Download eastern european alpine system and the carpathian

Tectonophysics, 63 (1980) 31-61
0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
31
EASTERN EUROPEAN ALPINE SYSTEM AND THE CARPATHIAN
OROCLINE AS AN EXAMPLE OF COLLISION TECTONICS
B.C. BURCHFIEL
Department of Earth and Planetary Sciences, Massachusetts Institute of Technology,
Cam bridge, Mass. 02139 (U.S.A.)
ABSTRACT
Burchfiel, B.C., 1960. Eastern European Alpine system and the Carpathian orocline as an
example of collision tectonics. In: M.R. Banks and D.H. Green (Editors), Orthodoxy
and Creativity at the Frontiers of Earth Sciences (Carey Symposium). Tectonophysics,
63: 31-61.
Although the two mountain belts of the Eastern European Alpine system (the
Dinaric-Hellenic
and Carpathian-Balkan
belts) form two bifurcating but continuous
mountain terranes their internal structural development is conspicuously diachronous and
is dependent upon the relative motion of at least three fragments of continental crust
which lay between the European and African plate. In Late Jurassic time the three fragments of continental crust were separated by narrow zones of oceanic crust and palinspastic reconstructions
of the continental fragments indicate they were considerably
larger than their present size and had different shapes. Convergence of continental fragments began in the Late Jurassic by subduction of oceanic crust, but upon complete disappearance of oceanic crust, convergence continued with subduction of small(?) amounts
of lower continental crust. During continent--eontinent
convergence, the upper lo-15
km of the continental crust were detached forming thrust sheets as the lower crust is
thickened and in part(?) subducted.
The Carpathian orocline formed by the complex suturing of continental fragments
against an irregular European plate boundary and developed during diachronous collision
of fragments which began in the Middle Cretaceous and continued to the Recent. Only
one narrow fragment of continental crust is oroclinally bent; the European plate to the
east and continental fragment to the west of the Eastern Carpathians are not bent.
The continental fragments do not behave as rigid plates, but are strongly internally
deformed. Thus, the motion of these small continental fragments cannot always be
treated in the same way as large plates. During suturing the three primary plates were
fragmented into more complex arrays of small fragments whose boundaries cross cut earlier boundaries. Furthermore, it is doubted whether meaningful plate boundaries can be
identified between the fragments and the fragments may be more realistically described as
continuously deformed fragments of continental crust.
The interpretation presented here suggests that the geologic data that remains in the
erogenic belts for determining the magnitude of convergence between fragments always
yields a minimum value. Much of the evidence for the magnitude of convergence between
fragments is lost during subduction and collision processes.
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The Alpine moun~n
system of Eastern Europe, consisting of the Eastern
Alps, Western, Eastern and Southern Carpathians, Balkan fountains,
Dinaric
and Hellenic Alps (Fig. l), forms two mountainous
belts with very different
trends. The Dinaric-Hellenic
belt contains northwest-trending
linear mountains whereas the Eastern Alps and the Carpathian-Balkan
belt form a great
Fig. 1. Generalized
tectonic
map of the Eastern European Alpine system. Black = ophiolites; vertical lined zone = Pieniny Klippen zone. EA = Eastern Alps; WC = West Carpathian; PK = Pieniny Klippen zone; EC = Baatern Carpathians;A
= Apuseni Mountains;
SC = South Carpathians;
BM = Balkan Mountains;
ND = North Dobrogea;
V = Vardar
zone; HA = Hellenic Alps; DA = Dinaric Alps; SA = South Alps; AA = Adriatic Sea; BS =
Black Sea; AS = Aegean Sea; M = Moesian plain; PB = Pannonian
basin; TB = Transylvanian basin; PP = PO plain; RP = Russian platform;
EP = European
platform;
RM =
Rhodopian
Mountains.
33
double looped trend of mountains. Structures within these two belts generally follow the two mountainous belts, and superficially the structures
appear more or less continuous, but nothing could be further from the
truth. Structures within the two belts have formed diachronously over a
period of nearly 150 m.y. and are the result of convergence and collision
between several different fragments of continental crust as small regions of
oceanic crust were eliminated between some of the fragments. None of the
continental fragments, and presumably the now disappeared oceanic crust,
behaved rigidly, thus it is difficult to call the fragments microplates in the
same sense that the term is used for the large plates which make up the firstorder plate tectonic mosaic of the globe, In want of a better term, I will refer
to these pieces of continental and continental plus oceanic crust as fragments, and not microplates, to emphasize that the fragments are severely
deformed internally and their movement and deformation does not obey the
same rules as relatively more rigid first-order plates. The characteristics of
the movement and deformation of these fragments will become evident
during the discussion of the evolution of the Eastern European Alpine
system.
Basic data to support the tectonic evolution presented here cannot be
fully documented in this paper. However, several summary articles on
various parts of the Eastern European Alpine belts are available, and provide
overviews and extensive biblio~phies;
the following references can be consulted: Eastern Alps - Tollman (1963, 1968) and Oxburgh (1968,1974);
Carpathians - Andrusov (1965, 1968), Sandulescu (1974) and Burchfiel
and Bleahu (1976); Balkan Mountains - Foose and Manheim (1975);
Dinarides - Aubouin (1973) and Aubouin et al. (1970); Hellenides Aubouin (1973) and Smith and Moores (1974). The quality of geologic
understanding in different parts of the belts is very uneven which means that
any synthesis may be subject to considerable revision as new data becomes
available.
