163
The southern Caspian: A compressional depression floored by a trapped, modified
~
oceanic crust
"... and a goat was drinking water from the
Caspian Sea of a geographic map." (Sohrab Sepehry)
2MANUEL
BERBERIAN
Departmentof Earth Sciences, Bullard Laboratories, University of Cambridge,MadingleyRise, Madingley Rd.,
Cambridge CB3 0EZ, England
Received March 4, 1981
Revision accepted July 14, 1982
The south Caspian intracontinental depression, floored by oceanic basement, is a relatively stable block, with minor
deformation, surrounded by active fold-thrust belts of arcuate form (Talesh, Alborz, and KopehDaghMountains),which have
undergone intense late Cainozoic crustal shortening. The basin is interpreted as a Neogene-Quaternary"compressional
depression," boundedby multi-role mountain-borderingreverse faults, and apparently floored by a late Paleozoic - Triassic or
late Mesozoic- early Tertiary "modified oceanic crust" trapped along an old geosuture. It maybe a relic of an old
(Paleozoic-Triassic) ocean, or else a marginal sea developedbehind a Mesozoic-Paleogeneocean, and analysis of geological
and geophysical data enables a schemeto be suggested. The general arcuate shape of the Alborz and the Talesh bordering
mountainbelts follows the pattern of the supposedrigid and thickened ocean crust of south Caspian depression.
A tectono-sedimentarystudy of the south Caspianregion, coupled with the body-wavemodellingof a recent earthquake along
one of the borderingfaults, maysuggest a possible flattening of the fault with depth(listric thrust), and that the estimate of the
focal depth of the regional earthquakesbased on teleseismic arrival time data is not accurate. Thedifference in elevation between
the depressionand the borderingactive fold-thrust belts is causedby a difference in crustal structure andreverse faulting during a
dominantcompressional tectonic regime.
Thestudy mayadd support to the idea that old continental deep-seated multi-role faults, whichhavecontrolled the sedimentary
facies and basins during different geological times and were responsible for the formation of the present physiographicfeature,
are the site of the present seismicactivity in the orogenicbelts.
Le foss6 tectonique intracontinental ~ socle ocranique de la rrgion Caspiennesud est constitu6 d’un bloc relativement stable,
peu drformr, entour6 par des zones de failles imbriqures actives de forme arqure (montagnesTalesh, Alborz et KopehDagh)
ota la cro~te fut considrrablement raccourcie au Crnozoiquesuprrieur. Oncroit que la drpression tectonique correspond ~ un
affaissement caus6 par compressionet limit6 en bordurepar des montagnesrrsultant de failles inverses ~ comportement
multiple,
et dont le socle serait apparamment
une "crofate ocranique modifire," d’~tge palrozoique suprrieur - triassique ou mrsozoique
suprrieur - tertiaire infrrieur, coincre le long d’une anciennegrosuture. I1 est possible que ce soit une relique d’un vieil ocran
(Palrozoique-Trias) ou autrement une mer marginale f0rmre derriere un ocran du Mrsozoique-Palrog~ne, et l’examen des
donnres grologiques et grophysiques fournit la trame pour l’explication des 6v~nements.Les chainons de montagnesAlborz
et Talesh en bordure prrsentent une allure grnrrale en forme d’arc et suivent la structure de cette prrsumre croOte
ocranique 6paisse et rigide du foss6 tectonique de la rrgion caspienne sud.
L’rtude tectono-srdimentaire de la Caspienne sud, secondre par une modrlisation basre sur l’interprrtation des ondes
sismiques d’un tremblementde terre rrcent le long des failles en bordure, semble suggrrer un aplatissement de la faille en
fonction de la profondeur (faille courbe) et que les hypocentres drterminrs ~ l’aide des tremblements de terre rrgionaux
en se servant des donnres des temps d’arrivre trlrsismiques sont localisrs avec imprrcision. La diffrrence en 616vation
entre le foss6 tectonique et la bordure des zones de failles imbriqures est causre par une diffrrence dans la structure de la
crofite et par les failles inverses rrsultant de la phase culminantedu rrgime de compressiontectonique.
La prrsente 6tude supporte l’idre que les failles continentales anciennes de grande profondeur et ~ comportement
multiple, lesquelles ont influenc6 les fossrs tectoniques et les facies srdimentaires durant diffrrentes prriodes grologiques,
et qui sont responsables des formes physiographiques actuelles, sont le lieu de l’activit6 sismique dans les chainons
orogrniques.
[Traduit par le journal]
Can. J. Earth Sci. 20, 163-183 (1983)
1CambridgeEarth Sciences Contribution No. ES 161a.
2Present address: Geological Survey of Iran, P.O. Box1964, Tehran, Iran.
0008-4077/83/020163-21 $01.00/0
©1983National Research Council of Canada/Conseil national de recherches du Canada
164
CAN. J. EARTHSCI. VOL. 20, 1983
Introduction
The Caspian Sea forms a deep midland basin, with
water depth up to 900 m, in the northern part of the
Alpine-Himalayanorogenic belt, and is surrounded in
the south by arcuate active fold-thrust belts (the Alborz
Mountains). The downwardmotion of the basin seems
to be linked with the uplift of the surroundingregion,
and appears to have started early in the Cainozoic Era,
with intense phases of activity in Neogeneand Pleistocene times. Thepresent depression can be divided into a
northern, a central, and a southern region, separated by
major basementfaults (Kornev etal. 1962; Fedynskyet
al. 1972; Glazunovaet al. 1977). The northern part is
partly floored by suboceanic basementdeveloped in the
Precambrian Russian platform, with north-south fold
trends that appear to havehad a controlling influence on
its development. The central part is floored by a
northwest-southeast trending Hercynian basement of
continental character, superimposedon the north-south
Precambrian trends. The southern part, which is the
most enigmatic, is floored by a basaltic basement,
15-20 kmthick, surroundedby granitic crust. It is this
part of the basin that is a geologicallyunusualfeature: an
oceanic basin that is situated betweenactive fold-thrust
mountains of the Alpine-Himalayan belt and has
escaped obduction during collisional orogenies.
The main purpose of this paper is to develop a
systematic approach to a better understanding of the
evolution of the south Caspian depression. Manyexplanations have been proposed for this region and are
discussed, but someeither conflict with the observations
or invoke arbitrary concepts. There are two particular
problems in understanding the south Caspian depression: firstly, howan intracontinental deep basin can
occur and exist within compressional regime, and
secondly, how an old ocean-like crust can remain
unsubducted in such an environment for such a long
period. To explain the first problemthe writer proposesa
"compressional depression" superimposed on a marginal basin, identifying some of the active faults
associated with it that have been mappedfor the first
time. For the second problem, the writer hypothesizes
that the floor apparently consists of a "modifiedoceanic
crust" rather than the commonlyassumedocean crust.
This hypothesismayexplain the thickness of the basaltic
layer and also suggests that it is sufficiently low in
density to have escaped subduction. The writer then
discusses the ancient and active tectonics of the region,
and suggests mechanismsresponsible for the developmentof the surrounding arcuate active mountainbelts.
basaltic basement15-20 kmthick, with P-wavevelocities of 6.6-7.0krn/s (Galperin et al. 1962; Malovitski
1968; Neprochnov 1968; Rezanov and Chamo1969;
Malovitski et al. 1970; Fedynsky et al. 1972). The
overlying sediments are 15-25 kmthick, with a refracting interface within them at a depth of 8-12km. The
sediments belowthis interface are thought to be Mesozoic and Paleogene in age, and those above, Neogene
and Quaternary in age (Neprochnov1968). Apparently
intensive sedimentation in the south Caspiandepression
took place in the Pliocene-Quatemaryperiod, with the
thickness of the Pliocene sediments exceeding 5-8 km
and that of the Quaternary sediments amounting to
1.5-2.0km (Fedynsky et al. 1972). The PlioceneQuarternaryincrease in the subsidencerate of the south
Caspian depression corresponds to the uplift, folding,
and thrusting of the bordering fold-thrust mountain
belts. The depression is characterized by near-normal
heat flow of about 0.8 x 10-6 to 1.4 x 10-6 Cal cm-2 s-1
(Lyubimovaet al. 1974). Theoretically calculated heat
flow showsan increase from 1.3 in the upper mantle to
1.75 at a dePthof 15 kmin the south Caspiandepression
(Ashirov et al. 1976), and a decrease to near normal at
top sedimentary cover presumably because of the rapid
sedimentation. The total crustal thickness in the south
Caspian depression ranges from 40 kmnear the periphery of the depression to 30km near its centre
(Yegorkin and Matushkin 1969). The depth to the
asthenosphereis around80 kmin the depression, whereas towards the Caucasus it increases to 120-150km
(Fedynskyet al. 1972). The depression is markedby
positive Bouguergravity anomaly, which changes to a
negative isostatic anomalytowards the southemcoastal
plain and then switches back to positive in the Alborz
Mountains(Fedynskyet al. 1972).
