The highest mountain and the deepest valley

Gausta_Norgeibilder-ingress
03.10.2016
The second mystery in this series of articles is – as far as I know – not defined in the literature of geoscience. It has to be based on your own experience. It came to my notice in 1978-1979 while I was studying at Stanford, California.

By Fridtjof Riis
This article has previously been published in Norwegian on www.geoforskning.no

For instance, if they spotted an intrusive dike in the field, they were not satisfied with describing all the properties of the dike; they also considered physical processes and talked about extension and stress fields. They admired Charles Hunt, the pioneer who both made painstaking observations and placed them in a geo-mechanical context: How do mountains actually form? And how does one collect mountains?

That approach reminded me that the Scandinavian mountain range with its high-altitude plateaus, such as Hardangervidda, had not yet been explained. And it was there that Jon Fink, who was then a PhD student, one day looked at me with wonder in his eyes and said: Why is the highest mountain always located next to the deepest valley?

It is an interesting question, but does it make sense to ask it? Is it just a coincidence when a high mountain is located next to a deep valley, or are there tectonic and/or morphological connections that we tend to overlook?

We went on excursions and did field work in many different parts of the western United States that year. We looked at rift valleys, volcanoes and the Sierra Nevada mountain range. Checking became a habit. Many times we nodded to each other: Oh yes, the mountain and the valley.

That simple question stayed with me. Soon I would be returning to Norway and would test it on the Scandinavian mountain range.

 

The Caledonian core

A great deal of Norwegian bedrock geology is dominated by the formation of the Caledonian mountain range in the late Silurian – early Devonian period.

The current Scandinavian range, however, has come into being through a series of phases with tectonic movements under various stress conditions, interspersed with long, calm periods with erosion. It provides many good examples of connections between mountains and valleys, structural highs and basins that have developed through geological time. Our mountain formations have been formed by means of erosion into old, wide geological surfaces that combine to form Norway’s roof (see the mystery about the mountain plants). The most striking of these surfaces is the top of the Pre-Cambrian bedrock – the Sub-Cambrian peneplain. For large sections of Scandinavia east of the Central Caledonides, we can use this as a reference level that has been lying more or less flat prior to the formation of the Caledonian range.

 

Left: In the collision of plates a foreland basin is developed on the subsiding Indian plate, whereas the mountain belt is formed in the Tibet plateau on the overriding plate. The highest peaks lie next to the foreland basin. The slope from the highest mountains to the deepest part of the adjacent basin is built up of overthrust rocks and is typically 150 km wide.  Right: The blue line shows the termination of the thrust nappes in the ancient Caledonian mountain belt. The red line shows the boundary to the central part of the orogen, where the basement is strongly deformed, and rocks from the subducting oceanic crust and the colliding plate are represented. The white line shows the outline of Triassic and early Jurassic highs. The scale of the two pictures is the same.

Left: In the collision of plates a foreland basin is developed on the subsiding Indian plate, whereas the mountain belt is formed in the Tibet plateau on the overriding plate. The highest peaks lie next to the foreland basin. The slope from the highest mountains to the deepest part of the adjacent basin is built up of overthrust rocks and is typically 150 km wide.

Right: The blue line shows the termination of the thrust nappes in the ancient Caledonian mountain belt. The red line shows the boundary to the central part of the orogen, where the basement is strongly deformed, and rocks from the subducting oceanic crust and the colliding plate are represented. The white line shows the outline of Triassic and early Jurassic highs. The scale of the two pictures is the same.

 

550 million years ago, the peneplain was nearly at sea level and covered enormous areas on the Baltic Shield and in North America. However, the blue line in the figure shows that there are now major altitude differences on this peneplain in Scandinavia.

In Estonia today, the peneplain is about at sea level. From there it sinks a few hundred metres in the Gulf of Bothnia before rising to around 500 metres towards the Scandinavian range. It reaches somewhat higher in the northern culmination of the range in the north of Nordland county and up to 1000-1500 metres at the Hardangervidda plateau in the southern culmination.

