PDF COPY - Manuel berberian

Central Iran [RoH. Slbson personal communication]
(Fig. 5). This could be considered as evidence
for a long continued activity and a large degree
of differential upllft and erosion (exposure of
the Precambrlan metamorphic rocks with mylonltes)
along the fault.
D) Accretlonary Flysch Belt
The Mokran ranges of southeast Iran, and the
Zabol-Baluch ranges of east Iran along the
Afghanlstan-Paklstan border, are post-Cretaceous
flysch belts which join together in SE Iran and
continue into the Pakistani Baluchestan ranges
(Fig. 2). The flysch deposits are believed
have been deposited on upper Cretaceous
ophlolltlc-melange in a continental slope environment.
The Makran ranges are a zone of east-west
Tertiary folds overthrust south with north-dlpplng
longitudinal imbricate reverse faults extending
from the Hinab fault in the west to the Ornach Nal
and Chsman faults of Pakistan in the east (Figs.
and 3). The flysch belt is shortened by a northsouth
compression system into steep E-W folds and
north-dipping thrusts. The isodepth contour map
of the Moho surface in the Makran ranges [Pelve
and Yonshln 1979a] shows a thick crust of about 50
km thick in the northern Mokran (southeastern active
continental margin of Central Iran).
Apparently it gradually thins southwards to a
crust of ~0 km thick along the coastal Mokran.
However recent investigations along offshore
Mokran indicated a much thinner crust [Niazi et
al. 1980, White and Louden 1981].
The Makran re,ion is an unusually wide deformed
accretlonary s~dlment prism formed from material
scraped off the northerly subductlng oceanic
lithosphere of the Gulf of O~an (Arabian plate)
underneath the Central Iranian continental margin,
at a rate of about 5 cmlyr [Stoneley 197q, White
and Klltgord 1976, Farhoudi and Katie 1977, Jacob
and Quittmeyer 1979, Oulttmeyer et al. 1979]. The
ophiolltlc melange of the Makran ranges cannot be
interpreted as an indication of continental
closure, since subduction of oceanic crust
preceded and succeeded its late Cretaceous
emplacement [Berberian and King 1981]. The area
has been an active convergent plate margin at
least since Cretaceous. Folding and stacking of
the surface sediments of the Oman abyssal plain is
thought to be due to the ongoing subductlon of the
oceanic crust along the Makrsn continental margin
[White and Klltgord 1976, White 1977, White and
Ross 1979]. Apparently the continental slope of
the Gulf of O~an off western Makran is underlain
by an oceanic lithosphere with thick pile of
stacked sediments. The subducting oceanic crust
has a thickness of about 6.7 km and is carrying
about 7 to 12 km of sediments into the
sceretionary wedge [Niazl etal. 1980, White and
Louden 1981]. Presumably the subductlng oceanic
lithosphere underneath the Makran ranges has a
gentle dip (less than 2 degrees) from the coastal
trench to the hingeline (approximately 170 km
inland) and deepens northward [Jacob and
Oulttmeyer 1979, White and Louden 1981].
Apparently the Bazman-Taftan-~oltan active
volcanic-arc, north of Makran, is associated with
this subduction zone. The presence of Pleistocene
and Molocene raised beaches and marine terraces
along the Makran coast indicates a tectonically
emerging coast [Falcon 19~7, Sneed 1969, 1970,
Little 1972, Vita=Finzl 1975, 1979b, Ghorashl
1979, Page etal. 1979]. Episodic uplift of the
convergent continental margin of the Makran coast
could be the result of the large-magnitude
earthquakes [Page etal. 1979]. The I&C and U-Th
dates on shells from the raised beaches of Iranlan
Makran, range from 11~,000 to 156,000 YBP, and the
Holocene rate of uplift on the coast is about
0.01-0,02 cm/yr increasing towards east [Page et
al. 1979].
The zone is characterized by a low and scattered
selsmlctty compared with the Zagros actlve foldthrust
belt [Berberian 1976a; Fig. 1]. Lack of
selsmlclty along the subductlon zone is not clear;
it could be due to the very shallow angle of
subductlon [White and Klltgord 1976]. The depth of
earthquakes in Makran apparently increases from
very shallow at the coast to 80 km inland [Jacob
and Quittmeyer 1979, Quittmeyer and Jacob 1979].
Fault plane solutions of the Mokran ranges of
southeast Iran (Fig. 5) show that the region
characterized by two seismic regimes: i) shallow
earthquakes with thrust mechanism (reactivation of
the reverse faults at the surface), and ii)
moderate to deep earthquakes with normal faulting
mechanism (deformation of the subduetlng oceanic
crust) [Jacob and Oulttmeyer 1979]. No normal
fault is observed at the surface in the Makran
ranges. The Jaz Murlan Depression, which is
situated in the northern part of the Makran
ranges, is considered to be downthrown along
normal faults [Farhoudl and Karlg 1977]. The South
Jaz Murian fault, bordering the southern part of
the depression and the northern part of the Makran
ranges (Fig. I), has a high-angle reverse
character at the surface.
The area has experienced two large magnitude
earthquakes (1945.11.27, Ms=8.0 and 19~7.08.05,
Ms=7.3) along the Mokran active subduction zone,
off the coast of Makran [~ondhl 1947, Oulttmeyer
etal. 1979]. Recent low level seismicity in the
region west of ~ that affected by 1945 event,
suggests that the western part of the coastal
Makran region may be the site of the next large
magnitude earthquake [Ouittmeyer 1979].
Recent Tectonics Models
Plate tectonics has provided a good description
of the active deformation of oceanic lithosphere
which is characterized by narrow belts of
selsmicity separating larger aselsmic rigid plates
[Isacks et al. 1968]. In contrast, continental
lithosphere in convergent zones with diffuse
selsmlclty behaves differently [McKenzle 1972,
60 BERBERIAN