PDF COPY - Manuel berberian

egional greenschist-facies metamorphism in the Tomd
area is thought to be the result of the Late Jurassic
movements (Hushmandzadeh et al. 1978), but there is
not enough evidence at present to prove it.
In the Ardakan area of Central Iran, the Jurassic rocks
are intensely folded, exhibit slaty cleavage, and are cut
by quartz veins. They are unconformably covered by
Cretaceous conglomerate (Haghipour et al. 1977). It
seems that the Upper Precambrian salts of the Ravar area
reached the surface as diapirs during the Late Jurassic
movements (Huber 1978) and were the source for the
Jurassic-Cretaceous evaporites. Evidence of continuous
Jurassic--Cretaceous sedimentation is only found in the
Shal Formation of the Talesh mountains southwest of
the Caspian Sea (Davies et al. 1972; Clark et al. 1975;
Table 2), and in the southeastern part of the Sanandaj-
Sirjan belt in the Calpionella limestone (Dimitrijevic
1973; Berberian and Nogol 1974).
H.4b~Central lranian active continental margin during
Late Jurassic time
The Jurassic rocks of the west Sirjan area along the
Central Iranian active continental margin (the Sanandaj-
Sirjan belt) are affected by an important schistosity,
which is not present in the Orbitolina limestone of Berriasian-Valanginian
age (135 Ma). This may indicate
Late Jurassic tectonic phase (Ricou 1974). The K/Ar
ages of the Abukuma type metamorphic rocks of the area
west of Sirjan (along the Sanandaj-Sirjan belt) range
from 186 to 89 Ma (Watters and Sabzehei 1970). Although
some of the metamorphic rocks are definitely
Middle Triassic (Section II.3a) and Late Cretaceous
(Section II.5.3b) in age, three samples indicate a Late
Jurassic - Early Cretaceous compressional movement
and metamorphism possibly related to subduction zone
processes. Haynes and Reynolds (1980) suggest 170 ---
5 Ma as the date of collision processes including metamorphism
and ophiolite obduction. This is based on one
date for hornblende taken from an amphibolite enclosed
by ultrabasic rocks in the area northeast of Minab (east
of the Minab fault). However, it seems that there is no
disruption in the sedimentation processes of the Zagros
and Central Iran during Middle Jurassic time and that
ophiolite emplacement took place during the Late Cretaceous
Epoch (Sections II.5.2b, II.5.3b). It is possible
that the radiometric age has been disturbed by later
retrograde recrystallization and argon loss.
The upper Paleozoic crustal extension, which led to
the development of continental rifting along the High-
Zagros and the Central Iranian continental margins,
apparently created marginal oceanic crust in late Permian
and Jurassic times. Subsequent pelagic sedimentation
(radiolarite, turbidite, black marl) during Late Triassic
and Jurassic times (Ricou et al. 1977) reflects
passive continental margin subsidence and the full
establishment of the High-Zagros Ocean between the
BERBERIAN AND KING 241
Central Iranian active and the Zagros passive continental
margins. The sedimentary evidence of the continuous
Upper Triassic to Upper Jurassic pelagic deposition
along the subsided continental margins apparently indicates
a long period during which undisturbed ocean floor
existed.
Presumably at the end of the Triassic Period and the
beginning of the Jurassic Period, an ocean extended
through the Great Caucasus (Khain 1977). Deposition
the Jurassic-to-Neocomian (190-140 Ma) radiolarian
cherts and volcanic activity (spilites and diabases) along
the Sevan-Akera ophiolite belt of the Little Caucasus
indicate the existence of oceanic crust during Jurassic
time (Knipper and Sokolov 1974). This ocean seems
be the western part of the Hercynian Ocean (Fig. 5).
During this time the northern slope of the Great Caucasus
was a continental margin of the Atlantic type, the
southern slope a marginal sea, and the northern Transcaucasia
an island arc (Adamia et al. 1977). The period
is known to be marked by a general extension and
subsidence of all tectonic units of the Caucasus.
After the Late Jurassic movements, the two separate
basins of Zagros and Kopeh Dagh continued their subsidence
with marine carbonate deposition.
H.4c--Kopeh Dagh basin during Late Jurassic time
Like the Zagros, the Kopeh Dagh basin started subsiding
in the Jurassic Period, and after deposition of the
Liassic Shemshak (Kashafrud) Formation, the sea deepened
along normal faults depositing the Chaman Bid-
Mozduran Formation (carbonates and marls; Afshar-
Harb 1969, 1979). The fault-controlled subsidence in
the Kopeh Dagh, from Jurassic to Oligocene times, allowed
up to 10 km of sediment to be deposited. The
Jurassic marine carbonates also covered the southern
part of the Turanian plate (Beznosov et al. 1978).
During late Jurassic time, the Kopeh Dagh basin
became shallower, and emergence took place over the
area, producing a continental red unit, the Shurijeh
Formation (Afshar-Harb 1970, 1979). The late Jurassic
regression of the sea laid down red clays and sandstones
of lagoonal and fluvial origin in the southern Turan
(Beznosov et al. 1978). The Jurassic rock units of the
Kopeh Dagh sequence in the south extend to the
Binalud-Aladagh mountains (geographic continuation
of Alborz in east; Fig. 1) and possibly indicate a united
landmass in the north (Central Iran - Alborz - Kopeh
Dagh - Turan plate) with more subsidence in Kopeh
Dagh than in eastern Alborz or Central Iran. The reason
for this ’greater subsidence’ is not clear and no Mesozoic
or Tertiary ophiolite belt is exposed along the southern
part of the Kopeh Dagh to assume a passive continental
margin regime.
H.4d--Zagros basin during Late Jurassic time
Following the deposition of the Neyriz Formation,
marine limestones and marls of the Jurassic - Lower