THE ALPINE EPISODE
The time span of events regarded as Alpine ranges from Triassic to Recent.
Continental configurations at the beginning of the Alpine episode are obtained by closing the Atlantic Ocean, and they show an eastward widening
ocean terrane, co~espond~g to the Tethys of Seuss (1893), between Europe
and Africa being approximately 1000 km wider at the longitude of Greece
than at present. The width of this oceanic terrane varies depending upon
which Atlantic reconstruction is favored. Throughout the Alpine system of
the Mediterranean west of Turkey, no Triassic rocks of deep oceanic character and no remnants of oceanic crust older than late Middle Jurassic have
been recognized. Early Mesozoic sedimentary rocks suggest that most of the
western part of Tethys was underlain by continental and not oceanic crust.
With the recognition that within the Alpine system of the M~ite~ean
34
there are several fragments of continental material which lie between Europe
and Africa, attempts have been made to rearrange many of these fragments
into a Triassic western Mediterranean
area floored by continental
crust (for
example: Hsu, 1972, and Dewey et al., 1973). Much of the Tethyan seaway
of the eastern Mediterranean
probably also was underlain by continental
crust, but the present extent of continental material in continental fragments
within the Alpine system is insufficient to floor most of eastern Tethys with
continental
crust. However,
from examination
of the structure
of the
Eastern European Alpine system it is clear that not only has oceanic crust
been subducted, but unknown amounts of the lower parts of the continental
crust have been subducted whereas the upper parts have been imbricated and
shortened.
Unravelling of the convergence
and shortening
of continental
fragments which has occurred in the Eastern European Alpine system leads
to the conclusion that the present shape and mass of the fragments are not
their original shapes and masses; they were much larger before collision and
thus may have floored much of eastern Tethys.
Remnants of oceanic crust (ophiolites) and sedimentary rocks that can be
interpreted
to have been deposited on oceanic crust are present in the Alpine
system. None of these rocks, however, can be dated as older than late Middle
Jurassic and most of them are Late Jurassic or Early Cretaceous in age. This
suggests that if the break-up of continental
crust within Tethys began in
Early Mesozoic time, no oceanic crust was formed until Middle or Late
Jurassic time. The early extension and transform stage of the Alpine system
will not be treated here. The focus of this paper will be on the convergence
and collision of fragments which began in the Late Jurassic shortly after
oceanic crust was formed in the eastern Mediterranean
region.
IDENTIFICATION
OF INTRA-ALPINE
FRAGMENTS
Within the Eastern European Alpine system during Late Jurassic time at
least four fragments can be identified (Fig. 2) which are referred to here as
the Apulian, Rhodopian,
Moesian and North Dobrogean fragments. These
fragments lay between first-order European and African plates and at various
times during their history, the fragments were attached to the plates and
moved with them. Remnants of Late Mesozoic oceanic rocks are present in
many places between the fragments and the geologic evolution of the area
indicates Late Mesozoic oceanic rocks were present between the Apulian and
Rhodopian fragments, and between these fragments and the European plate.
Late Mesozoic oceanic rocks were probably present between the Rhodopian
fragment
and the northern
and western parts of the Moesian fragment.
Whether oceanic rocks were present between the southern part of the
Moesian fragment and the Rhodopian fragment is uncertain. My preference
is that there was oceanic crust between the two, but the evidence in the
present day Balkan Mountains is equivocal. No oceanic rocks were present
along the boundaries
of the North Dobrogean fragment. Rocks in North
35
Fig. 2. Plates and fragments in the Eastern European Alpine system. E = European plate;
R = Russian plate; A = Apulian fragment; ND = North Dobrogean fragment; M = Moesian
fragment; R = Rhodopian fragment; OR = Oceanic remnants between the Apulian and
Rhodopian fragments.
Dobrogea have a very different pre4enomania.n history from the rocks of
the Moesian fragment and European-Russian plate and its boundaries are
probably fault zones of large strike-slip displacement. These fragments
involve the lithosphere and are bounded by zones of large displacement.
For most of the fragments, the remnants of oceanic rocks between them
represent boundaries along which narrow terranes (several hundred kilometers) of Late Mesozoic oceanic crust were subducted as fragments of continental crust converged and collided. During convergence the fragments
probably moved as part of the lithosphere, but at times shallow parts of
some fragments were detached from the rest of the lithosphere and formed
imbricated thrust sheets. Thus in Late Jurassic time, the fragments, except
North Dobrogea, consisted of both oceanic and continental crust and their
boundaries were entirely within oceanic lithosphere.
Times of deformation within and along the boundaries of the fragments
are shown in Fig. 3. From this compilation it is evident that deformation
was diachronous and shifted from one mountain belt to the other reflecting
the shift from one fragment boundary to another. The shifting of tectonic
activity from one boundary to another, or the changes in direction of
motion alond boundaries, appear in the geologic record as episodes or phases
of deformation. In order to reconstruct the structural evolution of the belts,
the convergent, transcurrent and extensional displacements were reversed in
sequence and by a magnitude thought to be consistent with the geologic
evidence. The results, progressing from the oldest to the youngest events, are
presented and discussed below; however, it should be kept in mind that the
reconstructions were made in reverse order.
36
Fig. 3. Times of deformation in the Eastern European Alpine system. I = Latest Jurassic
to pre-AIbian time; 2 = Albian time; 3 = Cenomanian to Turonian time; 4 = Coniacian to
Paleocene time; 5 = Eocene to Oligocene time; 6 = Miocene time; 7 = Pliocene time.