Towards the central part of the Caledonian mountain range, the shales above the peneplain are low-grade metamorphic, and in the Baltics they are in the oil window. Thus the foreland basin must have been buried deep below thrust nappes and sediments in the mountain range. The figure shows the analogy with the Himalayas. The altitude and the course of the peneplain today are due to later movements in the earth’s crust. Transferred to Himalaya, the blue line would be located inside the foot of the mountain range, while the red line would be in the area with the highest snow-covered peaks.

 

The second mountain range

In 1978 it took a full day to travel from San Francisco to Basin and Range, a rift area with mountain ridges and basins stretching over several states. For Charles Hunt, since 1930, surveying this area was the work of a lifetime. Today one can go there with Google Earth and quickly ascertain that the highest rift shoulders are indeed located next to the deepest basins.

People who interpret seismic data from the rift basins in the North Sea or study the topography of the mid-oceanic ridges, will recognise a similar geometry. And no wonder: the forces that are at work during an earthquake, make the footwall go up and the hanging wall go down in relation to the stable surroundings. There is a physical connection here: the more the footwall rises, the more the hanging wall sinks. The movement might be just a few metres in one earthquake, but the large faults we can observe are the overall results of innumerable earthquakes.

The belt that belonged to the central part of the Caledonides west of the red line in the figure, continues onto the Norwegian shelf and makes up the bedrock there. Rift tectonics in the Permian and Triassic periods formed deep sediment basins on the Norwegian shelf where the mountain range had previously been located.

The sediment volumes in these basins and the size of the faults indicate that the shelf basins were grabens which belonged to rift shoulders situated on what is now Norwegian mainland. Scandinavia had a new mountain range, along with new basins in the North Sea and Norwegian Sea formed by rifting of the partially worn-down Caledonian area.

The white line in the figure shows areas that have had a high altitude and received small amounts of sediments in the Triassic and up to the middle Jurassic period. There is good correlation between the extent of these heights and the extent of the current Scandinavian mountain range, but the topography in this second mountain range was mostly worn down in the Mesozoic period.

 

Rifted landscape in the western US with Death Valley and Panamint Range (left). Below: Structure map of the Nordkapp Basin in the Barents Sea showing salt structures and corresponding rim synclines (subsiding basins) at the Jurassic level. Illustration: Fridtjof Riis

Rifted landscape in the western US with Death Valley and Panamint Range (left).
Below: Structure map of the Nordkapp Basin in the Barents Sea showing salt structures and corresponding rim synclines (subsiding basins) at the Jurassic level.
Illustration: Fridtjof Riis

 

The mass balance

Rifting is ususally accompanied by volcanism. Liquid magma down in the earth’s crust can find its way up and form volcanic mountains. Similar structures can form when the salt in large salt basins is mobilized and rises up through the sediment layers above.  The principle of the highest mountain and the deepest valley holds true for such intrusions, too. The salt mass which is located in a salt dome originated in the area surrounding this dome. This surrounding source area will lose mass and subside when the salt moves up. The larger the salt dome or volcano that is built up, the greater and deeper the rim syncline or caldera around it will be.

The volume and age of the subsidence basin at the Jurassic level around the salt structures in the North Cape Basin can be used to calculate how much salt has intruded after the Jurassic period.

The volcanic cones in the Oslo field eroded a long time ago (the Oslo field is a graben, a subsided geological area, 45–75 km wide, stretching from Langesund in the south to Brumunddal in the north). But the calderas in the Oslo field are still there, ready to be modelled by anyone who is curious to know how large the volcanoes were.

 

The mountain, the valley and the erosion

The landscape in South Norway has many deep valley systems that are eroded into hard metamorphic rocks. Long-term erosion makes the valleys wider and deeper. While the sides of the valleys are eroded, the mountain plateaus between the valleys remain almost unchanged. Isostasy (the geological equilibrium between the lithosphere and asthenosphere) makes the mountains higher as the valleys are eroded.

Hans Reusch, a geologist of the same calibre as Charles Hunt, defined the palaeic surface based on the mountain plateaus between the glacial valleys. He assumed that this surface was the remains of a pre-glacial landscape.

With the benefit of today’s knowledge, one might say that the palaeic surface is composed of many different elements. If one is looking for pre-glacial forms, however, the mountain plateaus between the oldest valleys are where to look.

