The Chihteyn rupture (Fig,2,[3]) is located in the northern part of
the deep-seated Hovd fault and to the western side of the Achit
Nuur Basin. The main 27 km long rupture is accompanied by
short low-angle ravines and cracks both within the basin and the
intrabasin area. The movement of the major fault zone is
determined as right-lateral. The maximum height of the scarp
varies between 0.5 and 1.5 to 2.0 m.
my a - À d -
I INI m e
vafe, PES ST ae T e
| at. :
? 1
Figure 8. An enlargement of the KOSMOS data (see Fig.7 for
location) showing the exposed Tertiary clastics grey to dark grey
in tone. The active fault in the center is a thrust fault and shows
distinct light tone. Besides, the image shows characteristic offset
drainages, truncated fan and behaded drainages clearly indicating
neotectonic movement. In the field, some sagponds are also
evident.
The Ar Hótól rupture (Fig.2,[5]), which is the central part of the
Hovd fault Fig.2,[3] extends for more than 215 km. This rupture
trends NNW (330?) and is represented by scarps of almost 3 m
high, tension cracks, compression knobs, displacement of old
channels from which right-lateral displacement combined with
revers faulting could be inferred.
The Sagsay rupture (Fig.2,[2]) also trends NNW and occurs along
the reactivated Tolbo Nuur fault. The reactivated fault segment
extends for 35-37 km, has a width of 50 m and shows a regular
pattern of tension cracks and compressional jointing indicating
right-lateral strike-slip movement.
A linear crack 6-8 km long, striking mostly west-east, was
produced in the northeastern Mongolia Altay mountain range with
the Üüreg Nuur intermontane trough by the Üüreg Nuur
(Fig.2,[4]) earthquake of 15 May 1970 (N50.17 °,E91.25 ° and
M-7.0).
The Fu-Yun ruptures (Fig.2[1]) were formed during the Fu-Yun
earthquake (N46.89 °, E90.06 ^; M-8.0). The mapped surface
rupture trends N-NW (340 ^?) for a distance of 180 km (a bit
oblique to the Mongolian Altay direction). Right-lateral
displacement were observed along most of the fault. Several
measurements of 9 to 11m displacement were made in central
section, with a single maximum measurement of 14.6 m.
Measured displacement decreases to both north and south., with
an average of about 8 m (Zhang Peizhen, 1982).
The Bulgan rupture (Fig.2,[16]) system trends east-west, for a
distance of 30-32 km. The ruptures form the compressional hills
and scarps with a height of 2-3 m. This rupture is interpreted as a
left-lateral strike-slip with significance reverse component.
The Bij rupture (Fig.2,[6]) lies at the southernmost extremity of
the Mongolian Altay zone at its junction with the Gobi-Altai. The
seismic rupture trends nearly NS for more than 45 km. Its
morphological expression is a scarp as high as 1 m. In the field,
one can identify right-lateral movement combined with normal
faulting due to prehistoric earthquake.
b) Surface ruptures of the Gobi-Altai. The Gobi-Altay mountain
range, a southeastern continuation of the Mongolian-Altay range,
trends nearly 800 km to the WNW-ESE. It has a width of almost
100 km in the west and narrows constantly eastward to 50 km.
The average elevation exceeds 2500 m and several peaks are
higher than 3500 m.
Geologically, the Gobi-Altay is located in the border zone
between the Early Paleozoic and Late Paleozoic belts. Mostly the
Bogd fault constitutes the border between these two belts in the
western part. This border splits to the southeast and wraps the Ih-
Bogd and Baga-Bogd mountains along the southern foothills. It
could be assumed as one step during the closure of the Paleotethys
(Sengór at al, 1988) with subduction of oceanic lithosphere and
the collision of the ancient margin of the Eurasian continent.
One of biggest intracontinental earthquakes in this century has
occurred in the central part of the Gobi-Altay range (45.31 N,
99.2] E). In the noon on 4 December 1957 (h.. 39'48" by
International time) the Ih-Bogd and Baga Bogs mountains were
shocked by M-8.3 earthquake. The epicentre of the earthquake
was located at the western part of the Ih-Bogd massive. Although
Florensov and Solonenko (1965) referred to this earthquake as the
Gobi-Altay earthquake, this was not correct because a much
smaller portion of the Gobi-Altay was affected. Within Mongolia
as such, this earthquake is known as /h-Bogd earthquake and this
name seems to be more suitable for this event.
Apart from this earthquake, several major ruptures: Chandman
(Fig.2,[14]) and Fig.9,[6], Hujirtyn (Fig.9,[4]), Ehen Tógróg
(Fig.9,[3]), Myangayn (Fig.2,[13]) and Fig.9,5] and Tógróg
(Fig.9,[2]) trend west-east too.
The first three of these as well as the Gobi-Altai (Ih-Bogd)
ruptures are parts of the deep-seated Gobi-Altai fault and thus are
mostly left-lateral strike-slip faults showing some reverse
movements. Myangayn rupture and Fig.9,[5] show the same
features as the Gobi-Altai and lies roughly parallel to it. A scarp
marks the fault along much of its length, but the sense of vertical
slip varies along the fault. The maximum height reaches 3-4 m in
its centre and faces north. Probably, this scarp is the accumulation
of more than one earthquake. Tógróg rupture lies within Shargyn
Tsagaan basin and has a clear thrust component, similar to basin
ruptures in Ih-Bogd area.
The WNW-ESE trending Gobi-Altay range is controlled by
several parallel left-lateral strike-slip faults and consists of a
wedge shaped (in plan) mountains separated by narrow
intermontane basins. These huge mountain ranges and basin
systems arose on the territory of central Asia during Late Cenozoic
time. Many of the mountain fronts are marked by active fault
scarps, active alluvial fan deposits and other youthful
geomorphological features indicating current tectonic activity and
uplift. Most of these features are visible on Landsat TM, SPOT
and photo prints of Russian KOSMOS satellite stereo images. The
neotectonic activities of the mountain ranges is continuing
nowadays, and is a reason for the area's high seismicity.
c) Surface Ruptures of the Hangay. Within the Hangay dome,
there are few observed and several inferred ruptures of
622 International Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998
The
Tset:
8.4 [I
surfa
strike
high.
Two
occu
avera
the se
(N49
al, 1
conti
west-
displ:
decre
In th
(Fig.
create
trend
south
d) Su
know
(taigz
undet
The E
can be
Up i
5 km.