During convergence, oceanic crust was diacbronously subducted and continental collision occurred along different &agment boundaries at different
times. As suturing took place between continental fragments, the &agments
combined and enlarged, however, sometimes new boundaries developed
within fragments to form smaller units. Thus, the number and identity of
fragments changes temporally. In the following discussion the four fragments
present in Late Jurassic time will be followed through to the Recent,
although during various time periods different fragment nomenclature could
be used.
37
LATEST JURASSIC TO ALBIAN EVENTS
The position of the fragments in Late Jurassic time is uncertain. A, guess as
to their possible positions is shown in Fig. 4. The size and shape of continental crust in the fragments was considerably different than at present. By unrolling the intraplate deformation, a conservative estimate of the original
configuration of continental crust is shown in Fig. 4. Boundaries of the fragments along the Adriatic, Aegean and Black Sea coasts have no structural
significance as the fragments extend beyond these artificial boundaries.
The first geologic evidence to suggest convergence of fragments is present
in latest Jurassic rocks. Within what is now the Vardar zone (Fig. 1) separating the Apulian and Rhodopian fragments, is Late Jurassic flysch locally
containing volcanic detritus that was deformed and intruded by a small
granitic stock in latest Jurassic time. Metamorphism of the flysch under
conditions of high pressure and low temperature may have taken place at
that time, but the time of metamorphism is unclear. The Late Jurassic rocks
in the Vardar zone suggest that convergence of fragments began by subduction of oceanic crust to a degree sufficient to produce volcanic rocks
(perhaps 100-200
km). The polarity of subduction is uncertain, but eastward dipping imbricate thrusts in the Vardar zone suggest eastward subduction, however the east dip may be the result of later deformational events. If
the volcanic rocks are related to subduction, volcanism must have taken
place in an oceanic terrane, because Late Jurassic rocks resting on adjacent
continental crust do not contain volcanic detritus. The position of subduction zones and volcanic islands relative to adjacent continental crust is
unknown because later subduction and superposed deformation has greatly
obscured the early history of these rocks.
Geologic evidence for Early Cretaceous events (to Albian time) is more
widespread. Calcalkaline volcanic rocks and their detritus are present in
Early Cretaceous flysch along many parts of the oceanic terrane that lay
southwest of the Rhodopian fragment. At least part of this volcanic activity
may have been adjacent to the continental crust in the Rhodopian fragment
as Early Cretaceous plutonism and metamorphism occurred within continental crust at the southern end of the Rhodopian fragment. If the igneous
rocks are subduction derived, the polarity and number of the subduction
zones is uncertain. Only along the southeastern part of the Rhodopian fragment, does the plutonism and high temperature metamorphism east of the
high-pressure (glaucophane only) metamorphism in the flysch of the Vardar
Zone suggest eastward subduction beneath the Rhodopian fragment.
Convergent events of minor(?) significance are recorded by local underthrusting along the northeast side of Rhodopian continental crust by Late
Jurassic-Early
Cretaceous (pre-Barremian) flysch and local folding of Early
Cretaceous flysch along the southern part of the Moesian fragment. This
folding may be related to earlier phases of continent to continent collision
which continue into the Albian. Along the southwest side of the north end
38
Latest
Jurassic - Atbion
Fig. 4. Sugge8ted position8 of fragments
and location of structural
deformation
in the
time period West Jurassic to pre-Albiin. As long as regions of oceanic crust separate the
continental
part8 of the plates their relative location is speculative.
Heavy lines indicate
areas of thrusting
or folding documented
from remnants
preserved
in the present day
mountain
system. Open barbs indicate B-type subduction
(subduction
of oceanic lithosphere) and closed barbs indicate A-type subduction
(shortening and/or partial eubduction
of continental
crust). Triangles represent the suggested location of arc volcanism. S-shaped
lines are areas of metamorphigm
and black circke are locations of plutons. Dashed line at
top of diagram is the present location of the Carpathian-Balkan
front, solid linea are suggested extent
of continental
crust and dotted lines are the prerent size8 of fragment
boundaries.
Dashed lines dong boundaries
of the fragments are present day boundaries
along the Black, Aegean and Adriatic Seas and have no structural
significance.
Shaded
area indicates
possible extent of oceanic crust. E = European plate; R = Russian plate;
ND = North Dobrogea;
M = Moesian fragment; R = Rhodopean
fragment; A = Apuhan
fragment; TN = Transylvanian
nappes; Oph. = Ophiolite nappe. LJ = dated Date Jurassic
metamorphism.
39
of the Rhodopian fragment, wildflysch of Early Cretaceous age with basic
volcanic detritus and numerous large olistoliths of exotic pre-Albian
Mesozoic carbonate rocks is present resting on what was probably the
western edge of Rhodopian continental crust. The tectonic significance of
#is wildflysch event is uncertain.
One of the most significant events in the history of the Dinaric-Hellenic
belt, the emplacement of ophiolites onto the eastern edge of the Apulian
continental crust, occurred during pre-Albian time. The exact time of
emplacement is uncertain and may have been diachronous, but data from
Yugoslavia suggests a very Early Cretaceous age whereas data from Greece
suggest a Late Jurassie or Early Cretaceous age. The fact that these ophiolites
are dated as having formed in Late Jurassic time indicates they were obducted very shortly after they were formed. No evidence of arc-type volcanic
rocks are present at the top of the ophiolites suggesting eastward subduction
was minimal before the ophiolites were abducted. Probably the initial magnitude of ophiolite emplacement was between 50 and 100 km. Their present
structural positions have been greatly modified by later events.
