setting
very much like the Andes are today.
Could you explain what I type granites are and andesitic
volcanics? Yes, I type granites are ones
that are derived from melting and igneous protolith.
So they have a very distinct mineralogy and geochemistry from
granites that are formed by continental crustal thickening
like the Himalayas. The andocytic volcanics are very
much like the volcanoes you see today in Washington state and
Oregon. You know, Manson Helens is a
classic andocytic volcanic and that is sitting above the Sierra
Nevada baffilith, which is very similar geochemically to the
Gangesi baffilith. So we can say with a fair degree
of certainty what the geological setting of the Gangesi range
was, and it would have been something very similar to what
we see in the Andes or even the stretch from California up to
British Columbia. OK, let's talk about the
mountain ranges that you have studied in some detail, starting
with the Karakorum. What are it's salient geological
features and how do you compare these to the Himalaya?
The Karakorum are relatively complicated compared to the
Himalayas. The Karakorum have three major
terrains. The northern Karakorum are
composed dominantly of Paleozoic, early Mesozoic
sedimentary rocks. So these are rocks that span
from about 350 million years up to around 200 million years.
They are intruded by some older diarritic plutons.
For example, the most spectacular 1 you see on the
West face of Gashebrum 4, this spectacular mountain that you
see as you track up the Balsoro Glacier.
The central Caracorum is all composed of this amazing
backbone of the granite baphilith.
And the southern Caracorum is all a high grade metamorphic
terrain. These have classic Borovian
fascies. Cyanite, Solomonite, nices,
quinite, silimonite, and anderleucites are all aluminium
silicates, and they're all the products of crustal thickening
and regional metamorphism. Why is it?
Called Borovian. That's named after George
Barrow, who was one of the first geologists who traipsed over the
Scottish Highlands and first came up with the Borovian
sequence of metamorphism. This was over 120 years ago.
At least he was the first person to really say there was this
distinct mineralogy. As you increase pressure and
temperature that you get a distinct sequence of rock.
As you go from low pressure temperature green shish fascies,
you'll get to higher pressure temperature amphibolite fascies.
You go through a series of metamorphic isograds and Isograd
is where the first incoming mineral formed and that terrain.
So you go through from chloride to biotite to garnet, that's all
greenish fascies. And then storilite, kainite,
solomonite, which is the amphibolite fascies.
And at the top of the amphibolite fascies, when you're
getting up to temperatures in excess of about 700°C, you start
melting the rock to produce a migmatite, and that's the first
incoming melt that forms in that sequence when temperatures
become high enough to melt the rock.
In the Caricorum we have the most unique thing is this
incredible central zone of the whole backbone of the Caricorum.
This is enormous granite, Baphyllith, the Baltoro granite,
which are composed of S type granites, granites derived from
melting sedimentary rocks. They have a very distinct
mineralogy, totally different from the Gangesi.
The Baltoro Granites have garnet, tourmaline, muscovite
and biotite and they form some of the most spectacular peaks
along the Baltoro Glacier. I'm sure if anyone knows
anything about the Caricorum, they've heard of the Trango
Towers, which are these two three kilometre vertical walls
of granite. Well, these are made of the
Baltoro granite and they will have very high strontium 8786
ratios, which is indicative of crustal melting.
And the uranium lead zircon ages span around 21 to 13 million
years, so that's early Miocene. Bizarrely, they happen to be
exactly the same age as the Himalayan granites on the other
plates of the South, the high Himalayan leucogranates.
Can we then infer that they are in some sense the mirror image
on the Eurasian plate of what was going on to the South on the
Indian plate during the collision?
Well, yes, temporarily speaking, they were mirror images.
So it's sort of incredible that although the structure of the
Caracorum and the Himalaya are very different following the
collision, you've had the same 3040 million years of crustal
thickening and regional metamorphism producing these
Solomonite, quinite grade nices and migmatite.
And you've also had the end products of that, which is the
crustal melting to form the granite.
But the big difference is that the Baltoro granites are
absolutely enormous. And there's no doubt that it was
a continuous baatholith, whereas the Himalayas were all these
sort of individual pluton civil complexes produced from a
regional magmatite. So the structure is very
different. They're both pure crustal melts,
but the Balturo requires an extra heat source to explain the
massive volume of melt do. You think that heat must have
come from the mantle? Yes, it almost certainly came
from the mantle and we can get an idea of the involvement of
the mantle from the very scarce lamprophyric dikes that intrude
around the Balturo. So lamprifiers are these unusual
mantle derived melts. They're basaltic type rocks.