The idea that erosion mechanisms help place the highest mountain next to the deepest valley applies for both glacial erosion and river erosion, but the glacial erosion also contributes by creating valley systems and overdeepenings that are greatest where the mountains are highest. This is mostly due to the ability of water under pressure to transport matter away from overdeepened basins underneath the valley glaciers.

The Norwegian Petroleum Directorate has collaborated with the Geological Survey of Denmark and Greenland (GEUS) to correlate Tor Eidvin’s biostratigraphy in the Oligocene and Miocene sediments with regional geology. Sand deposits from the Oligocene and Miocene in Denmark and on the shelf fit well with the oldest valleys in South and West Norway. Then we are 20-30 million years back in time. The oldest Norwegian valley systems, such as the Sognefjord, might be even older:

The reservoir of the Troll field belongs to the Sognefjord delta which has reservoir sand in several levels from the Johansen formation (early Jurassic) to the Sognefjord formation (late Jurassic). The sand that builds up these formations comes from West Norway. The Stord basin has undrilled Jurassic deposits that also come from West Norway. The present day Sognefjord and Hardangerfjord were deeply incised in the Quaternary, but it is likely that the erosion to some extent followed the old Jurassic drainage systems.

The island of Utsira is another evidence that we once had a Mesozoic landscape in South Norway. This island consists of hard granitic intrusives (rocks formed when intrusive magma solidified in the subsurface) which had not been fully worn down before the Jurassic sediments settled. Glacial erosion has later on removed the overlying sediments, revealing the contours of the old landscape. The Gaustatoppen mountain consists of hard quartzite, towering 3-400 metres higher than any other peaks in Telemark county. It is interpreted that this was a mountain in the Oligocene and Miocene eras as well, perhaps even in the Mesozoic era.

The coastal areas in North Norway and some of West Norway have a landscape I will come back to in the next mystery.

 

Gausta mountain. The peak lies between the deep glacial valley of Vestfjorddalen to the north and the valley Gausdalen to the west, and towers more than 600 m above the plateau north of Vestfjorddalen. The mountain and the valleys were carved out in during a long period of time, which probably started a long time before the Quaternary glaciations. View from the north. Digital model from www.norgeskart.no (Kartverket-Geodata AS)

Gausta mountain. The peak lies between the deep glacial valley of Vestfjorddalen to the north and the valley Gausdalen to the west, and towers more than 600 m above the plateau north of Vestfjorddalen. The mountain and the valleys were carved out in during a long period of time, which probably started a long time before the Quaternary glaciations. View from the north. Digital model from www.norgeskart.no (Kartverket-Geodata AS)

 

Rising from the sea - for the third and fourth time

Deposition of sand and silt from the Scandinavian mountain range shows that it was built up in the Palaeocene and Eocene epochs. The volume of sediments was greatest next to the uplift centres in the northern part of West Norway and the northern part of Nordland.

Simultaneously, central parts of the North Sea subsided, creating a greater sea depth and more room for sedimentation. The large expanse of uplift and subsidence areas indicate that these processes encompassed the earth’s crust in all of Scandinavia. The mountain/valley principle works at this scale, too: subsidence and uplift go together.

A tectonic phase with compression early in the middle Miocene led to a new uplift in South Norway and subsidence in the central North Sea. At the same time, the Lofoten-Vesterålen segment rose up while the Lofoten basin sank, and a reactivation and new formation of anticlines and synclines occurred in the Norwegian Sea and the southern North Sea.

During the ice ages, the mountain range and basins have been powerfully impacted by isostatic forces, through ice sheets retreating and advancing, erosion from mountain areas and a rapid addition of sediments in the basins. We are able to reconstruct the course of ice ages, ice thicknesses and mass balance, but there is still a lively debate about mechanisms. is still vigorous.

It seems that those landscapes in Scandinavia, Greenland and elsewhere that have been subjected to repeated major glaciation have a more dramatic topography than one might expect based on just erosion and isostatic forces.

Is there an X factor that has been left out of the modelling, which helps repeated glaciations reinforce a topography with high mountains around the inland ice and deep central depressions?

 

The mountain, the valley and the X factor

Remember Jon’s old question next time you go for a mountain hike or sit down to interpret data. Use it to recognise geological processes that work together and to see forms and structures in a larger context.

And don’t be surprised if some X factors turn up that will inspire us to continue our search into the mountains.


Topics: Geology