The age of the basaltic basementof the depression is
uncertain. Accordingto some(Amurskiet al. 1968)it is
pre-Liassic (older than 195 Ma); according to others
(Shikalibeily and Grigoriants 1980) it is Jurassic and
overlain by Cretaceous volcanic rocks. Adamia(1975)
and Adamiaet al. (1977, 1980)believe that the basaltic
basement is Albian-Eocenein age. Despite the uncertainty in the age, most authors agree that the basaltic
basement extends eastwards to the southweatern foredeep of the Great Balkhan and to the eastern coastal
region of the southern Caspian Sea (Amurski et al.
1968;Fig. 2).
The deepest well drilled by the National Iranian Oil
Company
in the southeastern (Gorgan) coastal plain
the Caspian Sea was abandoned in Lower Jurassic
sediments (the coal-beating ShemshakFormation) at
2. Structure of the south Caspian depression
depth of about 6 km(Stocklin and Nabavi1973; Stocklin
Deepseismic sounding data suggest that the southern 1974b; A. Afshar-Harb and I. Yassini, personal compart of the Caspian Sea (the south Caspian depression; munication, 1980). It demonstrates the absence of the
Fig. 1) lacks a granitic layer and that the relatively thick Mesozoicand (or) Cainozoic layer in the southeastern
undeformed sedimentary cover rests directly on a coastal plain of the south CaspianSea. Paleoreconstruc-
BERBERIAN
165
192a,o2
19
FI~. 1. Documented
faults (heavylines) and epicentral regions(stippled areas) of the majorknown
earthquakesin the south
Caspianregion and its borderingactive fold-thrust belts. Theoceanic part of the south Caspiandepression, coveredby
Mesozoic-Cainozoic
sediments(Komev
et al. 1962)is hatched(Shikalibeilyand Grigoriants1980).Contoursof the baseof
sedimentary
coverin the southCaspiandepression(Malovitskiet al. 1970)are shownin kilometres.Basedmainlyon the present
study and the seismotectonic
mapof Iran (M.Berberian1976a,b, 1981)after modificationand correction.Sources--historical
(pre-1900)earthquakes:Ambraseys
(1974); Berberian(1976b,1977), Melville(1978;only for the 874and 1436events);
1895.07.09earthquakealong the southernmarginof the Turanplate: Ivanovski(1899), Gorshkov(1947), Savarenskyet al.
(1953), Petrushevsky
et al. (1954), Masarskii(1961), Lindenand Savarenskii(1961); the 1946.11.04earthquakein the
region along the KopehDaghfault: Rezanov(1955); epicentral regions of 1945.05.11, 1957.07.02, and 1971.08.09
earthquakes: Tchalenko(1974); epicentral region and Ipak earthquake fault of 1962.09.01: Ambraseys(1963, after
modification); epicentral region of 1970.07.03earthquakein the KopehDaghfold-thrust belt: Ambraseys
et al. (1971);
epicentral regionsof 1913.04.16and 1924.02.19earthquakesin the westernpart of the south Caspiandepression:Malinovski
(1939,1940);epicentreof the 1953.02.12
Torudearthquake:Abdalian(1953).Fault-planesolution(equal-areaprojectionof
lowerfocal hemisphere)
of the 1978.11.04earthquakeis fromthis study; others are fromShirokova
(1962),PetrescuandPurcaru
(1964), McKenzie
(1972), Jacksonand Fitch (1979). Compressional
quadrantsare shownin black and dilatational in
Heavybarbedlines with triangles = thrust; with short lines at right angle = high-anglereversefault. TransverseMercator
projection.
tion of the Iranian region based on the available
1968; Fedynsky et al. 1972), only 800m of the
Neogene-Quaternarysediments was deposited along the
paleogeographic data (Berberian and King 1981) indicates that if the basaltic layer is oceanicit couldbe a relic northern flank of the rising AlborzMountains,bordering
of either the Hercynian(closed in Triassic) and (or)
the southern part of the Caspiandepression(Sussli 1976;
Fig. 3).
Mesozoic (closed in Cretaceous) ocean, or else
marginal sea developedbehind an island arc. The region
3. Previous interpretations
has been mostly under compression at least since
There
are
several hypotheses on the origin and
Pliocene-Pleistocene times, and no long-term extensional regimeis knownto haveoccurredin the area since developmentof the south Caspian depression (Milanovsky 1963; Kosminskaya and Cheinmann1965; Rezanov
then (see Section 4).
It is important to emphasizethat while 10kin of the and Chamo1969). ManyRussian earth scientists beNeogene-Quaternary sediments (20-0 Ma) was accu- lieve that the oceanic-like seismic character of the
mulating in the south Caspian depression (Neprochnov depression basementresulted from basification of for-
166
CAN.J. EARTH
SCI. VOL.
20, 1983
lOOkm
~
I
CENTRAL
IRAN I
FI6.2. Physiographicfeatures of the south Caspianregion. Topographic
contourlines are in metres. Post-Neogene-early
Quaternary
fold axesin the Caspiandepression(Solovyev
et al. 1959;Glazunova
et al. 1977;ShikalibeilyandGrigoriants1980),
andin Iran (M.Berberian1976a,1981;Huber1978)are shownby dashedlines. SouthCaspianbasaltic layer (Shikalibeily
Grigoriants1980)is hatched.Majorfaults are shownbythick lines. ThePaleozoicshearedophiolites(blackpatch)andbasic
ultrabasic plutons (cross) in the Taleshmountains,and the southeasterncontinuationof the Sevan-Akera-Qaradagh
Cretaceous
ophiolite m61ange
belt (heavyhatches)are shownin the westernpart of the CaspianSea.
I
mer continental crust initiated by downwarpingduring
the Tertiary Period. It has also beensuggested that the
basementis a remnantof the ancient Tethys Sea (Kornev
et al. 1962; Deweyet al. 1973). Rezanovand Chamo
(1969) and Shikalibeily and Grigoriants (1980) regarded
the south Caspiandepression as an eroded rigid massof
continental character and a zone of multiple rifting.
They explained the absence of a granitic layer in the
depression as being due to the lack of Paleozoic and
Mesozoicrocks and the 6.6-7.0 km/s layer was interpreted as a pre-Paleozoic metamorphic basement of
continental compositionbut with a velocity similar to
that of basalt. Accordingto Adamia(1975) and Adamia
et al. (1977, 1980) the oceanic-like crust of the south
Caspian results from Late Cretaceous - early Paleogene
rifting processes, with Middle Eocene basalts in the
lowercrust alone, and is not the relic of an older ocean.
Apol’skiy (1975) believed that the south Caspian depression initiated during Late Jurassic - Early Cretaceous times at the southeasternend of a left-lateral fault,
whichalso created the BlackSea at its northwesternend.
Clark et al. (1975) have suggested that the westernpart
of the south Caspian depression might be bounded by
normalfaults buried beneaththe sedimentsof the coastal
plain, and that the depression is downthrown
along these
normal faults. Geological cross sections across the
southern (Huber 1978) and the western coastal plain
of the south Caspian depression (Davies 1975) show
that the basin is downthrown along normal faults
dipping north and east, respectively, but neither of these
references shows the faults on the geological maps.