They have ages in the Karakoram that overlap exactly with the
granite, so they're all early Miocene, suggesting that there
was heat coming out of the upper mantle at the same time as the
melting of the lower crust. Also in the Pamirs, which is the
northern part of the Karakoram extending off to Tajikistan,
there are spectacular series of ultra potassic volcanics and
cyanide intrusions which are very similar geochemically and
temporally to the lamprifiers that we have in the Boltura.
And they all have ages around 11,000,000 years in the Pamiers.
And these mantle derived intrusions are part of a huge
range of similar rocks intruded through the plateau of Tibet.
How do we know that the lamprophyric dikes and the ultra
protasic cyanide intrusions originate from the mantle?
Is it because of that chemical makeup?
Yes, they're geochemically very distinct.
They're highly alkaline, the silica under saturated so the
opposite end of the scales are granite, and they're ultra
potassic, so they're indicative of a highly enriched upper
mantle source. Cyanides are like granites.
There's coarse grained rocks with alkali feldspar.
They form in thick continental crust by low degrees of partial
melting, and they have to have a mantle component from the
geochemistry. Do we have an idea why that
extra heat from the matter was injected in the Caracorum and
not elsewhere? Well, that's a very intriguing
question and we don't really have an answer to it.
I mean we know from the concentration of these highly
alkaline lamprophyric rocks that they are an important components
of many orogenic belts. They usually intruding at a very
late stage, so whatever happens during the continental
collision, the mantle has got an anomalous highly alkaline
component to it right at the end of that origenic process.
How do the Pamir relate to the Caricorum range which lies to
their South? Well, the Pamir generally show
older geology, more Paleozoic, lower Mesozoic rocks.
There's nothing as young as we see in the Himalayas, but they
also show very large scale post India Asia collision
metamorphism. So similar to what we see in the
southern Karakorum and in the Himalayas.
Their pressure temperature conditions are very similar in
all three mountain ranges and their uranium lead monocyte
ages, which give you an indication of the timing of peak
metamorphism are again very similar to the Karakorum and the
Himalayas early Miocene. So they occur in a series of
what we call these compressional core complexes along the
Southern premier and they are directly related to the
metamorphism in the southern Karakoram.
Isn't it a bit surprising that the metamorphism in the Premier
reflects similar pressure and temperature conditions to that
of the Karakoram, even though the Premier, being further
north, are much further from the collision zone with India?
Well, it's not that surprising because the Southern Pamirs are
geologically part of the same continental place as the
Karakoram. There's a big suture that runs
right through the central Pamirs.
So the northern Pamirs are part of a different plate, but the
Southern Pamirs and the Karakoram were contiguous with
the same plate. OK, let's talk about the Hindu
Kush. Well, the Hindu Kush is really
the western extension of the Karakoram, runs right from
northern Pakistan. Then it swings S along the
Afghan border. And although it's a similar
geological plate to the Karakoram, it has a very
different geology. Most of the rocks in the Hindu
Kush are again older sedimentary rocks.
So Paleozoic, early Mesozoic sedimentary rocks.
They have very few collision related granites.
So whereas the Karakoram has enormous volumes of Baltoro
granite, there's only really one intrusion near Chitral in
northwest Pakistan, which is a Miocene post collisional
granite. The rest of the granitic rocks
up there are older and deformed and metamorphosed amphibolite
rocks which started life off as I type granite.
It's very similar to the Gangesi, but they've undergone a
huge metamorphism and defamation after intrusion.
The Hindu Kush also interestingly, has the world's
deepest known continental seismic zone.
So despite having this old geology and not a lot of
indications of crustal melting post collision, it has this
continental seismic zone in which earthquakes extend down to
about 250, even 300 kilometres. And these earthquakes define a
very narrow zone. It is the deepest continental
seismic zone we know on the planet.
There's a second seismic zone to the north called the Pannier
seismic zone, which defines a shallower S dipping zone.
So you have two converging continental subduction zones,
the deep Hindu Kush and the shallower Pamir.
And it's probably the convergence of those two
subduction zones that lifts up the whole of the Pamir Plateau.