In summary, the earliest convergent events from Late Jurassic to Albian
time recorded in the Eastern European Alpine system generated subduction
within a largely oceanic terrane and produced small volcanic islands chains
southwest of the Rhodopian fragment. During this time period, perhaps
ZOO-300 km of oceanic crust was consumed to form the volcanic islands
and another 100 km to abduct the ophiolites onto the Apulian plate. Convergence was probably more or less continuous and during this time period
there was significant closing of the oceanic terrane southwest of the Rhodopian fragment. Although the other boundaries of the fragments were undoubtedly active as transform or accreting boundaries, no direct evidence for
the nature of these boundaries remains. It is clear that at this time the fragments consisted of both continental and oceanic crust, but evidence for the
position, polarity and convergence vector of subduction zones has been
largely destroyed by later events.
In the Eastern European Alpine system the geological evidence suggests
that during convergent events, where fragments or plates consist of both
oceanic and continental crust, subduction is first developed in the oceanic
terrane, where subduction of oceanic lithosphere is the most favored process, and as convergence continues, much of the evidence for the structural
history of the oceanic terrance is lost. During and after collision of continental crust, only very fra~ent~
evidence for earlier oceanic events remains,
thus it is extremely difficult to reconstruct the earliest convergent events.
Where convergent events affect continental crust, the record is more complete, because continental crust is not readily subducted.
ALBIAN
EVENTS
The first continent to continent collision occurred during Albian time,
and is best documented in the Southern Carpathians where the Rhodopian
40
and Moesian fragments collided along a west-dipping subduction zone (Fig.
5). Debris eroded from the Rhodopian continental
crust and from oceanic
rocks that had lain between the two fragments is present in the Albian and
younger sedimentary
rocks deposited on the Moesian fragment. If a major
collisional event occurred to the south in the Balkan mountains, then the
convergence
must have occurred at about Albian time as Cenomanian sedimentary rocks rest on deformed
and metamorphosed
Mesozoic and older
rocks. The Moesian continental
crust may have under-thrust the Rhodopian
continent
crust by at least 100 km, as suggested by Tollmann (1968). Evidence for an event of this magnitude in the Balkan Mountains is weak, however most workers in the Balkan Mountains are prone to play down the role
of thrusting in favor of deep faults, vertical tectonics and rhegmatic shears.
Along its northern part, the continental
crust of the Rhodopian fragment
was narrowed by internal thrusting by as much as 60-100 km. Within what
is now the inner crystalline zone of the Eastern Carpathians there are three
E
,r
--
----.\
Albian
Fig, 5. Suggested position of fragments and areas of thrusting and volcanism during Albian
time. Stippled region is area of intra-cratonic A-type subduction. For explanation of used
symbols see Fig. 4.
41
east-directed
thrust plates each involving pre-Mesozoic crystalline rocks and
a fourth and highest thrust plate, probably
emplaced by gravity sliding,
which involves only Mesozoic cover rocks. This thrust complex. is itself
underthrust
along the eastern side by Late Jurassic and Early Cretaceous
flysch that was probably deposited on oceanic crust and partially subducted
in Albian time. All of these thrust-faults and associated folds are overlapped
by Cenomanian molasse. The amount of subduction along the eastern margin
of the Rhodopian fragment is probably less than 100 km as no evidence of
contemporaneous
volcanism is present. Only the metamorphism
in the
eastern Balkan Mountains might suggest deep seated plutonism.
Narrowing of continental crust, as in the northern Rhodopian fragment, is
a characteristic
for most fragments and plates in the Alpine system. The
thrusts which involve pre-Mesozoic
crystalline
continental
crust are commonly no more than lo-15
km thick and the Mesozoic rocks they carry
range from shallow water or non-marine
sedimentary
rocks to deep water
pelagic rocks. During deposition of these sediments, crustal thickness must
have been about 30 km for the shallow water sediments and about 10 km or
less for the pelagic sediments which were deposited on attenuated continental
crust (Trumpy, 1975). None of the thrust sheets carrying crystalline rocks in
the Eastern European Alpine system contain rocks of types characteristic
of
the lower continental
crust (such as are present in some thrusts in the
Western Alps [e.g., Giese, 19681). Thus in palinspastic reconstructions
these
thrust sheets must have had rocks of the lower continental
crust beneath
them over a width of from 60 to 100 km. Sedimentary rocks in the northern
Rhodopian fragment were deposited mostly in shallow water suggesting normal crustal thickness in this area prior to thrusting. If all of these lower
crustal rocks were still present below the Carpathians a crustal thickness of
more than 60 km would be expected. Crustal thicknesses of between 25 and
47 km are now present in this area, suggesting that some lower continental
crustal rocks have been subducted into the mantle by processes perhaps different from subduction
of oceanic crust. Throughout
succeeding episodes of
convergence in the Eastern European Alpine system, this structural problem
re-occurs: crystalline thrust sheets of 10-100 km displacement are detached
from within the upper 15 km with the continental crust and the lower parts
of the continental
crust
cannot be accommodated
in the present crustal
thicknesses. Helwig (1976) has argued that all continental
crustal material
can be accounted
for by thickening during collision and deformation.
He
points out that much of the continental
crust in the Eastern Alps may have
been thin crust, attentuated
during geosynclinal rifting, and thus the amount
of lower crustal rocks that need to be accounted
for in the root of the
orogen is much less than normal crustal thickness. While the data are insufficient at present to prove subduction
of lower continental crust along with
the rest of the lithosphere,
it is a process that must be considered and one
that perhaps has occurred in the Eastern European Alpine system.