Most of these interpretations
conflict with the
geological-seismological data and with the structural
evolution of the region, and their shortcomings are
discussed in the following sections.
4. Deformationhistory of the region
The western and the southern margins of the south
Caspian depression are delimited by the Talesh and the
Khazar (Caspian) mountain-bordering active reverse
faults (Fig. 1). The Talesh fault, about 400 kmlong,
truncates the eastern flank of the Talesh and the Little
BERBERIAN
h
Vp
Alborz
MI. Mo
S.C.D.
,
,
U. Pe
~ 650
F~. 3. Comp~ison
of the simplified column~sections of
¯ e sou~Caspiandepression(S.C.D.) ~d ~e Alborzbordering fold-t~st mountNnbelt. Mainlycompiled~omNeprochnov(1968), Stocklin (1974a), Sussli (1976), ~d
(1978).
Caucasus fold-thrust belts and has juxtaposed the
Paleozoic-Mesozoic and Quartemary sediments of the
Caspian coastal plain. Uplift of the northern flank of
Alborz Mountainalong the 600 kmlength of the Khazar
reverse fault in the south (Figs. 1, 2) has brought the
Precambrian Gorgan schists (Gansser 1951; Huber
1957; Jenny 1977a,b; Salehi-Rad 1979; Delaloye etal.
1981) in contact with the Quaternary sediments of the
southern coastal plain of the CaspianSea, indicating the
large magnitudeof the vertical uplift. Thesharp escarpmentand abrupt changein elevation betweenthe level of
the Caspian Sea (26 mbelow meansea level (msl))
the eastern flank of Talesh Mountainin the west (about
1800m above msl), and the Caspian Sea and the
northern flank of Alborz Mountain(about 2000m) in the
south, are striking topographic breaks that might have
been caused by differences in crustal structure and
reverse movements
along the faults shownin Figs. 1 and
2, which have been postulated as normal faults in some
geological cross sections. In both cases the faults were
not mappedearlier.
167
Since the age of the oceanic basement of the south
Caspian depression is uncertain at present, analyses of
geological, structural, and geophysical data mayenable
a scheme to be suggested. The present Talesh and
Alborz fold-thrust mountainbelts (bordering the western and the southern parts of the south Caspian
depression; Fig. 1) were the site of two major faultcontrolled subsiding sedimentary basins (with Gondwanian biostratigraphy) during the Paleozoic and the
early Mesozoicregional extensional phases, whenthe
whole Iranian basement was attenuated and stretched
(Berberian 1979a; Berberian and King 1981). Apparently during late Paleozoic - Triassic time the
Gondwanianfragments (including Alborz and Talesh)
split from Gondwanaland,crossed the Hercynian Ocean
(Paleo-Tethys), and collided with the Asian block
what is nownortheast Iran (Berberian and King 1981).
The Triassic suture zone is indicated by the Herat
(Tapponnier et al. 1981), southern Kopeh Dagh
(Stocklin 1974b, 1977; Majidi 1978, 1981), probably
northern Alborzand eastern Talesh (Clark et al. 1975),
and the southern Pontides line (Sengor et al. 1980;
Sengor and Yilmaz 1981; Tekeli 1981; see Figs. 4,
10d).
Following the Middle Triassic (220 Ma) collisional
orogenies, compressionalmovements,and uplift of the
whole region, the onset of the Mesozoicextensional
phase was markedby Rhaetic rift volcanism (tholeiitic
basaltic lava flows), estuarine, deltaic, and littoral
conditions, and deposition of the coal-beating Shemshak Formationof Rhaetic-Liassic age (with Eurasiatic
fauna and flora) in the Talesh (west), the Alborz(south),
and the KopehDagh(east) regions. TheseEarly Jurassic
extensional phases and subsidence, which were also
recorded in the Caucasus, resulted in the spreading of
the oceanic basin along the Sevan-Akerasuture zone
(Adamiaet al. 1977; Figs. 4, 10jO. Conglomerateswith
large detrital fragments in the ShemshakFormation of
the Alborz and Talesh Mountains (Assereto 1966a),
together with Liassic transgression of the formationover
the PrecambrianGorganschists (in the northern Alborz)
and over the pre-Triassic metamorphicrocks and the
Permo-Carboniferoussediments (in the Talesh), indicate that the northern Alborz(south Caspianregion) was
emergedduring the Triassic, and was subject to intense
denudation. This observation is also documentedin the
Russian side of the south Caspian region (Vereshchagin
and Ronov1968). Jurassic andesitic porphyrite dykes
cutting the ShemshakFormation, together with andesitic tufts, have been found in the upper levels of the
ShemshakFormation in the Talesh Mountains (Clark
et al. 1975). Sedimentaryconditions changedto a transgressive marine environmentwith deposition of carbonates during the Late Jurassic and Cretaceousin the Talesh
and Alborz basins.
168
CAN. J.
EARTHSCI. VOL. 20, 1983
LATE CRETACEOUS
0
200
~
FiG. 4. Present distribution of the Mesozoiccalc-alkaline Andeantype magmaticarc (stippled) along the southern active
continental marginof Central Iran (the Sanandaj-Sirjan belt (SS), north of the Zagrosgeo-suture), and the active continental
margin of Caucasus, northwest Iran (north of the Sevan-Akera-Qaradagh
geosuture). Note that the south Caspian depression
region (markedby dotted line) is situated in the back-arc region of the Sevan-Akera-Qaradagh
subduction system (marked
dashed line), and is boundedin the north and" the south by the Hercynianand Triassic geo-sutures. TheMesozoicsubduction
zones (Zagros, Sevan-Akera-Qamdagh,
and Sistan geo-sutures) are markedby thick lines with triangles showingthe direction
subduction. The Joghatai-Doruneh(in the northeast) and the Nain-Baft(in centre) Red Sea type spreading regions (with oceanic
crust) are markedby a line with diverging open arrows. Absenceof a wide arc-trench gap north of the Zagros geo-suture may
indicate a steep Mariana type subduction system during the MesozoicEra. Lambertconformal conic projection. Inset map:
Modifiedreconstruction (Mercator projection) of the region during the Late Cretaceous Epoch, showingthe active subduction
zones and the plate tectonic frameworkof the area. Oceanic crusts are stippled (H.Z.A. Ocean= High-ZagrosAlpine Ocean).
Magmatic
arc is represented by crosses, subductionsystemsare shownby thick lines with triangles, and spreadingridges by a line
with small bar perpendicularto it. Thedashedline in the south Caspiandepressionrepresents the area of the Mesozoicfailed-rift
system in the back-arc region of the Sevan-Akera-Qaradagh
geo-suture zone in northwest Iran. Dotted lines are the present
continental shorelines. See also the schematic cross sections in Fig. 10 d-g, which complementthis figure and showthe
evolution of the area at various stages during the MesozoicEra. Basedon Adamiaet al. (1977, 1980), Berberian and Berberian
(1981), M.Berberian etal. (1981), F. Berberian et al. (1982), Berberian and King(1981), Tapponnieretal. (1981), Tirrul etal.
(1982), and other referen6es cited in the text.
BERBERIAN
Augite-olivine diabasic flows of Late Jurassic Early Cretaceous age are developed in central Alborz
(east of Damavandvolcano and in the Chalus area;
Gypsum-Melaphyre Formation and member 1 of the
Chalus Formation; Allenbach 1966; Steiger 1966; Cartier 1971; Sussli 1976) and were followed by the
deposition of Lower Cretaceous limestone. This sequencemaybe the lateral equivalent of the alkali basalttrachyte lavas with salt- and gypsum-bearing
terrestrial
deposits of the northern Little Caucasus(Adamiaet al.
1977). An approximately 40 m thick flow of olivine
basalt is recorded in the lower Karsang Memberof the
LowerCretaceous Tizkuh Formation in central Alborz
(Assereto 1966b).