Wow, that's very interesting. Continental subduction is fairly
rare, and here you have two of them going on in opposite
polarities right in the same part of the world.
How can continental crust subduct to these enormous
depths, given that it's a lot lighter than what it's
subducting into? Yes, well, good question.
Most deep seismic zones are in the oceans.
Of course, in the oceans all around the Pacific Ring of Fire,
you have very deep oceanic subduction zones, which is the
subduction of oceanic lithosphere.
But in the continents, it's very uncommon to get these deep
earthquakes. So along the Himalayas and S
Tibet, the deepest earthquakes occur around 70, maybe 80
kilometres depth. So there's several factors you
need to make this deep seismic zone.
You need rapid subduction. You need old cold material being
subducted and you need to keep the temperatures low enough to
make the earthquakes. Earthquakes only occur at
temperatures less than five 5600°C.
So how on earth can you get low temperatures 250 kilometres deep
in the mantle? The answer is the only way you
can do it is by this very narrow subduction of old cold
continental crust and that is most likely going to be lower
Indian crust subducting northwards.
So we know in the geological record there's over 15 or 16
ultra high pressure terrains. They're all from the Cambrian
onwards and they occur on all the continent.
So we do know that it's possible to subduct continental crustal
rocks to these depth. And the rocks you've formed down
there are first kozite bearing, which is a high pressure
polymorphic quartz kozite eclogites and the other are
micro diamonds. So diamond difference exclagites
we see in places like the Western Nice region in Norway.
And we know they occur in subducted continental rocks and
that is exactly what's happening under the Hindu Kush today.
The other thing you need to do, apart from getting them down
there very quickly, is to exhume them very rapidly to get them
back to the surface. So you do need some sort of slab
break off. You need to release the anchor
that has dragged them down to mantle depths so the buoyancy
forces take over and these rocks will pop straight up to the
surface. If these ultra deep earthquakes
in the Hindukur stem from the subduction of low Indian crust,
wouldn't we also expect to see such Jeep earthquakes further E
under the Karakoram or Tibet? Well, that is a very good point.
And the answer is we don't see that people have speculated on
this for a while. So in the central part of the
Himalayan, the Tibet, we don't see this deep structure.
What we do see is the lower Indian crust, which is all the
Archaean Indian shield under placing the Himalayas, under
placing the Tibetan plateau. And this was a mechanism that
Emil Argan, the great Swiss geologist, suggested over 100
years ago, that rather than the deep subduction zone, you have a
much shallower under placing of lower Indian crust underneath
southern Tibet. So there's a kind of bifurcation
really between the mode of subduction you have under the
Hindu Kush on the western end, which goes very deep, and the
shallow under plating of the Indian plate further E which
just slides underneath. Do we have any idea as to why we
get these two different modes and what causes the Indian plate
to do one at one end and the other at the other?
Well, there's something very special about the two syntaxes,
which is the northwest corner and the North East corner, where
the Himalayas go from east West to north-south.
All of the structures swing around through 90°, down through
the Chaman ranges of the Pakistan Afghanistan border on
the West and down through the Indo Berman ranges on the east.
And at both of those syntaxes there are 78000 meter mountains
which have the youngest metamorphism known anywhere on
the planet. So the Nangoparbat metamorphism.
Nangoparbat is this 8000 meter peak in the northwestern part of
Pakistan is actually the western end of the Himalayas rather than
being in the Karakoram. And the northeast is a mountain
called Namchi Bawa, which also has extremely young
metamorphism. In both of these areas, the
rocks were formed less than one to two million years ago and
were exhumed extremely rapidly. So you have this enormous
exhumation of deep crustal metamorphic rocks in a very
shallow time. And the reason we think is
structural that the rocks in the main part of the Himalayas are
extruding to the South channel flow.
But in the Syntaxes region, they are having compressive stresses
of everywhere. So there is nowhere else to go
but straight upwards and that's exactly what we see so.
Although India collided with Asia about 50 million years ago,
it continues its northward advance to this day, albeit at
about half the pre collision speed.
How is this movement accommodated and is that part of
the story of the ongoing uplift of the mountain ranges we've
been discussing? Well, yes, there's two theories
of how the northward push of India was and is continuing to
be absorbed in Asia. Firstly, there was the one that
was first proposed by Emil Argand in 1924 where he
suggested that India was under thrusting the whole of the
Tibetan plateau and jacking it up.