Bally (1975) has proposed that subduction which involves underthrusting
42
of oceanic crust be called B-type (or Benioff-type)
subduction and subduction which involves the underthrusting
of continental
crust below continental crust be called A-type (or Alpine- or Ampferer-type).
These terms are
useful to distinguish the two types of subduction and will be used here. It
can be seen from the discussion above that continental
fragments, or plates
for that matter, that have oceanic crust around them, begin convergence by
B-type subduction
and after collision continue convergence by A-type subduction with most of the structures formed during the earlier B-type subduction becoming strongly modified or subducted.
All the thrusts within the northern part of the Rhodopian continental crust
are of the A-type with continental crustal material carried under other continental crust (A-type subduction),
but the mechanism for producing these
thrusts is not clear because no collision of continental crust took place at this
time in the northern part of the Rhodopian fragment. Three possible mechanisms can be proposed for thursting within continental crust: (1) continued
convergence during and after collision of continental fragments at a subduction
boundary; (2) partial subduction and imbrication of a continental fragment
which was carried into a former B-type subduction zone; and (3) imbricate
thrusting of continental
crust above a B-type subduction
zone. The first
mechanism is easily visualized, because once all oceanic crust has been subducted between continental
fragments, but convergence continues, yielding
can only take place within continental
crust. This mechanism does not
appear applicable to the thrusts in the northern part of the Rhodopian fragment as this continental
fragment was probably surrounded by oceanic crust
at this time. It is possible, but unlikely, that other continental fragments, not
shown in the reconstructions
because they have been nearly wholly subducted, collided with the west coast of the Rhodopian
continental
crust
along a west-dipping subduction zone. Remnants of these continental
fragments might now be represented
in the highest structural
units of the
Eastern Carpathians (i.e., the Tmnsylvanian Nappes). The second mechanism
would suggest that a westward dipping B-type subduction zone was present
west of the Rhodopian fragment into which the Rhodopian continental crust
passed with the lower part of the continental
crust becoming detached and
being subducted as the upper part was deformed and imbricated
but not
subducted.
This mechanism is supported
by the fact that flysch sequences
with rare volcanic rocks were present in the oceanic area west of the Rhodopian fragment.
However under the proposed
model, the abduction
of
oceanic crust into the Rhodopian continental crust would be expected but is
not present. Thus, this mechanism is not completely
supported by the evidence. The third mechanism is more complex but consistent with most of
the geologic evidence. B-type subduction
beneath continental
crust may
generate sufficient horizontal
compression
in the overriding plate to cause
either synthetic (i.e., thrusting parallel to and in the same sense as the subduction) or antithetic (i.e. thrusting along faults dipping opposite to the dip
of the subduction zone and whose sense is opposite to that of the subduc-
43
tion zone) thrusting. In the Eastern European Alpine system both types are
present. Detachment
of upper from lower continental crust is probably facilitated by intrusion of magma into the overriding plate raising the level of the
brittle-ductile
transition within the crust. Detachment of thrusts is localized
near the brittlelluctile~transition
and the lower parts of the continental crust
are partially or wholly subducted with the remainder of the lithosphere. Geologic evidence supports westward subduction of oceanic lithosphere beneath
the Rhodopian
fragment as flysch sequences were underthrust
westward
beneath Rhodopian continental crust at this time, and elevated temperatures
are suggested by low-grade Alpine metamorphism
at the base of some crystalline thrust sheets which in this case are synthetic to the main B-type subduction. The key question is whether intracratonic
crustal shortening (Atype subduction)
requires continent-continent
collision, or sufficient stress
can be generated in the subduction
process itself to cause thrusting. The
latter is supported
by the geologic relations in the northern part of the
Rhodopian fragment; however, the problem re-occurs again during later convergent events.
Within the oceanic terrane southwest of the Rhodopian fragment, structural events are recorded, but are of unknown significance. Minor volcanic
activity suggests subduction,
but of lesser magnitude
than in pre-Albian
times, and an unconformity
between Cenomanian
and Early Cretaceous
rocks is widespread. Along the northern boundary of the Apulian fragment
Albian deformation
is recorded locally. Flysch units in the Eastern Alps contain detrital chromite which suggests erosion of ultramafic rocks that might
have been exposed during subduction.
The significance of these data is unclear and is still being examined, for example see the discussions in Trumpy
(1975),
Tollmann
(1963) and Oxburgh
(1974). The data suggest convergence was active for the first time along the northern boundary of the
Apulian fragment, and the motion between Apulian fragment and the European plate may have changed to a more northerly (convergent) direction and
increased the importance
of transform relative to subduction motion along
the boundary in the oceanic terrane to the east. This seems to be clearly the
case in Cenomanian to Coniacian time (see below).
In post-Tithonian
pre-Cenomanian
time folding and thrusting occurred in
North Dobrogea and the Moesian fragment was joined to North Dobrogea:
the steep northwest
trending fault that separates these two units is overlapped by Cenomanian
molasse. Deformation
may be Albian or as old as
latest Jurassic and is the westernmost
continuation
of deformation
in the
Caucasus Mountains. No oceanic crust appears to have been present adjacent
to North Dobrogea and the structural
development
is intracratonic.
The
North Dobrogean deformed zone is a foreland belt, external to the main
Alpine erogenic belt, and the amount of deformation
in North Dobrogea is
small and perhaps negligible compared to the deformation
within the main
part of the orogen. The deformation
is, however, a result of movement
between the Moesian and North Dobrogean fragments and the Russian plate,
44
and probably was the result of dextral(?) transform motion with a compressional component.