In Azarbaijan (northwest and west Talesh Mountains,
south of the Russian border) thick Jurassic-Cretaceous
alkali basalt (hawaiite, mugearite), rhyolite, andesite,
and tuff (Vach6 1968; Babakhani et al. 1977; Riou
1979), and Senonianalkaline subsaturated (Blairmorite)
lavas (Didon and Germain 1976) were laid down.
Discovery of the Late Cretaceous pelagic limestones,
pillow basalts, and ophiolites along the "Qaradagh
geo-suture" in northwest Iran (Berberian et al. 1981)
illustrated the southeastern continuationof the Mesozoic
Sevan-AkeraOcean in Iran (Fig. 4). The position and
geometryof the Qaradaghgeo-suture (Berberian et al.
1981) with respect to the south Caspian oceanic basement may throw new light on the development of the
depression.
Aptian-Cenomanian (member 3 of the Chalus
Formation), Cenomanian-Turonian (number 4),
Turonian (member5 of the Chalus Formation) amygdaloidal pyroxene-olivine porphyrite to diabase and tuff
(Sussli 1976) and Aptian-Albian and Senonian tholeiitic basaltic andesites are found in the western and
central Alborz Mountains(Annells et al. 1975). Late
Cretaceous (Senonian-Maastrichtian) andesitic (with
somebasaltic) lavas and tuffs, and estuarine sediments
cut by small dykes of porphyrites (similar composition
to the andesites) are dominantin the Talesh Mountains,
west of the Caspian(Clark et al. 1975). Thesevolcanodetrital facies seem to be the southeastern continuation of the UpperCretaceous shallow-marineterrigenous carbonates and volcanics of the Transcaucasian
island arc, whichis developedonly in the area north of
the Sevan-Akera suture zone (Adamia et al. 1977,
1980; Fig. 4). In eastern Alborz, Upper Cretaceous
basalt, spilitic lavas, and andesitic basalts are recorded
in a few places (GSI 1975; A. Saidi, personal communication, 1981; see Fig. 4). Triassic, Jurassic, and Cretaceous intrusive bodies are also exposedin the Talesh and
western Alborz .Mountains (Fig. 4, Berberian and
Berberian 1981).
A Mesozoicactive continental margin with Andean-
169
type magmatic arc may be deduced to have been in
existence along the Transcaucasus-Talesh - Western
and Central Alborz belt. It should be noted that the
Mesozoicvolcanic activity diminished towards central
and eastern Alborz(Fig. 4). Volumetrically significant
calc-alkaline volcanic rocks are restricted to the Azarbaijan, Talesh, and west-central Alborz Mountains.
This may indicate that the Sevan-Akera-Qaradagh
rifting and the subsequent northwardsubducting system
terminated towards eastern Alborz. No evidence of
completerifting and subduction-related volcanismis yet
found in eastern Alborz. This mayimply that the pole of
rotation for Sevan-Akera-Qaradagh oceanic rifting
was close to the eastern end of the suture zone, i.e.,
east-central Alborz, for which no data are available.
Alternatively the suture zone was possibly displaced by
a transformfault.
The Jurassic and Cretaceous facies variations have
produceda clear regional zonation parallel to the major
structural trends of the Talesh (Clark et al. 1975) and
Alborz regions (Sussli 1976), with a considerable
increase in thickness of the sediments. The "marked
sedimentary zonations" and "differences in facies and
thickness" of the sediments (deposited in a short
geological time) are clear consequencesof the considerable downwarpingof the sedimentary basin and the
controlling effect of the major normalfaults along which
the Talesh and the Alborz sedimentary basins were
formed. Similar conditions have been documentedin the
Shotori fold-thrust belt of Central Iran (Berberian
1979a, 1982).
Apparentlynear Late Cretaceous time, the northwestern Central Iranian and Caucasian blocks collided and
ophiolites were obducted along the Sevan-AkeraQaradaghgeo-suture zone in the Little Caucasus and
northwestern Iran (Adamiaet al. 1977, 1980; Knipper
1980; Berberian et al. 1981). The resulting plate collision, whichaffected the region during Late Cretaceous
- Paleocene time, produced substantial compressional
deformation, as well as perhaps somehorizontal movementsoblique to the direction of compression,for which
no data are yet available. Becauseof these convergent
movements,the motion of the Talesh and Khazarfaults
that border the eastern and the northern parts of the
Mesozoic, fault-controlled,
subsiding sedimentary
basin (of the Talesh and the Alborz) possibly changed
from normal (in the Mesozoic) to reverse (in
Cainozoic). Gradualrising of the early Talesh and early
Alborz Mountains along these faults inherited from
older geological times is evident by "lack of the
Paleogene and the Neogenesedimentation" over most of
the Talesh and Alborzhighs (Berberian and King 1981).
No Paleogene volcanic activity is recorded in the
Talesh or Alborz Mountains(Fig. 5). Strongly alkali
170
CAN.J. EARTH
SCI. VOL.20, 1983
’.,
LATE OLIGOCENE
COMPLEX
MAKRAN
~
f
MAKRAN TRENCH
FIG. 5. Present distribution of the Central Iranian Paleogenecalc-alkaline magmaticarc (stippled) and the synchronousalkali
rift volcanism(randomdashed-line pattern). The complexdistribution of the Paleogenecalc-alkaline volcanism(stippled)
indicate that a simple subduction system was not the only mechanismresponsible during this period. The wide zone of the
Paleogenearc-trench gap north of the Zagros geo-suture mayindicate a low-angle subduction system for this period (see Fig.
10h). The Paleogenecalc-alkaline magmaticarc gradually gives wayto the synchronousalkali-rift volcanismin the north. The
line with diverging open arrowsand patches of alkali-rift volcanics indicates approximateareas of the Paleogeneactive back-arc
extension in northwestIran (Azarbaijan) and south of AlborzMountain.Theline with filled arrows indicates the formation of the
south Caspian back-arc (marginal) basin and development of the oceanic crust. The Zagros geo-suture (subduction zone
terminated in early Neogenetime) and the presently active Makrantrench are markedby thick lines with triangles showingthe dip
of the subduction zone. The Late-Cretaceous-emplaced ophiolite m61angecomplex is marked by black patches. East-west
parallel lines represent the Late Cretaceous - Tertiary accretionary complex along the continental margins. J. D. =
Joghatai-Doruneh ophiolite m61angecomplex in the northeast. Lambert conformal conic projection. Inset map: Modified
reconstruction (Mercator projection) of the region during Late Oligocenetime, showingthe formation of the south Caspianand
the Black Sea marginal basins developedin the back-arc region of the Zagros subductionsystem. Crosses indicate the Paleogene
Andeantype magmaticarc, and the black patches are the Late-Cretaceous-emplacedophiolites. Thick line with triangles is the
subduction zone, and the lines with perpendicular bars indicate back-arc spreading systems. Thepresent continental shorelines
are markedfor reference by dotted line. (See Fig. 10h, which complementsthis figure.) References as with Fig. 4 and those
mentionedin the text.
BERBERIAN
171
silica-undersaturated lavas (analcimites, phonolites) (Vindobonian-Sarmatian; 10-5 Ma) are at an elevation
with silica-saturated or only slightly undersaturated of about 2000 m along the northern flank of Alborz
lavas (ankaramites, alkali basalts, porphyritic latites,
Mountain(Stocklin 1974a; Sussli 1976; Huber 1978).
latites, alkali trachytes, andesites) of Paleocene-Eocene Similarly the Caspian-facies subaquatic continental
age are dominantin the Azarbaijan area west of Talesh series (Pliocene; 5-1.8 Ma)lies at about 1000 mabove
Mountain (Babakhani et al. 1977; GMSI1978; Riou sea level along the northern flank of the Alborz
1979; Babakhani 1981; Fig. 5). About 4kmof Eocene Mountains, south of the Khazar fault. The same beds
alkali basalt and basanite with Late Eocenealkali basic were discoveredby deep drilling in the southern Caspian
and ultrabasic dykes and small intrusive bodies was coastal plain (north of the Khazar fault) below some
reported from the northern part of Talesh in Russia 1600-2000m of Pliocene and Quaternary sediments
(Azizbekov et al. 1978). Eocene-Oligocene analcime (Faridi 1964; Mostofi and Paran 1964; Paran and
basanite lavas were also developed in the southern Crichton 1966), indicating a rapidly subsiding basin.
