Download Building an intracontinental mountain range: active deformation of

Building an intracontinental mountain range: active deformation
of the Mongolian Altay Mountains
Laura C. Gregory
University of Oxford
January 23, 2012
Figure 1: Photo of Tsengel Hyarhan Mountain, one of the highest peaks of the Central Altay
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Deformation in the Altay Mountains
To build a mountain range, the earth’s crust is subjected to immense stresses that break the
crust into long fractures, or faults, which move in earthquakes. Over longer time scales, repeated
movements on faults cause uplift of the earth’s surface to form mountains that are then modified
by erosion to produce the landscape that we see today. The evolution of a mountain range occurs
on significantly different time scales, and it is difficult to compare measurements of processes
over such vastly different lengths of time: from the scale of hundreds to thousands of years for
an earthquake cycle, to tens of thousands of years for changes in small-scale topography, to
millions of years for the uplift and eventual erosion of a mountain belt. My research is focused
on quantifying these processes, and comparing the rates and styles of tectonic deformation that
occur on time scales of different orders of magnitude. Fieldwork is essential for this researchduring our months in the field, our team collects samples used for several methods of measuring
rates of deformation, and we make observations of the surface expression of active tectonics
and erosion that are crucial for understanding the styles and mechanisms of mountain building.
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DEFORMATION IN THE ALTAY MOUNTAINS
0
6000
500
Siberia
Sayan
4000
2
The Altay
2000
Û
0
Al
ta
y
Ha
ng Mongolia
ay
Gobi
Tien
Û
han
Altay
S
50˚
Kunlun
Quilian
Ulaangom
Shan
>0.5
Tibet
Himal
Û
China
aya
India
Kh
Û
Û
Û
Û
lgi
y
Hovd
l
~0.9
>1.2
tö
Hö
Ar
48˚
1.3
Ö
city
(pop. >20,000)
Nuur
Û
ur
say
Û
Olgiy
Nu
Sag
a
sHar-U
ot
on
Û
1-4
EQ
rupture
<2.4
~0.7
n
Yu
Fu
3000
2000
Existing slip-rate studies
Our on-going studies
46˚
0
84˚
100
86˚
200
1000m
300
88˚
90˚
92˚
94˚
96˚
Figure 2: ASTER GDEM shaded topography with active Altay faults mapped in black, based
on field work and remote sensing. All known and ground truthed surface ruptures are mapped
in red (including ruptures that occured within the past century). Our active slip-rate sampling
sites are indicated by blue dots.
This work not only improves understanding of the fundamentals of continental deformation (i.e.
how rates and styles may or may not change over time) but also helps to understand the human
hazards that exist due to the presence of active faults.
The Altay Mountains in western Mongolia (Figure 2) are ideal for studying active tectonics
because the regional climate is arid and cool, allowing for excellent preservation of features that
form as a result of active faulting. Faults generally move at a rate of millimetres per year, or
metres per thousand years, with the accumulated motion on the fault released episodically as
slip during earthquakes. This slow motion is recorded in well preserved landscape features, such
as abandoned alluvial fan surfaces, that are offset by tens of metres as a result of motion along
the fault (Figure 3). Despite optimum preservation conditions, the active deformation that has
occurred over the last 5 million years or so in the Altay is poorly understood because the region
is remote and field work can be logistically difficult. Research conducted in the Altay before
summer 2010, including our previous field seasons, has been focused on faults in the eastern
Altay, which appear to be moving relatively rapidly averaged over the last few tens of thousands
of years (e.g. Baljinnyam et al. (1993); Cunningham et al. (2003)). However, the faults in the
very centre of the range, along the Mongolian-Chinese border, had yet to be investigated. By
comparing fault rates and the expression of faulting in landscapes across the whole range, it
is possible to recognise patterns of deformation across the Altay. It may be that activity has
migrated from west to east, or instead that faults on the eastern margin of the range have
initiated without a reduction in activity on faults in the interior. My work is focused on testing
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FIELD METHODS
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Reverse Faults:
Strike-slip faults:
Fault scarp
Fault
-
Offset river
+
Fault plane
100 m
Fault intersection with the surface
Alluvial fan pushed up by fault
Figure 3: Schematic diagrams of faults affecting the landscape, with satellite imagery and field
photos of examples of both strike-slip (horizontal motion) and reverse (vertical motion) faults.
these hypotheses, the result of which will have general importance in understanding how the
earth deforms, especially in the context of how faults move (for example, in earthquakes) over
large regions of the continents and through time. My work on the faults will also be interpreted
in the context of our ongoing study of the history of mountain building over the past few million
years using low temperature apatite fission track (AFT) thermochronology, and the structural
rotation of the mountains revealed in magnetic signatures (‘paleomagnetism’) recorded in the
rocks.
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Field Methods
An average fault slip rate can be estimated by dating a landform that has been displaced by fault
movements over the past few tens of thousands of years (Figure 3). Because organic material
for radiocarbon dating is rare in the arid climate of the Altay, my primary dating technique is
terrestrial cosmogenic nuclide (TCN) dating in profiles down the sediment column or of in-situ
surface boulders. TCN dating utilizes the production and accumulation of
10 Be
ions in quartz
as a result of bombardment of cosmogenic rays at the earth’s surface (Gosse and Phillips, 2001).
Cosmogenic rays only penetrate the top few metres of the earth, and thus concentrations in
quartz grains can be used to estimate an exposure age for either boulders sitting on the surface
or the age of sediment at depth based on the decay of
10 Be
below the surface. Where possible,
I also use uranium series dating techniques on in-situ formed carbonate, which utilises the
decay series of uranium to lead to measure when crusts of carbonate formed on pebbles in the
sediment. This method gives the minimum age for a displaced landform surface, because the
sediment has to be stabilised before a crust can form. In order to produce accurate results, it
is best to use as many methods as possible to date the abandonment of the displaced surfaces.