In summary, the major events during Albian time are: (1) the collision of
the Rhodopian and Moesian fragments accompanied
by considerable internal
deformation,
bending and narrowing of the Rhodopian
continental
crust;
and (2) the deformation
and final movement
between the Moesian and
North Dobrogean fragments. Convergent activity continued from pre-Albian
times in the oceanic terrane southwest of the Rhodopian fragment although
the rate of convergence may have lessened as the Apulian fragment began to
move in a more northerly
direction, with respect to Europe. The evidence
indicates that significant convergence shifted from areas southwest to areas
northeast of the Rhodopian fragment during Albian time.
CENOMANIAN
TO CONIACIAN
EVENTS
Following the collision between the Rhodopian
and Moesian fragments
the rate of convergence apparently declined in this part of the Carpathians, but
increased along the northern margin of the Apulian fragment (Fig, 6). From
centraI
Romania
(Apuseni
Mountains)
westward
through
the Western
Carpathians and into the central Eastern Alps, a major north-directed
thrust
complex developed within the northern part of the Apulian fragment. All
the thrusts are intracratonic
and formed by A-type subduction. Total magnitude of displacement
is loo-150
km which narrowed the northern part of
the Apulian continental
crust by nearly that amount as only a small part of
this displacement can be accounted for by gravity sliding.
Geologic evidence indicates that the A-type subduction within the Apulian
fragments was accompanied
by southward dipping B-type subduction along
its northern margin. Evidence from central Austria (Tauren window) and the
Western Alps suggests the earliest phases of high-pressure, low temperature
metamorphism
began at this time as oceanic rocks were subducted southward
beneath Apulia. In the Western Carpathians,
where thrusting is best developed, evidence for south-dipping
B-type subduction is largely lacking as the
location
of the proposed
subduction
zone would he beneath younger
Cretaceous and Tertiary flysch. Rare small plutons in the Carpathians, one
of which has been dated at 90 m-y., may be the only surface manifestation
of subduction.
If southward-dipping
B-type subduction
was present, the thrust faults
within the Apulian fragment are synthetic to this subduction, and only two
of the mechanisms for A-type subduction presented above are applicable: (1)
cont~ent~ont~ent
collision; or (2) synthetic detachment
of thrust faults
related to stresses within the overriding plate. In the first mechanism, the
colliding fragment of continental
crust may have been the terrane, now
largely subducted,
represented
by the Pieniny Klippen belt (Fig. 1). This
would place another fragment (or fragments) of continental
material north
45
E
,A---‘\
Cenomanian-Turonion
Fig. 6. Suggested location of fragments and areas of thrusting during CenomanianTuronian time. Stippled region is area of intra-cratonic A-type subduction which modifies
the northeastern boundary of continental crust of the Apulian fragment. Arrows indicate
change in position of the boundary. For explanation of used symbols see Fig. 4.
of the Apulian fragment and south of the European plate. An additional
continental fragment is not strongly supported by the geology. In the second
mechanism, synthetic thrusting within continental crust in northern Apulia
would have been related to differential stress difference generated by B-type
subduction large enough to produce intracratonic shortening and A-type subduction in an overriding plate, Detachment near the b~~l~uct~e
transition is suggested by recent age dates on synkinematic metamorphic minerals
near thrust planes in Austria. The large magnitude of the thrusts (100-150
km) and present crustal thickness strongly suggests lower parts of the continental crust were subducted. The geologic relations suggest the second
mechanism may be the correct one.
Significant tectonic activity in other areas is not obvious, however during
collisional processes in the Alpine system evidence for the plate boundary
46
activity in oceanic areas is largely destroyed.
As will become obvious as the
evolution of the Eastern European Alpine system is discussed further, the
“significant”
tectonic events are those for which evidence remains after collision and most of these are within continental
rocks. Nevertheless, the available evidence suggests little or no subduction occurred in the oceanic terrane
east of the Apulian fragment at this time, as only rare volcanic detritus
appears in flysch of this age. Thus, it is inferred that the motion of the
Apulian fragment was more northward
relative to Europe than in earlier
deformations
and that right-slip transform motion took place in the oceanic
terrane to the east.
CONIACIAN
TO PALEOCENE
EVENTS
Exceedingly
complex events that resulted in collision of the Apulian and
Rhodopian
fragments took place during latest Cretaeeous time. Timing of
events during this period is not easily determined,
because rocks of the
proper age are not present over broad areas, particularly
in the eastern part
of the Dinaric belt. Convergent structures are present along the eastern margin of the Rhodopian fragment and along the eastern and northern parts of
the Apulian fragment (Fig. 7). In addition considerable rotational strain took
place within the central part of the Rhodopian
fragment as the eastern
Carpathian loop began to take shape.
The most significant displacements
of Late Cretaceous age were the result
of convergence between the Apulian and Rhodopi~
fragments. A well developed Late Cretaceous volcanic and plutonic belt is present throughout
the
Balkan Mountains and Southern Carpathians largely within the Rhodopian
fragment, and contemporaneous
east-dipping thrust faults are present in the
Vardar zone which together indicate eastward subduction
in the Vardar
zone. Probably throughout
the early part of this period most of the subduction was B-type, but collision occurred between the two continental
fragments during the latter part of this time period and subduction changed to
A-type. The greater volume of igneous rocks and longer period of igneous
activity in the southeastern
part of this magmatic belt suggests increasing
amounts
of subduction
to the southeast
and probable counterclockwise
rotation of the Apulian fragment relative to the Rhodopian fragment, Following continental
collision, the eastern parts of the Apulian fragment were
imbricated on east-dipping faults, first in the Vardar zone then progressively
westward.