Alborz Mountains(Stalder 1971; Annells et al. 1975). The total vertical movementacross the Khazar fault
The extensive "Paleogene alkaline volcanism" devel- since late Neogene time (2Ma), as indicated
oped in northern Iran (northern Talesh, Azarbaijan, comparing the amount of throw in the Caspian-facies
southern Alborz)indicates a "deep faulting and rifting" sedimentsdisplaced by the fault, is therefore estimated
phase during an overall compressionalregime in Central to be at least 3000m. Total displacements of 2 km(for
Iran (Fig. 5). Paleogene alkaline volcanism generally the Quaternary) and 600 m(for the past 300 000 years)
lessened toward the south (i.e., Central Iran) where are reported from the southern coastal plain of the
more "synchronous calc-alkaline volcanics" (mainly Caspian Sea (Paluska and Degens 1979).
andesitic) predominate (Tarakian 1972; Dimitrijevic
Similar to the formationof the Tabasand other Iranian
1973; Amidi 1975; Caillat et al. 1978; Fig. 5). The majordepressionsin responseto peripheral uplifts of the
Central Iranian Paleogene volcanic activity was fol- fold-thrust mountainbelts (Berberian 1979a, 1982), the
lowed by development of the northwest-southeast
south Caspian depression underwentgradual subsidence
Andean-type intrusive arc of Oligocene-Miocene age along the mountain-bordering reverse faults (Khazar
(F. Berberian 1981; Berberian and Berberian 1981; and Talesh in the south and the west) during the
Berberian et al. 1982). The occurrence of synchronous Pliocene-Pleistocene compressional orogenies. It subcalc-alkaline volcanics (a subduction product) in the sided as the Alborzand the Talesh fold-thrust mountain
south and the predominantnorthern alkaline volcanism belts rose. The subsidence started in Eocene time
mayindicate the formation of a "marginal basin" during (55 Ma) and continued to the present, with climaxes
early Tertiary time along the Talesh - south Caspian in the Pliocene-Pleistocene. The Pliocene-Pleistocene
elevation of the bordering fold-thrust mountainbelts is
region (Fig. 5).
During Middle and Late Miocene times marine therefore a manifestation of crustal shortening and
Caspian-facies sediments covered only the northern thickening, and is not a basin-and-range type of su’uclimb of the rising AlborzMountains(deposition of about ture. The fluctuations and elevation changes of the
520mof Vindobonian-Sarmatian sediments; Sussli
Caspian terraces and shorelines (Federov 1961; Leon1976). These sediments were unconformably covered tyev 1961, 1964; Kvasov1968; Ehlers 1971; Annells et
by the Pliocene continental series (5-1.8 Ma)along the al. 1975; Paiuska and Degens 1979) are proof of
northern foothills of the Alborz fold-thrust belt. The continuing Vertical movements
of the region throughout
Pliocene-Pleistocene compressional movementswere the QuaternaryPeriod. Despite an increasing subsidence
involved in folding, reverse faulting, and further
rate in Neogene-Quaternary
times, no volcanic activity
elevation of the Talesh and AlborzMountainsrelative to developed in the south Caspian depression during that
the south Caspian depression. The Alborz fold-thrust
period. The Pliocene-Quaternary tectonics (uplifting
mountainbelt was elevated by northwardthrusting in the and folding of the bordering Alborz and Talesh Mounnorth and southwardthrusting in the south of the range. tains) and the present seismic activity (Fig. 1) indicate
Similarly the Talesh fold-thrust mountainbelt rose along that the region is still under compressionalregime.
eastward and westwarddipping reverse faults bordering
the mountain belt. Surface geology (Stocklin 1974a; 5. Active tectonics, the Siahbil earthquake, and the
Clark et al. 1975; Huber1978)indicates that folding and nature of the faults bordering the south Caspian
depression
thrusting of the Alborzand the Talesh fold-thrust belts
represent a minimumhorizontal shortening of about 25
Nowroozi (1972), McKenzie (1972), Deweyet al.
and 20%during the Pliocene-Pleistocene compres- (1973,) and Sborshchikovet al. (1981) postulated the
existence of a south Caspian plate in their modelof
sional movements.
At present the folded Caspian marine sediments active tectonics of the area. Thereis no surface structure
172
CAN. J. EARTHSCI. VOL. 20, 1983
O0 O0 ¯
¯
¯
I,:.5.".
¯
-
¯
o~
¯ O0
¯
-.
50*
¯
¯
Oo Oq-o -o q~O
. .
~
O0
..
~0
¯
¯
¯
¯
O0
¯
~o
FIr. 6. Twentiethcentury instrumental epicentre (ISS 1918-1964;ISC 1964-1981)mapof the south Caspianregion
indicatinghigh, shallowseismicactivity in the borderingactivefold-thrustbelts andlowerlevel of activity at the southCaspian
depression,especiallyin the region of the basaltic layer. Theseismicityfollowsthe pattern of the fold-thrust belts. Early
epicentral locations are poor, and epicentres of the 1970’shavea location error of about15 km(Berberian1979a,b). Mb
3.5-7.2; h = shallow.
that could correspond to the western and southern
boundariesof the introducedplate. Moreover,the recent
folding (Fig. 2), faulting (Fig. 1), and seismic activity
(Fig. 6) are not restricted to, or evenconcentrated on,
the boundariesof the proposedplate. Scattered historical earthquakes and the 20th century instrumental
earthquake locations of the south Caspian region lie
along the bordering fold-thrust belts of the Talesh (in
the west), the Alborz(in the south), and the KopehDagh
in the east (Figs. 1, 6; see also M. Berberian 1976a,
1981). The inner part of the depression has a low level
of seismic activity, and the region cannot be divided in
the manner proposed by Nowroozi (1976). Regional
geo!ogy, tectonics, and seismicity do not showeast or
north dipping faults in the westernand southern borders
of the Caspian Sea. The Talesh (Astara in Berberian
1976a) and the Khazar multi-role basement reverse
faults are major active features associated with several
damagingand destructive earthquakes during the last
1100 years (Fig. 1). Nofault-plane solutions have been
available for earthquakes along the active faults described. The recent earthquake of Siahbil (1978.11.04)
provided interesting results concerning the seismotectonic evolution of the area and the nature of the Talesh
fault bordering the western part of the south Caspian
depression. The earthquake of magnitude Ms= 6.0 (Mb
= 6.1) took place on November4, 1978 at 15:22:19
CUT(Coordinated Universal Time) without any recorded foreshocks. The instrumental epicentre was reported by the National Earthquake Information Service
of the United States Geological Survey (USGS)
37.674°N,48.90 I°E, with a focal depth of 34.0 km. The
earthquake destroyed and damagedabout 20 villages
along the Talesh fault (Fig. 1) and, unlike the previous
earthquakes (Berberian 1979b), fortunately caused
casualties (RLSO1978). The 1978 earthquake was not
the first recordedseismicactivity alongthe Talesh fault.
Thenorthern segmentof the fault was associated with an
earthquake of Mb= 5.2 on April 16, 1913 (Malinovski
1940;see Fig. 1).
Fault-plane solution of the 1978 earthquake using
long-period vertical component P-wave first motion
shows a low-angle (09°WSW)plane and a high-angle
(81 °ENE)plane striking 168° (Fig. 7). Regionalgeological information, Landsat imagerystudy, damagedistribution (RLSO1978) along the Talesh fault (Fig. 1),
the epicentral location of the earthquakeon the hangingwall block of the fault (NEIS1978)all mayindicate that
the plane dipping west-southwestis the fault plane (Fig.
