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4.1 NEOTECTONIC IMPRESSIONS ON SATELLITE
IMAGES
Western Mongolia (Fig.1) and its surroundings have been one of
the most seismically active intra-continental region of the world in
this century. Many surface ruptures of paleo- and recent
earthquakes represent the seismic activity of this area. Indirect
evidence based on geomorphological, stratigraphic or pedological
criteria and historical record of earthquakes with structural
disruption and displacement in rock units of age less than 11,000
years are some of the criteria used to denote active new tectonic
movement in the region.
4.1 Indirect Evidence Based on Geomorphological Features.
Mapping of present day morphological features provide important,
though indirect clues for delineating active faults related to
neotectonism. Peculiar patterns, for example, bending and off-
setting of streams, ridges, sagponds, springs, scarps, hanging and
headless valleys, river capture, open rifts and prominent scraps,
and their alignments in certain directions, can indicate recent
movements. These features may be relatively difficult to decipher
in the field, and more readily observed on remote sensing images,
due to their advantage of polar synoptic overview.
The common feature of most active fault is their sharp topographic
expression often coinciding with three different types of
landscape.
a) Structures lying within the mountain massifs. The surface
rupture formed within the mountain massifs often coincide with
narrow V-shaped valleys of mountain rivers and streams. The
active fault strikes are concordant with the strikes of the mountain
fronts. They commonly define smooth curves, with surface
ruptures often forming in places of maximum curvature.
Figs.3a,b is a KOSMOS image of SW Mongolia where the
Mongolian Altay meets the Gobi-Altai range in its south junction.
Traditionally, the geographic boundary between the two Altay is
somewhat arbitrary. The opposite senses of strike-slip
displacement however, make it difficult to treat the two Altays' as
a tectonic continuation of one another. The southern end of the
Mongolian-Altay is taken to be just south of the Shargyn Tsagaan
basin (Figs.2,3), where mountain ranges trend east-west. This is
a convenient boundary because, unlike the situation in the
Mongolian-Altay where right-lateral strike-slip faulting is
pervasive, left-lateral strike-slip faulting with components of
reverse faulting, occur on easterly trending planes that bound the
mountains of the Gobi-Altay. Several parallel intermontane (ramp)
and narrow wedge-shaped mountains are laced with WNW-ESE
trending left-lateral strike-slip faults. These faults bound blocks
with remarkably flat-topped mountains and basins with
asymmetric cross sections. Their geometry does not strike over a
long distance except for a great structure bordering the south of
the Gobi-Altay range, from where the great Gobi desert starts.
The fault that ruptured the Hujirtyn Gol earthquake is easily
depicted from KOSMOS data (Fig.4) and lies within the
Mongolian-Altay region (45.70°N, 95.40°E), in the place with
maximum curvature to the south. Field evidence shows the
presence of very recent scarp following the southern edge of a
narrow river valley bearing the same name.
b)Structures following the base of mountains. Most surface
ruptures related to active faults follow the base of large mountain
massifs. Many of the known active faults such as, the Shuvuut
fault, show remarkable NW-SE alignment. The offsetting of
streams and the alignment of morphological features are very
conspicuous . The drag effect imply a right-lateral sense of
displacement along the neotectonic fault.
Another example, the Boga-Bogd fault zone (Figs.5a,b) - the main
structure in the Gobi-Altay, has a slightly curved shape in plan
view. The most wedge-shaped mountain massifs are associated
with the restraining bends of the fault zone, whereas in the areas
of the releasing bend, small secondary basins within the main
depressions occur. On the Landsat TM (Fig.5a) and SPOT data, it
was possible to analyze the behavior of the surface rupture
developed during the Ih-Bogd earthquake of 4 December 1957.
Although the rupture itself are not visible on the satellite imagery,
the general behavior of the ruptures were studied in relation with
geomorphology, lithology, relief and field data. The main rupture
follows the northern foothills of the Ih-Bogd and Boga-Bogd
mountain ranges and are clearly associated with borders of
basement rocks and basin sediments. Some secondary thrust
scarps forming a typically hilly uplands within the basin, can be
followed along the down foothills of these structures (see Fig.5b).
From the main rupture, several splits are observed mostly forming
an acute angle with the main one. One of the longest and biggest
split rupture constitute the Toromhon Overthrust which form an
angle of 70^ with the main rupture. This thrust scarp splits
towards southwest from the main rupture near the outcome of the
Toromhon Sair and further follows the contact zone of the
Mesozoic basalts (darker in tone on Fig.5a) and the Paleogene
clastic sediments for up to 50 Km. The Paleogene clastic rocks are
easily delineated from all remotely sensed data by their different
tones and textures.
Alluvial fan-apron deposits form the piedmonts of the Ih-Bogd
and Baga-Bogd massifs. These deposits form a zone along the
foothills of the mountains. Most upper part of the apron deposits
are cut by faults, appearing as clear linear features between the
apron deposits and the basement rocks. The fan-apron deposits as
seen from Landsat TM and SPOT images show characteristic fan-
form in plan view, and is possible to determine the relative age of
the fans by overlying each other. The younger fans have a lighter
image tone. They have brown smooth surface dissected by
numerous diverging dry streams - "sairs". In the southern
piedmont of the mountain massifs, the alluvial fan-apron deposits
are much wider than along the northern slopes. The alluvial plain-
fluvial deposits form the central part of the basins. The colour of
this unit varies in the light tones with pink and grey hue. The
eolian sands form separate overlaps within the basin. It has a
rough texture due to sand dunes and in some places it is possible
to see the dominant wind direction by sand stripes.
On the peripheries of the Ih-Bogd massif, prominent shutter ridges
are present. These include a line of frontal, elongated hills along
strike-slip fault protruding above the piedmont zone. They are
separated from the main massif by a wide (up to 3 km) elongated
depression. The hills bearing the local Mongolian name
"zereglee", are intersected by dry streams which descend from the
main massif. The ridges reach an elevation of 100-150 m. Such
"Zereglee", as locally known, is symmetrically distributed along
the northern and southern piedmonts of Ih-Bogd and has an
obvious neotectonic origin.
The Baga-Bogd massif differs from the Ih-Bogd massif by its
marked dissection and incision, fault line scarps, triangular facets,
steep slopes and rocky crests, and the poor state of preservation of
its flat mountain tops. The main pyramid-shaped peak of the
Intemational Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998 617