The second model is that of continental extrusion and this
was first proposed by Peter Molnar and Paul Tapini back in
the 1980s. This was based on when Landsat
photography first became available and when you laid out
all these Landsat photos of Tibet and Central Asia, the
thing that really strikes you immediately are these huge
strike slip faults, San Andreas type faults, but they're going
on for thousands of kilometres across Central Asia.
So Moulnar and Tapani proposed that the thickened crust of
Tibet was being squeezed out to the east to make way for India
pushing towards the north. They wanted, at the time, over
1000 kilometers of offsets on these bounding strike slip
fault. So what observations can help us
distinguish between these two models?
Well, the continental extrusion model is very easily testable
because these strike slip faults are exposed to the surface and
all you need is a pinning points of a well dated granite or
sedimentary rock or anything that you know was once
continuous and is now being offset by X amount of slip.
And when this extrusion model was first proposed, there were
no geological maps. There was no constraints on the
amount of slip of any of these faults.
So we went into the Karakoran Fault back in the 1980s and
carried out very detailed work along the Karakoran Fault in
Ladakh, where it's beautifully exposed and a whole series of
granites. And by getting very accurate
uranium lead zircon ages on each one of these granite, we were
able to map the dextral offsets. And we found that the offsets of
these myosin granites was no more than 120 kilometers on the
Karakoran Fault, one of the most active strikes at faults of
Tibet, which happens to be exactly the same as the course
of the ancient Indus River, which was captured by the fault,
dragged for 120 kilometres before it cuts S back into
northern Ladakh. Well, that would seem to be
pretty conclusive evidence. Do the proponents of the
extrusion tectonics accept these arguments?
Do you have to? Basically because it's good
proper science. But the problem was on the other
faults. We couldn't learn in the Alton
tag and nothing like as clear cut as the Karakorum.
So there were no definitive pinning points and you're
reliant more there on the extensive the ductile slip along
those faults. So the take home message of all
of this is that continental extrusion was working as a
process, but it was nowhere near the amount that Peter Molnar and
Paul Taponiere originally proposed.
It was more like each one of these faults had somewhere
between 150 and maybe 3 or 400 kilometres maximum.
So if we have this lower amount of extrusion going on, the
remainder of the accommodation of the northern part of the
Indian plate moving up is accommodated by this flat slab
subduction and uplift and folding throughout the crust to
the north. Yes, and all the Himalayas
indicate hundreds of kilometers of homogeneous crustal
shortening. That's folding, thrusting.
That shortens the crust and thickens the crust and then you
get the metamorphism. But all of the rocks in the
Himalayas are Proterozoic or younger.
There's nothing older than about 1000 million years.
So we know that the lower crust of India, which is dominantly
the pre Cambrian Indian shield, these are Archaean rocks, which
are much, much older. They're very old, high grade
granulite fascies rocks. So they're dry and they're
competent, and it's the shallow subduction of the lower part of
the Indian plate that is jacking up the Himalayas, and it's
jacking up southern Tibet, if not most of Tibet itself.
And that act of fault is the one that all the big earthquakes
along the Himalayas occurred today.
So the main Himalayan fault is where, for example, the Gawker
earthquake 2015 set off from the Great Assam earthquake.
All of those are on the southern boundary of the Himalayas, which
is where the Indian shield is subducting beneath the Himalayas
and jacking it up. Do other mountain ranges, such
as the Alps or the Andes, have the kinds of satellite mountain
ranges that we've been talking about today with respect to the
Himalaya? Well, all mountain ranges are
different. The Alps is part of the Tethian
collision zone. It's the same as the Himalayas
temporarily, but it shows totally different structures.
The Alps have much more high pressure, ultra high pressure
rocks than we see in the Himalayas.
There's very little regional metamorphism in the Alps,
whereas in the Himalayas it's huge.
Most of the greater Himalayan sequences all that the Alps have
almost no crustal melt granites associated with the young
collision zones. So there's nothing like channel
flow. The Andes are completely
different from the Himalayas. The Andes show a geology very
similar to the Gangesi. You know I type granites related
to subduction of oceanic lithosphere beneath the
continent. Big explosive andesitic
volcanics like Manson, Helens and porphyry copper deposits
typically. So yeah, all mountain ranges are
different, which makes geology all the more interesting.
Mike Sowell, thank you very much.
Thank you, Oliver. It's been a pleasure.
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