The progressive collision and imbrication
of continental
sedi.
mentary and crystalline rocks in the eastern part of the Apulian ment
is
very similar to convergent events presently shaping the modern Zagros (Bird
and Toksiiz, 1975).
East of the Vardar zone, the Rhodopian
fragment was thrust e&ward
over the Moesian fragment along a fault in a position similar to those developed in mid-Cretaceous
time. Remnants
of the Late Jurassic and Early
Cretaceous oceanic rocks were carried more than 60 km eastward below this
47
1
c
/
I
\
t
.. . .
\
i
\
\
,
\
Coni&&
?’
3--.
-_
<
\
\
\
-
_I
- Paleocene
Fig. 7. Suggested location of fragments and areas of deformation, magmatism and intracratonic A-type subduction during Coniacian-Paleocene
time. Pz = Pieniny Klippen
zone, For explanation of used symbols see Fig. 4.
thrust of largely crystalline rocks. Like all the other thrust faults which
involve crystallinerocks, only the upper 10-15 km of the crust was involved
suggestingA-type subduction. Magnitudesof Late Cretaceousdisplacements
appear to increase northward from the Balkan Mountainsinto the Southern
Carpathians, indicating the Rhodopian continental fragment was rotated
clockwise as it was moulded around the western end of the Moesian fragment. The position and polarity of the thrustingin the Southern Carpathians
indicates this thrust belt developed antithetically to first B-type then Atype subduction in the Vardar zone. Furthermore, igneous activity both
precedes and follows thrustingsuggestingthat de~c~ent of the crystalline
thrust faults was controlled by a shallow brittle-ductile transition zone in
the crust.
North of the Vardar zone, in the northeast part of the Apulian fragment,
no major Late Cretaceousdisruption of the fragmenttook place. Insteadthe
convergence of the two Eragmentsafter collision appearsto have been taken
up east of the Rhodopian fragment where flysch units were imbricated in a
west-dipping B-type subduction zone. Becausethese flysch units were imbricated on west-dipping thrust planes, the geometry of convergent displace-
48
ments was that of two subduction zones of opposing polarity connected by a
diffuse transform
zone which displaced the northeastern
part of Apulian
fragment eastward as the Rhodopian
fragment was deformed into a large
dextral arc. The Apuseni Mountains and fragments of the Vardar zone were
transformed
eastward and juxtaposed
against the Southern
Carpathians.
Thus the apparent anomalous relation of north and south vergent crystalline
thrust sheets rooting in an oceanic terrane in this part of the C~pathians is
explained as the result of transform juxtaposition
of thrusted terranes of different ages which were not formed
in their present relative positions.
Furthermore,
this collisional event is responsible
for the eastward limit,
within the Apuseni Mountains, of the Mesozoic paleogeographic
element and
early Late Cretaceous deformational
zone of the northern Apulian fragment
and its nearly right-angle juxtaposition
with the late Early Cretaceous deformational zone in the Eastern Carpathians. Dextral coupling of subduction in
the Vardar zone and the flysch terrane of the Eastern Carpathians
was
probably
facilitated
by the oceanic terrane present north of the Moesian
fragment.
The geometry of thrust faults along the east side of the Rhodopi~
fragment is the result of west-dipping, antithetic A-type subduction changing to
west-dipping
B-type subduction
in the Eastern Carpathians.
Much of the
dextral
bending
of the Rhodopian
fragment,
its moulding
around the
Moesian fragment and the eastward motion of the Apuseni Mountains was
completed
by Early Tertiary time as the youngest (Danian and Paleocene)
igneous rocks of the Late Cretaceous m~atic
arc form a belt that trends
across the zone of dextral bending without major displacement. Crossing of
the zone of dextral displacement
by these igneous rocks indicates a change
to a more northerly
direction of subduction
in the Dinarides (continued
counterclockwise
rotation) so that subduction occurred beneath the former
dextral transform
zone. This change in motion continued
into the Early
Tertiary (see below).
Along the northern
border of the Apulian fragment, B-type subduction
accompanied by minor synthetic thrusting took place. Along the eastern part
of this border synthetic imbrication and narrowing of the Apulian fragment
took place within the Pieniny Klippen belt (Fig. 1 and 7). Parts of the
Klippen belt may represent exotic fragments underlain by continent
crust,
but its more southerly parts are clearly part of the Apulian continental crust.
Further west, B-type subduction
was present, but the evidence for subduction is less clear.
In summary, the collision of the continental
parts of the Apulian and
Rhodopian fragments, antithetic thrusting of the Rhodopian fragment in the
Southern
C~ath~s
and east movement
of the northern part of Apulia
causing dextral bending of the Rhodopian
fragment were the major structural events at this time. Late Cretaceous-Early
Tertiary igneous rocks
crossed all the structural
elements of the Southern
Carpathians
and the
Apuseni Mountains,
and demonstrates
that the concave-east
part of the
49
Carpathian orocline, moulded around the Moesian fragment, was developed
by Early Tertiary time. Furthermore, these events clearly show the non-rigid
behavior of the fragments.
