PDF COPY - Manuel berberian

graphic maps. We present both sets of maps to allow the
significance of the data on which our reconstructions are
based to be easily assessed.
This examination of the paleogeography and the paleotectonics
of the region has clearly excluded some of
the previous reconstructions, which scarcely considered
important geological constraints. Our reconstructions,
which are geometrically as simple as possible, are more
consistent with the available data, but the data are limited
and more work will be required to confirm some of
our suggestions and extend others. In particular, large
strike-slip motions such as those now occurring in Central
Asia and Turkey could have occurred, but are undetectable.
In this study we assume that ophiolites may
represent the site of either a large old ocean or a small
Red Sea type ocean. Evidence to distinguish between
these for our reconstructions comes from large-scale
geometric contraints and not from any known difference
in the field observations of features left by their closure.
Our account of the region, which starts in late Precambrian
time, is divided into two sections. The first
section describes the general evolution of Iran and the
significance of various geological features in its interpretation
and is intended to be an overview that is easy to
read. The second section provides a detailed geological
review and contains the data on which the interpretations
of the first section are based. The major Iranian tectonosedimentary
units together with their characteristics
and the localities cited in the paper are given in Figs. 1
and 2. Correlation charts of the Paleozoic, Mesozoic,
and Tertiary formations, and sedimentary gaps and unconformities
discussed in the text are given in Tables 1
to3.
1--Evolution of the region
I. 1--PRECAMBRIAN
It is not possible at present to produce a paleogeographic
map and continental reconstructions prior to the
Upper Precambrian. However, some features of Iran are
recognizable from this period. These include a possible
fossil island-arc (Chapedony, Posht-e-Badam), late Precambrian
deformation followed by alkali-rift volcanism,
the Hormoz Salt deposits with epicontinental red
elastics, and some acid magmatism. Some major structures
such as the Main Zagros, High Zagros, Chapedony,
Posht-e-Badam, and Nayband faults apparently
formed facies dividers in the Upper Precambrian and
Lower Paleozoic (Figs. 3 and 10; see also Section II. 1).
Like the Arabian craton, the Precambrian basement of
Iran may be the crust from Precambrian calc-alkaline
island arcs (see Section II.1), and if this is so their
cratonization must have taken place prior to the deposition
of the Upper Precambrian - Lower Cambrian salt,
red detritus, and carbonates.
BERBERIAN AND KING 211
The Hormoz Salt was deposited in basins on the
peneplaned Arabian shield during Late Precambrian-
Early Cambrian time. The distribution of these sedimentary
facies suggests that during the Late Precambrian,
Central Iran and Zagros together with the Salt
Ranges of Pakistan and Arabia were all part of the same
landmass and were partly covered by a common shallow
sea (Table 1). The present Main Zagros reverse fault
probably marks the site of a normal fault controlling the
sedimentation (see Fig. 10) and was associated with the
formation of a passive continental margin to the north,
recognizable by the Cambrian (Fig. 3). The Late Precambrian
orogeny (around 850-570 Ma) and its associated
magmatism represent an earlier compressional
phase much before the Hormoz Salt deposition. The
Upper Precambrian acid and basic alkali-volcanics
(Figs. 3 and 10) are subsequent to the Late Precambrian
orogeny and presumably developed during the rifting
that formed the sedimentary basins in which the Hormoz
Salt and the Upper Precambrian - Lower Cambrian
sediments were deposited, and may be associated with
this rifting.
1.2--PALEOZOIC
The first recognizable tectonic event in Iran occurred
near the end of the Paleozoic Era, with the onset of the
Late Paleozoic (Hercynian) orogeny. Prior to this time,
the whole region was a relatively stable continental platform
with epicontinental shelf deposits, and lacked
major magmatism or folding.
After deposition of the Upper Precambrian Hormoz
Salt-dolomite, shallow-water red arkosic sandstones
and shales of Cambrian age were deposited over a wide
area from Arabia in the south to the Alborz mountains
in the north. These deposits also occur in Pakistan,
Afghanistan, and Turkey (Fig. 3; Table 1; Section
II.2a). The red sandstone sedimentation was followed
by the deposition of dolomite, marl, and shale (with salt
pseudomorphs) in shallow sea conditions. The first fully
marine carbonates were deposited in the Middle and
Late Cambrian Epochs, and in the Ordovician or the
Silurian the marine transgression was terminated with
the deposition of sandstone (Table 1).
All of these terrestrial to very shallow marine depositional
environments are consistent with a passive and
continuously connected continental margin at least between
600 and 400 Ma. The fragments of the margin
that we can now identify may not have been in the
positions we suppose in our reconstruction. However,
this would require large strike-slip motion to have occurred
subsequently, for which, as yet, we have no
evidence. During the same period the Asian part of the
Caucasus, south Caspian, and Kopeh Dagh (north of the
Hercynian suture line, not shown in Fig. 3; see Fig. 10)