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magmatism associated with it would have ceased not
much more than 10 Ma after the end of subduction
(about 65 Ma), with the absorption of the downgoing
slabs, the cause of this extensive ’post-collision’ volcanic
activity is not clear. Because of the wide-ranging
composition of these volcanics (rhyolite, dacite, andesite,
ignimbrite, and basalt) it is difficult to explain them
simply as crustal melts due to rapid uplift and erosion,
and it is necessary at least in part to invoke a lower crust
or mantle source (Section II.6b). Although the nature
this volcanism is enigmatic, volcanism of the same type
continues to the present day (Figs. 17, 18) and is evidently
not related to subduction. It is suggested in the
absence of alternative hypotheses that this volcanism
might be related to processes of crustal shortening or to
strike-slip faulting and sheafing, or to both (see Sections
II.6b, II.Tb, and 11.8). During the period of most active
volcanism Iran was subject to an overall right-lateral
shear and relatively little shortening (comparing Figs.
and 7).
Assuming either of the foregoing mechanisms it is not
clear why the volume of volcanics has diminished with
time. It may be due to changes in the amount of shearing.
Alternatively the source conditions may alter as the
crust thickens or, if volcanoes cease activity when they
reach a maximum height, larger volumes of lava will
erupt when the crust is near sea level than on crust more
than 3000 m above sea level.
The marine carbonate and marl deposition in the narrowing
sedimentary basin of the Zagros continued after
the Late Cretaceous collision (Figs. 6 to 9 and 14 to 17),
with the folded and uplifted Central Iranian active continental
margin (the Sanandaj-Sirjan belt) acting as
barrier between the Central Iranian shallow basins in the
north and the Zagros basin in the south (Figs. 6 to 9).
The late Alpine orogenic events followed continuously
from the Middle Alpine and extended to the present.
Progressively more of Iran became land with separate
mountain-divided narrow basins (Figs. 8 and 9).
Neogene time (10 Ma), continental deposits supplied
from the rising orogenic belts characterize the sedimentation
in Iran (Fig. 9).
BERBERIAN AND KING 221
During the Middle and Late Alpine orogenic movements,
folding and uplift occurred followed by subsidence
in central and northern Iran (Tables 2 and 3). The
episodes of major activity defined in the literature and
discussed in Section II refer to the unconformities associated
with subsidence and marine transgression. Thus,
although the overall relative motion of Arabia and Asia
caused compression and uplift, there are clearly defined
diachronous episodes of subsidence and extension. This
indicates that the tectonic forces were not supplied from
the Asian-Arabian motion alone, and presumably must
have resulted from motions in the upper mantle or lower
crust. However, throughout the period, the major fold
belts grew in size, with fold axes continuing to form
parallel to those initiated during the Late Cretaceous
movements.
1.5--DISCUSSION AND CONCLUSION
Iran and some of the surrounding countries were connected
to Arabia and Africa from the late Precambrian
until the late Paleozoic. At that time these fragments of
continental crust split from Arabia, crossed the Hercynian
Ocean, and collided with the Asian block. During
this passage and the subsequent subduction of ocean
crust to the south of Iran, the continental crust was
stretched. At the time of onset of continental compression
(about 65 Ma) Iran was entirely below sea level and
marine sedimentary conditions prevailed. This is consistent
with the crust being thin. Post-colIisional convergence
could then have resulted in progressive crustal
thickening and shortening by folding, reverse faulting,
and the gradual rise of the mountain belts above sea
level. Redistribution of material laterally by sediment
transport and large-scale strike-slip motion could also
have occurred.
If the Late Cretaceous crust was nominally 20 km
thick and 100 or 200 m below sea level, a compression
by a factor of two would double its thickness to that at
present and account for the present mean elevation of the
Iranian plateau of 2-3 km. This simple view assumes
that thermal changes have not altered the density of the
crust or mantle. The thermal processes associated with
coal-bearing sandstones and shales of the Shemshak Formation with Asiatic flora and fauna covering Iran and southern Eurasia
via Kopeh Dagh belt. 4. Continental clastics with marine intercalations. 5. Sea marginal flats, sabkhas, and shallow marine
deposits. 6. Shallow water marine carbonates and shales. 7. Shallow to moderately deep marine sediments of the Great Caucasus
(miogeosyncline basin). 8. Volcanic arc of Pontian-Transcaucasian (P-Tc). 9. Upper Triassic - Jurassic intrusive rocks.
Upper Triassic - Jurassic andesitic-basaltic volcanic rocks. 11. Approximate boundary between different sedimentary facies.
12. Spreading centres. 13. S ubduction zone with triangles on the upper plate. 14. Reverse faults with bars on the upper plate. 15.
Major normal faults activated during the late Triassic rifting phase. 16. Middle Triassic regional metamorphic rocks along the
active Central Iranian continental margin, the Sanandaj-Sirjan belt (SS). 17. Present continental shorelines. P-Tc:
Pontian-Transcaucasian island arc; C-C: Crimean-Caucasian marginal sea; SS: Sanandaj-Sirjan belt.
Principal sources of data: Reconstruction (Mercator Conformal Projection) is modified from Smith and Briden (1977).
tectono-sedimentary data within the boundaries of Iran are based on our Fig. 13. Data outside Iran come from Vereshchagin and
Ronov (1968), Razvalyayev (1972), and Adamia et al. (1977) for the northwesternmost part, Bein and Gvirtzman (1977),
Biju-Duval et al. (1977) for the westernmost part.