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southwestwards with distance from the High Zagros
in the northeast. The southern and southwestern
limbs of the Zagros folds (steeper than the
norheastern limbs) are locally overturned, and in
places overthrusts have been developed. The recent
age of the folding is implied by anticlines
forming topographic highs with Juvenile erosion
patterns superimposed upon them. Upper Precambrian
salt has risen dlaplrlcally through the whole
sedimentary sequence and several salt. horizons
decouple the upper structures from those at depth
(Fig. 6). At present the Zagros active foldthrust
belt becomes more compressed and narrower
in the NW (near the impinging zone of the Arabian
plate with the northwestern pert of the Iranian
plateau) and has a width of about 200 km. The belt
widens towards southeast to about 350-400 km.
Apparently this pattern is influenced by the shape
of the Arabian-plate margin and the geometry of
the collision zone along the northern margin of
Arabia.
The isodepth contour map of the ~oho surface in
the Zagros belt [Peive and Yanshin 1979a] shows a
50-55 km thick crust along the High Zagros (the
northern margin of the belt). This gradually thins
out towards southwest to a crust of about 35 km
thick along the Zagros foothills at the northern
shorelines of the Persian Gulf. A thicker crust
along the High Zagros marginal belt could be due
to the fact that this marginal belt has undergone
two major collislonal orogenies during the late
Cretaceous and the late Miocene-Pleistocene times;
whereas the rest of the belt was only affected by
the latter phase. Moreover, the High Zagros was
the northern edge of the Mesozoic passive margin
of the Arabian-plate and it underwent larger
amount of subsidence and sedimentation.
The belt is a broad zone of continuing
compresslonal deformation which is marked by a
high level of shallow crustal seismicity (Fig. I).
Most of the earthquakes in the Zagros active foldthrust
belt are shallower than 30 km (Fig. I) and
their hypocentres do not appear to increase in
depth ~owards the northeastern margin of the belt.
Some of the deeper focal depths reported for this
belt (Fig. 12) are either real deep shocks or are
poorly recorded events [Berberian 1979b, c;
Jackson 1980b]. The entire belt is intensely
seismic and there is no evidence for seismic
shortening on a single northeast dipping plane.
The intense selsmlcity is limited to the Main
Zagros reverse fault in the northeast and the
southwestern edge of the Zagros folds in southwest
(Figs. I and 2). The Main Zagros reverse fault
believed not to be seismically active, but the
local damage zones of the earthquakes of
1965.06.21 (at HaJiabad; 28.30N, 55.91E),
1973.08.28 (at Gondoman; 31.87N, 51.15E),
1973.11.11 (at Oeshlag; 30.56N, 53.03E); and
finally the Lice earthquake in south Asia-Minor on
1975.09.06 with a 30 km surface rupture and thrust
mechanism [Toksoz and Arpat 1977], apparently
belie this Judgment (F18. 5 and Table III).
The belt is truncated to the SE by the Minab
fault, north of the Straight of Hormoz, but the
selsmicity stops west of this fault (Fig. I),
along the Hormoz-Jiroft seismic trend [Berberian
1976c]. The Minab fault (Fig. 5) has been
important tectonic structure at least since the
late Cretaceous time, separating the continentcontinent
collision in the west (Zagros) from the
oceanic-continent plate boundary in the east
(Makran; Figs. 3, 4 and 5). The belt
characterized by a high proportion of low to
medium magnitude shallow earthquakes and very few
earthquakes of magnitude . Series of earthquakes
resembling swarms are also characteristic of this
belt.
Fault plane solutions of the 20th century
earthquakes in the Zagros are consistent with
predominantly high-angle faulting; this indicates
that the present deformation appears due to NE-SW
thrusting along several NW-SE trending buried
reverse basement faults. Recent folding and
uplifting of the belt is already evident in the
deep river valleys cutting the anticlines, fossil
(raised) beaches, the height of Quaternary
alluvial terraces and uplift of historical canals
[Lees and Falcon 1952, Vita-Finzi 1979a]. The
minimom average rate of uplift of the Zagros is
estimated to be about I mm/yr since the early
Pliocene time [Falcon 1974, Vlta-Finzl, 1979a].
Based on surface geology, the thrusting and
folding of the Zagros mountains represent a
shortening of about 20 per cent across the present
250 km width of the belt since the early Pliocene
time.
B) Kopeh Dash Active Fold Belt
This northeastern border fold belt of the
Iranian plateau (Figs. 3 and 4) is formed on the
southwestern margin of the Turan continental crust
(southern Eurasia) and is presumably underlain
Hercynian metamorphosed basement. The sedimentary
cover consists of about 10 km of Mesozolc-lower
Tertiary sediments deposited in a subsiding
sedimentary basin during the Mesozoic extensional
phase. Although the Kopeh Dagh basin did not
evolve into an oceanic crust, the thickness of the
Mesozoic sediments implies that its crust was
strongly stretched.and thinned along normal faults
at that time. Like the Zagros belt, the sediments
were folded into long linear NW-SE folds
perpendicular to the trend of the relative motion
between Arabia and Eurasia, during the last phase
of the Alpine orogeny. The minimum horizontal
crustal shortening during Neogene, was about 15
per cent on the Iranian side of the fold belt
(with the present width of 70 km). Folds
western Kopeh Dagh have a gently dipping northern
flank whereas the southern flank is steep and
thrust faulted. The folds in eastern Kopeh Dagh
are symmetric and cut by numerous transverse
conjugate shear faults. These shear faults
displace some longitudinal reverse faults and
folds, and a few (like the Baghan-Germab fault;
Fig. 5) have been recently active. The belt is
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