7). Unfortunately no surface break was observed in the
field and no aftershock study wascarried out to constrain
this argument. To myknowledgeno east dipping fault
(the high-angleauxiliary plane) has been reported in the
epicentral area. The auxiliary plane is well constrained
BERBERIAN
173
1978.11.0~
51AHBIL(Tolesh)
C~syn
obs
ANP
1.2
¯
¯
~=168’
SHL
~_~
-I 20s I-
F~G.7. Synthetic (top) and observed(bottom) P-waveforms
at 15 WWSS
networkstations for the Siahbill 1978.11.04
earthquake
alongthe Taleshfault, borderingthe westernpart of the southCaspiandepression.Eachstation canbe identifiedby its
letter code.Thelong-periodP first motionpolarity is representedby the equal-areaprojectionof the lowerhalf of the focal
sphere.Solidcircles are compressional
first motion,openare dilatational. T = tensionaxis; P = pressureaxis. Thefoci are taken
to be in the crust witha P-wave
velocityof 6.8 km/s.Thehorizontalprojectionof the slip vectorsis shown
bylines witharrows
andnumbersoutside the focal sphere. Synthticwaveforms
matchedthe observedwell at a focal depthof about20 ± 4 km(see
Fig. 8). Thepredictedkink in the downward
synthetic waveforms
at WINandNAIis the P arrival, which is not present in the
observedseismogram.COP,PTO,and ESKseemto have somebackgroundnoise.
by the long-period first-motion observations, but the
fault plane is less constrained(Fig. 7).
In order to constrain the fault planes and to determine
the source depth of the earthquake, far field waveform
modelling was used (Jackson and Fitch 1981). For
shallowearthquakesthe shapeof the first 10-20s of the
long-period vertical components of the teleseismic
P-waveformsis largely determined by direct P and its
two surface reflections pPand sP. The polarities and
relative amplitudes of these phases are determined by
the fault-plane solution and their relative arrival times
are controlled by the source depth (Helmberger1974;
Langston and Helmberger 1975; Helmberger and Burdick 1979; Jackson and Fitch 1981).
The comer frequency of the spectrum of the source
function affects the waveformsin an identifiable way
and is adjusted to give a goodfit. Thusthe initially
determined fault-plane solution, the depth, and the
comer frequency were the only source parameters
altered in the modelling process. Figure 7 showsthe
waveformschosenfor the best-fitting fault parameters.
These have a fault of 168°, a depth of 20km, and a
comer frequency of 0.1 Hz. The effect of changing
comer frequency is shown in Fig. 8 and is firinly
controlled by the pulse width to 0.1 Hz. Figure 9 shows
the effect of depth change, and suggests an accuracy of
better than 4 kmin the source-depth estimate (20 km).
This is a slightly deeper event comparedwith similar
observationsin other parts of Iran, and like other cases
indicates that focal-depth estimates based on teleseismic
174
CAN. J.
cop
CHG
EARTHSCI. VOL. 20, 1983
STU
MAT
obs
d(kt=)
|0
20s
CHG
COP
STU
~
~
~
FIG. 8. Variation of synthetic waveforms
with changeof
comerfrequencyffc) at four selectedstations in Fig. 7. The
observed waveformsare shownat top and bottom of each
column,fc = 0.10 Hzseemsa reasonablefit withthe observed
waveform.
arrival-time data of the Iranian earthquakesare inaccurate (McKenzie 1972; Berberian and Papastamatiou
1978; Berberian 1979c; Berberian et al. 1979; Jackson
1980; Jackson and Fitch 1979, 1981). Similar variations
of modelswith changing fault orientations suggest that
the fault planes are well constrained. In conclusion a
focal depth of 20 -+ 4 kmwith a fault plane at source
dipping 09°WSW
was constrained by detailed examination of the long-period teleseismic waveformmodelling
of the Siahbil earthquakealong the Talesh active reverse
fault, bordering the southwestern margin of the south
Caspian depression. Despite the low-angle faulting at
depth, the steep escarpmentand linearity of the fault at
the surface reflect a mountain-bordering high-angle
reverse character for the Talesh fault. Comparisonof
different dips at the surface and at depth mayindicate
that the fault dip decreases considerably with depth.
6. Discussion of the models
The mechanism for generating the south Caspian
depression is poorly understood, and most of the
proposedideas fail to agree with the regional observations. This is mainlydue to the uncertainty in the age of
the ocean basementand absence of detailed geological
and petrochemical studies of the bordering mountain
belts in Iran. Theessential conditionsto be fulfilled are
that the compressionalregimewith reverse faulting and
folding have dominated since at least Late Miocene
time, and the pre-Jurassic (Amurski et al. 1968),
Jurassic (Shikalibeily and Grigoriants 1980), or the
Cretaceous - early Tertiary (Adamiaet al. 1977) thick
basaltic crust was not subducted.
It is difficult to assumea very thin (15 km), rigid
continental basementcrust (i.e., metamorphicbasement
at granulite or amphibolitegrade) for the south Caspian
depression with P-velocities of about 6.6-7.0km/s.
obs
~ 20s
I--
FIG. 9. P-wavesynthetic seismogramsat four selected
stations in Fig. 7 with varying depth (d). The observed
waveformsare shownat top and bottom of each column. A
source depthof about20 kmseemsa reasonablefit with the
observed waveform.
Metamorphic basements exist in northern Iran and
southern Russia, but none of themhave the characteristics of the south Caspian basement,and the latter does
not belong to either of them. The idea of diapiric
intrusion of dense asthenospheric material into lower
continental crust (oceanization of continental crust;
Muller 1978) acting as a weight for lowering the
basementis favoured by several Russian authors. The
increase in the lower crust density by the invasion of
subcrustal volumesof densebasic or ultrabasic intrusive
rocks (Beloussov 1960, 1962, 1972, 1977), or
metamorphismto the granulite or eclogite facies
(Haxby et al. 1976) could be a possible initiating
mechanism for the formation of the south Caspian
depression. Several areas for which such an idea was
proposed are now explained by other mechanisms.
Moreover, it is not clear why such a mechanismwas
initiated only in this special location. The dynamic
effect of the mantle processes (Tamrazian 1976)
unlikely to have been involved in the subsidence of the
south Caspiandepression, since it is difficult to understand whysuch a mantle activity should exist beneath
the south Caspian depression in particular rather than
elsewhere. An upward migration of the Mohorovi~i6
discontinuity (Moho) due to phase changes and sedimentary loading (Rezanov and Chamo1969) does not
appear plausible, because it is nowbelieved that the
Mohois not a phase change (McKenzie1978b).
The Baykal (Tapponnier and Molnar 1979), Hornelen
175
(Steel 1976), Maritime (Leeder 1976), and Dead-Sea
rhombgraben (Garfunkel 1981; Garfunkel et al. 1981)
type rifting associated with the large strike-slip movements seems not to be applicable to the south Caspian
region. Large-scale strike-slip motions such as those
now occurring in central Asia and Asia Minor could
have occurred during previous geological times, but are
not detectable by present paleogeographic and surface
geological data. The detachment and sinking of the
lower part of the lithosphere (the "cold blob" modelof
McKenzie1978b) are unlikely to be applicable because
the crust was not thick enoughduring early Tertiary time
when the main loaded subsiding phase started. The
present depth of the Mohounder the south Caspian
depression is about 40km. By subtracting 20kmof
sedimentary thickness, 20 kmof oceanic crust is obtained. Comparingthis amountwith the depth of the
Mohounder the Alborz Mountains(in the south) or the
Caucasusbelt (in the west), which is nowabout 45
(Peive and Yanshin 1979; Dehghani1981), requires
large amountof stretching. The timing for subsidence
caused by an extensional tectonic regimeand stretching
of the crust is therefore critical, and shouldbe consistent
with the geological history of the region.
