Arkell.1956.Jurassic..

374 RANGES OF SOUTH-WEST ASIA
THE ZAGROS AND OTHER SOUTH-WEST MARGINAL RANGES OF PERSIA
AND KURDISTAN
From the surroundings of Lake Urmia two great systems of foldmountains
diverge: the Elburz running eastwards and showing in its
internal structure signs of folding and thrusting directed towards the
sunk foreland of the Caspian Sea; and the Zagros and its parallel and
continuing ranges running south-eastwards and folded and thrust towards
the sunk fore-deep of Mesopotamia and the Persian Gulf.
On the second and greater mountain arc, which extends from near
Lake Urmia for 1300 miles to the Arabian Sea, a flood of light has been
thrown by geologists of the Anglo-Persian (later Anglo-Iranian) Oil
Company. Their late Chief Geologist, Dr G. M. Lees, F.R.S., most
generously provided, especially for this book, transcripts of the Company's
unpublished reports dealing with Jurassic stratigraphy, and the map of
Jurassic outcrops (fig. 53). Where not otherwise stated, the facts summarized
below are taken from reports by J. A. Douglas, B. K. N. Wyllie
and P. E. Kent, which synthesize information obtained by Messrs G. M.
Lees, F. D. S. Richardson, B. K. N. Wyllie, J. V. Harrison, K. W. Gray,
N. L. Falcon and other geologists. The identifications of ammonites were
by Dr L. F. Spath. He also supplied identifications for the Iraq Petroleum
Company, whose geologists have done considerable work in the northern
areas, especially in Kurdistan.
Structurally, these mountains are divided longitudinally into two parallel
bands along a line running approximately NW.-SE. near the towns of
Kermanshah and Shiraz (Lees & Richardson, 1940). The mountain
ranges to the SW. of this line, and the foothills, constitute the normally
folded zone. They consist of large anticlines and synclines similar to those
of the French Jura but on a larger scale, and there are also some great
thrust faults which appear to have developed from simple folds, but these
are subordinate. The ranges to the NE. of the divide constitute the zone
of overthrusting, where the whole country is carved out of overthrust
nappes and the complicated structure and topography contrast sharply
with those of the simple fold-mountains farther south-west. Metamorphic
rocks, largely of unknown age, comprising schists and sheared crystalline
limestones, partly Palaeozoic and partly Jurassic (Dehghan, 1947), occupy
much of the NE. part of the zone of overthrusting, and in places are thrust
south-westwards over part of the normally folded zone. The thrusting
occurred at two distinct periods, Upper Cretaceous (probably Senonian)
and Upper Tertiary.
Differences of facies and stratigraphy in the Mesozoic rocks show that
at least as early as the Jurassic these two tectonic zones were becoming
differentiated from one another, and from the third and much wider
tabulate zone of Arabia where the sediments were laid down on the
gently-shelving margin of the Arabo-Nubian massif or shield (see p. 284).
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