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THE FIELD!
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Figure 4: Photos of our group: on the left in nice weather, there is Nara, Tulga, Deaggi, and
Ed. On the left, with snow in the background, is our whole team, with me second from the left.
Quaternary slip-rates can be compared with evidence of large-magnitude earthquakes that
ruptured the surface in the past few hundreds of years. The exceptionally arid conditions in
Mongolia have allowed ruptures as old as potentially 1500 years to be preserved. These ruptures
are visible in high-resolution satellite images and can be observed and mapped in the field to
measure the length of rupture and amount of slip, from which the approximate magnitude of
the earthquake can be calculated. This information helps to interpret the behaviour of the
faults over the past few thousand years, and gain an understanding of the potential for large
earthquakes in the near future (e.g. Jackson (2001)). Prior to the start of this summer’s field
work, I identified two potential earthquake ruptures in satellite imagery that had not been
previously reported. These ruptures are along the Tal Nuur and Hoton Nuur faults in the
central Altay (Figure 2), and much of the field season was be spent along these faults, mapping
their surface expression and determining Quaternary fault slip-rates at sites identified remotely.
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The Field!
Our team spent 5 weeks in Mongolia, with a total of over 8 days just getting to and from
western Mongolia on the difficult Mongolian roads (mostly dirt tracks). We wild camped for
the duration of the trip, except when there were high winds and snow. The team comprised Ed
Nissen, a postdoctoral research and collaborator from the University of Cambridge, Gantulga
(‘Tulga’) Bayasgalan, a Mongolian geologist and our translator, Nara, our Mongolian driver,
Deaggi, our Mongolian cook and field assistant, and myself (Figure 4). The highlights of the
trip included a very good climb up a remote mountain, successful sampling of a slip-rate site,
walks up glacial valleys, and documenting very clear ancient earthquake ruptures. The weather
varied from warm and sunny ‘t-shirt’ weather to cold, very windy and snowy weather blown
down from Siberia (Figure 4). I even woke up in my tent one morning to find that the water in
my water bottle had frozen!
One of our best sites was less than 10 kilometres away from the Mongolian-Chinese border,
along the Hoton Nuur valley. In this area we documented fissures in the ground from an
earthquake that probably happened over 500 years ago (Figure 5). We also collected the tops of
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THE FIELD!
5
N48˚ 11’ 42.5” E88˚ 48’ 43.9”
N48˚ 10’ 54.0” E88˚ 49’ 29.3”
fissure
fissures
(a)
(b)
(c)
(d)
Figure 5: (a) Fissures from an ancient earthquake observed near our camp. (b) The displaced
moraine from which we collected dating samples. The white dotted lines point out the top of
the moraine surface that has been moved by about 30 metres to the right by motion along the
fault. The black dotted line is along the bottom of the moraine surface, displaced by about the
same amount. (c) Ed wearing his safety goggles, after hammering off part of a boulder. (d)
At the top of Buyant Mountain, where we collected samples of rock that will elucidate when
the mountain began to form. We think that very few westerners have gone up this mountain
because it is so remote, perhaps we were even the first!
REFERENCES
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boulders that were sitting in a glacial deposit (a moraine, Figure 5c) for dating with the TCN
method. The samples are now being measured, and they will reveal an average slip-rate for the
fault in millimetres per year, representing the last 20,000 to 50,000 years of fault motion. They
will also give some information about the climate history of the region, because the age of the
exposure of the glacial deposit should correlate with the melting of glaciers in the region.
This is just a snapshot of our successful field trip, but I am writing the work up into several
scientific publications that should be available in the next year. This includes documentation of
the ancient earthquake ruptures, rates of slip on faults that weren’t even known to exist before
our trip, and measurements of the uplift of the mountain range. My research has changed our
view on the kinematics of the Altay mountains, because we now know that faults in the middle
of the range are still active, and moving relatively quickly, whereas before it was thought that
only the faults on the edges of the range were active. This is important for our understanding of
the fundamentals of deformation within the continents, as it is an example of an area where the
force of continents colliding is expressed in a distributed manner on a number of fault structures
across an entire mountain range.
I want to thank the Explorers Club for granting me an Exploration Fund Student Grant for
my fieldwork in the summer of 2010. Without their funding, we would not have been able to go
to western Mongolia, and visit regions of the Altay Mountains that had never been geologically
explored. I also want to thank my fantastic and hard working team- the trip certainly would
not have been successful without them!
References
Baljinnyam, I., A. Bayasgalan, B. A. Borisov, A. Cisternas, M. G. Dem’yanovich, L. Ganbaatar, V. M.
Kochetkov, R. A. Kurushin, P. Molnar, H. Philip, and Y. Y. Vashchilov (1993), Ruptures of major
earthquakes and active deformation in Mongolia and its surroundings, in Geological Society of America
Memoir, vol. 181, pp. 1–61, The Geological Society of America.
Cunningham, D., A. Dijkstra, J. Howard, A. Quarles, and G. Badarch (2003), Active intraplate strikeslip faulting and transpressional uplift in the Mongolian Altai, in Special Publications, vol. 210, pp.
65–87, Geological Society, London.
Gosse, J. C., and F. M. Phillips (2001), Terrestrial in situ cosmogenic nuclides: theory and application,
Quaternary Science Reviews, 20, 1475–1560.
Jackson, J. (2001), Living with earthquakes: know your faults, in Journal of Earthquake Engineering,
Special Issue 1, vol. 5, pp. 5–123, Imperial College Press.