All existing data confirm that the south Caspian
fault-controlled depression is underlain by an old basementof oceanic character: (i) lack of similarity with
either the northern Iranian or the southern Russian
geology;(ii) the special position of the depressionalong
an old (late Paleozoic - Triassic) geo-suture, with
obducted Hercynian-Triassic ophiolites (Talesh in the
west (Clark et al. 1975) and Mashhad-Herat
farther east
(Majidi 1981;Tapponnieret al. 1981)); (iii) the depression’s situation in the back-arc region of the Late
Cretaceous Sevan-Akera-Qaradagh ophiolite-radiolarite belt (Grigoryevet al. 1975; Adamiaet al. 1977,
1980; Knipper 1980; Berberian et al. 1981); and (iv)
having the Black Sea oceanic crust (Adamiaet al. 1974,
1977; Letouzey et al. 1977; Biju-Duval et al. 1978)
farther northwest in a similar situation. These four
factors give somecredibility to the assumptionthat the
south Caspian depression formedquite differently from
its adjacent blocks, but its formation was linked with
consumption of an oceanic crust. Absence of the
Lg-wavephase propagation across the deep parts of the
south Caspian depression (Kadinsky-Cadeet al. 1981)
supports the oceanic type character of the depression.
Becauseof its thickness (about 15-20km)it is difficult
to consider it as being a renmantof a simpleoceancrust.
Oceanic crusts are muchthinner (about 6 km), and the
old, cold oceanic crusts are denser and gravitationally
unstable with respect to the underlying hot mantle rock;
therefore, they usually subduct in convergent zones
(Parker and Oldenburg1973; Parson and Sclater 1977;
McKenzie 1977, 1978b; Molnar and Atwater 1978).
However,there are pieces of old oceanic crusts, like the
Mesozoic oceanic plate of the eastern Bering Sea
(Cooperet al. 1976a,b), that did not subduct into the
mantle.
The relatively thick and nonsubductedbasaltic crust
could be analogousto a"modified oceanic crust" such as
the oceanic plateaus (Carlson et al. 1980), Iceland type
crust (Jacobyet al. 1980),or to the island arcs that never
sank into the mantle. There are two sets of obducted
ophiolite sequencesof different ages in the westernpart
of the south Caspianoceanic crust:
(1) The older sequence, consisting of sheared
ophiolites-serpentinites, basic to ultrabasic plutonic
bodies (gabbro and peridotite), LowerCarboniferous
and Permian andesitic volcanics, and Paleozoic metamorphic rocks, has been found in the Talesh Mountains
(Davies et al. 1972; Clark et al. 1975; Davies1975; see
Fig. 2). Unfortunately the radiometric age of the
ophiolites of the Talesh Mountainsis not known,and the
evidence is not conclusive, but someof themare thought
to be of probable late Paleozoicage; detrital fragments
of the sheared serpentinites and metamorphicrocks are
present in the LowerJurassic conglomerate(Clark et al.
1975). The Talesh late Paleozoic - Triassic ophiolites
appear to mark the western continuation of the
Mashhad-Herat Triassic suture line (Majidi 1978;
Tapponnieret al. 1981; Figs. 2, 4). If the age of the
oceanic crust of the south Caspian depression is preJurassic (Amurski et al. 1968) and the Triassic geosuture delineates its southern boundary, then it is
plausible to consider the south Caspian depression
basement as a part of an intra-oceanic island arc
associated with the northwardsubduction of the Hercynian-Triassic ocean crust, which was possibly trapped
along the suture zone. In this case the crust of the island
arc, which is composed of volcanics and the arc
sediments (Dickinson 1970, 1974; Mitchel and Reading
1971), could have a lower density than that of the upper
mantle. This density difference, together with its thickness, presumablypreventedits completesinking into the
mantle. An obducted fossil island arc with similar
thickness was recently discovered in the Kuhistanregion
along the India-Eurasia suture zone of northern Pakistan (Tahirkheli et al. 1979; Tahirkheli and Jan 1979).
This discoverymayindicate that it is difficult to destroy
the ancient island arcs that have lowerdensities than the
mantle.
(2) The younger ophiolite sequence is the "SevanAkera-Qaradaghophiolite m61angebelt" of Late Cretaceous age (Figs. 2, 4, 5), which was emplaced
along the Little Caucasusmountainrange, west of the
south Caspian depression (Gdgoryev et al. 1975;
Yegorkina et al. 1976; Satian 1979; Knipper 1980;
Berberian et al. 1981). The south Caspiandepression is
situated in the "back-arc region" of the Sevan-Akera-
176
CAN.J. EARTHSCI. VOL; 20, 1983
~
GONDWANALAND
S
SQ~pre 650Ma
?
"
b) 650-Z’00 Me
d) 270-220 Ma
"’"’-.
~HrZAGROS
.:
~
~
" ..........
hz 80Ma.
RedSea
OCEAN
~
.....................
g) 85-60M~
NE
:
9
’
HERCYNIAN
~~
c)270Ma
LAURASIA
NE
i ~-Turan
9
OCEAN
PRECAMBRIAN
ss60~nb60Ma
"
/
""
~
~
~ .
~ sTqlOOMa
~
"
:
¯ -..
jd
~ ".
f~ff
SOUTH
CASPIAN
NE
.. :
i)5Ma-0
FIG.10. Schematic
sequentialcross sections showingthe large-scalekinematicsand the simplifiedevolutionof the Iranian
crust duringseveral collisional orogenics. Thecross sections complement
Figs. 4, and 5 and the text. Northward
motionof
Central Iran, closure of the HercynianOceanin the northeast, and openingand closure of the High-Zagros
AlpineOceanin
southwest[ran duringvariouscollisional orogeniesprovidea plate tectonic framework
for the structure of Iran anddevelopment
of the southCaspianoceanicbasement.Widthsof the oceanbasins(black) are not representedat scale. 1 = metamorphism;
2
emplaced
ophiolites; 3 = alkali-rift volcanism:4 = calc-alkaline magmatic
arc; hz = High-Zagros
(the northernmarginof the
Zagrosbelt, southwestof the Zagrosgeo-suture);ss = Sanandaj-Sirjan
belt, the southernactive continentalmarginof Central
Iran, northeast of the Zagrosgeo-suture;al = AlborzMountains,northernmarginof Central Iran, south of Caspian;nb =
Nain-Baft.jd = Joghatai-Doruneh
ophiolite belts in Central[ran (also see Figs. 4, 5).
Qaradaghgeo-suture (Fig. 5). The southern continua- AlborzMountains(see Fig. 5 and Section 4). This linear
tion of this geo-suture was recently discovered in Iran belt was therefore a complexactive margin during the
(Berberian et al. 1981). This geo-suture and the wide- Mesozoic Era, and the complex volcanism (Section 4)
spread Mesozoicmagmaticarc along a linear zone can could have been connected with the formation of an
be traced from the Transcaucasianisland arc (Adamiaet island-arc-marginal-basin system.
al. 1977, 1980) southwardto the Talesh and parts of the
Based on the geological data (Section 4) the south
BERBERIAN
Caspian depression is situated in a special region that
has undergone at least two extensional phases during
the Mesozoic(Fig. 4) and Paleogene(Fig. 5) with widespread developmentof alkali basalts and basanites. The
Mesozoicextensional phase(s) was (were) related to
formation of the Sevan-Akera-Qaradaghoceanic crust
and its northern back-arc (marginal) basin development.
The south Caspian depression could be considered as a
failed-rift systemsuperimposedalong the Triassic collisional suture during the MesozoicEra, and its evolution was possibly terminated because of the Late
Cretaceouscollisional orogenies (closure of the SevanAkera-Qaradagh ocean).
Widespreadoccurrence of thick Paleogene alkaline
volcanic rocks in the area west of the south Caspian
region (northwest Iran, northern Talesh, and southern
Alborz) may indicate development of the Paleogene
extensional regime and "marginal basin" behind the
Central Iranian Andeantype calc-alkaline magmatic-arc
belt (Figs. 5, 10h). Developmentof the "predominant
Paleogenealkali volcanics" in nothern Iran and in the
Caspian region (far from the Zagros trench) and the
"synchronous Andean-typecalc-alkaline volcanics" in
Central Iran (close to the Zagros trench), with
"transitional volcanic province" between them, becomes an interesting problem. Recent petrochemical
and isotope study of the magmaticrocks in Central Iran
(F. Berberian1981; Berberian et al. 1982) revealed that
the Andeantype magmatism
related to the subduction of
the High-ZagrosAlpine Oceancrust underneath Central
Iran lasted until early Neogenetime, whenan Arabian
- Central Iranian continental collision took place. The
Paleogenecalc-alkaline, transitional, and alkaline volcanic provinces in the north can be related to this
subduction (Figs. 5, 10h). Apparentlythe south Caspian
Mesozoicfailed rift (superimposedon the Triassic collisional suture zone) evolved as a true marginal basin
during the Paleogeneback-arc rifting behind the Central
Iranian Andeantype calc-alkaline magmaticarc. Presumablythe south Caspian basementsubsided further as
a result of continued cooling and sedimentloading.
Since at least Pliocene-Pleistocene time the south
Caspian depression evolved as a depression boundedby
major reverse faults. Data presented in this study
indicate the establishment of the compressionalregime
and introduce the bordering active reverse faults with
their possible listric behaviourat depth. Theconvergent
movementsand lithospheric response to loading of the
depression (Watts and Ryan1976; Sweeney1976) with
thick sedimentary deposits over a cold oceanic crust
presumablyaccentuated the subsidence. At present the
oceanic type basementof the south Caspian depression
is limited by and downthrownalong the Khazar(in the
south; this paper) and Apsheron-Balkhan
(in the north;
Shikalibeily and Grigodants 1980) reverse faults (Fig.
177
1). The Khazarfault coincides with a line dividing the
northern zone of substantial negative isostatic anomalies from the southern (Alborz) zone of monotonous
positive anomalies (Fedynskyetal. 1972). The Apsheron-Balkhan fault is clearly revealed by a gravity
gradient and the pattern of the geomagnetic field,
introducing a deep structure separating the south Caspian oceanic crust from the platform regions of the
central Caspian (Fedynskyet al. 1972; Shikalibeily and
Grigoriants 1980).
The low level of seismic activity (Fig. 6) and the
relatively undeformedthick sediments abovethe basaltic layer (Fig. 2) in the south Caspiandepressionindicate
that, like the oceanic crusts, the old basementof the
depressionis "inherentlyrigid." In contrast to this strong
crust with minor deformation, the Talesh, Alborz, and
the KopehDaghperipheral active fold-thrust belts with
weak(continental) crust and inherited planes of weakness are folded, thrusted, and have higher seismic
activity levels (Figs. 2, 6). The old oceanic type
substratum of the depression presumably has not been
remobilized during later deformational phases and
possibly resisted fragmentation, althoughit was subject
to the same forces as its bordering active fold-thrust
belts. Minor folds observed in the upper sedimentary
cover of the south Caspian depression should be
involved in detachment tectonics with a d6collement
zone at the base. Apparentlythe general arcuate pattem
of the Alborzand Talesh bordering fold-thrust mountain
belts (Fig. 2) follows the shape of the trapped modified
ocean-crust basementof the south Caspian depression.
Just as in the experimentalstudies on strain fields around
rigid inclusions (Ghosh and Sengupta 1973; Ghosh
1975; Ghosh and Ramberg 1976), the Neogene-Quaternary fold patterns, with swerving of the fold axes
around the south Caspian depression and bowingof the
AlborzMountains(Fig. 2), mayoutline the strain field
around the relatively rigid ocean-crust type basement.
The superficial morphologyis somehowsimilar to the
Tarim basin of central Asia (Molnar and Tapponnier
1978), whererifting does not occur and the depressionis
surroundedby active folded and elevated belts.
7. Conclusions
Notwithstanding manyassumptions due to the limited
data available from Russian literature, an approachhas
been attempted to tackle some of the seismotectonic
problemsof the south Caspian region. A "compressional
depression" superimposed on a "marginal basin,"
boundedby "reverse (listric?) basementfaults" inherited
from older geological times and floored apparently by a
"modified ocean-crust basement" seems to be a plausible mechanismfor the formation and elevation of the
south Caspian depression during the convergent movementsof the Arabian and Eurasian plates. The stratig-
178
CAN. J. EARTHSCI. VOL. 20, 1983
raphyand structural history of the region showthat the
subsidencewas associated with uplifting of the bordering fold-thrust mountain belts. Basementfaults that
controlled the boundaries of the depression were formed
prior to or during the initial subsidence. Apparently,
throughout the Mesozoic and Cainozoic Eras, the
basementfaults were reactivated with different mechanisms in response to different tectonic regimes produced
by continental movements.
The south Caspian depression opened as a marginal
basin behind the Mesozoic Transcaucasian-Taleshwestern Alborz and the Paleogene Central Iranian
Andeanarcs. Eventscan be divided into several overlapping and synchronousstages (Figs. 4, 5, 10):
(a) Mesozoicrifting and formation of the SevanAkera-Qaradagh ocean between the European plate
(active continental margin of the northern Little Caucasus) and the Central Iranian plate (passive continental
marginof the southern Little Caucasus)(Fig. 10j’).
(b) Mesozoic Andean-arc stage, characterized
calc-alkaline volcanism north of the Sevan-AkeraQaradaghgeo-suture (Figs. 4, 10f).
(c) Late Cretaceous collisional orogenyin northwest
Iran and the formation of the Sevan-Akera-Qaradagh
collisional geo-suture (Figs. 4, 10g).
(d) Paleogene synchronous Andean-arc and back-arc
stages, characterized by calc-alkaline volcanism(north
of the Zagros geo-suture) and alkali basalt volcanism
(northwesternIran), respectively. (Apparentlythe intraplate block faulting and predominantlyalkali basaltic
volcanism(back-arc spreading stage) led to the formation of the south Caspianoceanic crust (Figs. 5, 10h).)
(e) Neogenecollisional orogenyin southwestIran and
formation of the Zagros collisional geo-suture, and
overall shortening and thickening of the Iranian continental crust (Fig. 10i).
(f) The south Caspian depression, with its modified
oceanic crust, remaining open and subsiding further
perhapsas a result of the continuedcoolingof its oceanic
substratum and overthrusting of the bordering foldthrust mountainbelts (Fig. 10i).
The assumedmodifiedocean-crust region of the south
Caspiandepression, whichis not seismically active, is
surrounded by arcuate active fold-thrust belts with a
high seismic activity level. This study has demonstrated
that the present physiographicand tectonic features are
strongly influenced by the older tectonic elements. The
axial direction of the arcuate Alborz and the Talesh
fold-thrust belts bordering the trapped modified ocean
crust was inherited at the onset of the regional compressional regime. These belts follow the shape of the
trapped oceanic basement (which behaved as a rigid
block) and apparentlystill control the orientation of the
contemporaryfolding. Theactive tectonics of the area is
dominatedby compressionalregime, and seismic activ-
ity is widespread along several inherited mountainbordering reverse faults. Regional Pliocene-Quaternary
geology,active faulting, and fault-plane solution of the
earthquakes show predominantly reverse faulting and,
therefore, crustal shortening and thickeningof the area,
as well as the whole Persian plateau. With the limited
data at hand, it is hopedthat the present reviewand the
newdata presented in this paper on the bordering active
reverse faults and the active tectonics of the region will
initiate somefurther detailed study of the region.
Acknowledgments
This work was supported by the Departmentof Earth
Sciences, CambridgeUniversity, and the Geological
(and Mineral) Survey of Iran. I wouldlike to thank
Berberian, G. C. P. King, K. Louden, D. P. McKenzie,
P. Molnar, and C. Soufleris for critically reading the
manuscript and for valuable discussions and comments.
I benefited from discussions with P. C. England, D.
Karig, R. H. Sibson, M. Sinha, S. Tanner and C.
Williams. I acknowledge with thanks the comments
received from X. LePichon. I also wish to thank an
anonymousreviewer for helpful commentsand suggestions. Gratitude is also expressed to the Departmentof
the Armenian Affairs of the Galuste Gulbenkian
Foundation (Lisbon), to British Petroleum, and to the
British IBMfor donating separate small grants during
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