EOCENE AND OLIGOCENE EVENTS
During Eocene and Oligocene time coun~rclo~k~se
rotation of parts of
the Apulian fragment continued and the motion of Apulia with respect to
Europe changed to a more northwesterly direction (Fig. 8). There was also
a shift in tectonic activity from the Carpathian to the Dinaric-Hellenic belt.
Whereas in Late Cretaceous time some imbrication and probable narrowing
of the Apulian fragment took place, in Eocene and Oligocene time massive
intracratonic thrusting and narrowing occurred. Although thrusting in the
external (western) parts of the belt are reasonably well dated those in the
inner (eastern) parts, particularly in Yugoslavia, are very poorly dated and
some thrusts shown in Fig. 8 may be latest Cretaceous in age. Enough data
are present however to demonstrate that the thrusts developed earlier in the
east than in the west. Proven magnitudes of thrusting appear to increase
Eocene - Oligocene
Fig. 8. Suggested location of fragments and regions of deformation, intra-cratonic A-type
subduction, magmatism and metamo~h~m
during Eocene-Oligocene
time. SV-Strimon
valley. For explanation of used symbols see Fig. 4.
50
southward along the chain and they may reach more then ZOO-300 km in
southern Greece but appear to die out in northern Italy. All of the thrust
faults involve continental
rock sequences including either crystalline or sedimentary
pre-Mesozoic
rocks or Mesozoic carbonate
and flysch terranes.
Ophiolites and associated oceanic rocks are involved, but these terranes were
initially abducted
in pre-Albian time and were carried with or were overridden by thrust faults of Eocene and Oligocene age. The thrusts have greatly
imbricated
pre-existing
paleogeographic
units. In the Dinarides, ophiolites
emplaced in pre-Albian time have been overridden by carbonate rocks that
were initially structurally
below the ophiolites.
Furthermore,
some ophiolites lay within the upper plates of thrust faults so that locally they rest on
Eocene rocks although their initial emplacement
was pre-Albian.
Thrusting
within the Din~ic-Hellenic
belt was A-type subduction
and
involved mainly sedimentary
rocks as old as Paleozoic in the Dinarides and
Mesozoic
sedimentary
rocks and pre-Mesozoic
crystalline
rocks in the
Hellenides. Palinspastic reconstruction
requires that continental
crust that
underlay the thrusted terranes was partly incorporated
into the sialic root of
the orogen, but because of the magnitude
of thrusting, also partly subducted. If greater subduction took place in the south, it follows that counterclockwise
rotation of the Apulian fragment which began in Late Cretaceous continued
into Eocene or Oligocene time. From geologic evidence a
rotation
of at least 20 degrees is suggested during Eocene and Oligocene
time. During this time period, the rotation was absorbed in the disruption
and narrowing of eastern part of the Apulian fragment. The amount of Atype subduction indicated by thrusting is indeed large and palinspastic reconstructions seem to demand large volumes of continental
crust which cannot
be accounted for in the present volume of crustal rocks beneath the Eastern
European Alpine system.
Associated temporally
and spatially with thrust faulting was a belt of volcanic and plutonic rocks which trends parallel to the thrust belt and partially
overlaps it in Yugoslavia and Greece. Not all the igneous rocks are dated,
however rocks of Eocene, Oligocene and Early Miocene age are known. If
this magmatic belt was genetically related to A-type eastward subduction, its
spatial relations are not obvioulsy explained as igneous rocks were intruded
very near to what would have been the surficial position of the subduction
zone. This relationship may be a function of A-type subduction where continental crust rather than oceanic crust is being melted to form igneous
rocks. Not enough data are presently available to determine if this volcanic
belt differs geochemically
from other belts derived from melting of subducted oceanic crust.
East of the Vardar zone suture, parts of the Rhodopian and Moesian fragments (now combined) were disrupted and absorbed some of the convergent
motion. The Rhodopian continental crust was thrust eastward at least 25 km
along the Strimon valley (Fig. 8). Age dates on metamorphic
minerals from
crystalline rocks west of the Strimon thrust fault suggest amphibolite grade
51
metamorphism of Eocene-Oligocene
age. The Strimon thrust thus formed
antithetically to the main A-type subduction and its localization was probably related to an increase in ductility of continental crust caused by magmatic intrusion. The Strimon thrust is used by some workers to divide the
Rhodopian fragment into two parts: the Rhodopian and Serbo-Macedonian
massifs to the east and west respectively. It does not however mark a major
boundary between fragments but a second-order break. Father east along
the northern part of the Balkan Mountains northeast thrusting of crystalline
rocks took place and flysch of Cretaceous and Early Tertiary age north of
the thrusts was folded with north vergence. Thrusting appears to be localized
just north of the Cretaceous suture between the Rhodopian and Moesian
fragments and was antithetic to the main subduction in the Hellenides.
Thrust faults of small(?) magnitude are present between the deformed zone
in the Balkan Mountains and the Strimon thrust fault and they are vergent
both to the northeast and southwest. This suggests the crust in this area was
under horizontal compression, but contained no significant anisotropy which
controlled the direction of thrusting. Total shortening of the terrane east of
the Vardar Zone may be of the order of 50-75 km and is completely intracratonic .
The second major event that occurred during this period was the collision
of the northwest part of the Apulian fragment with the European plate (Fig.
8). The northward vergence of thrust faults, overriding of ophiolites by the
Apulian continental crust and a weakly developed magmatic belt suggest the
following sequence of events: (1) initial south-dipping B-type subduction
continued from Late Cretaceous time; (2) collision; and (3) south-dipping Atype subduction, which imbricated continental crust of both the Apulian
fragment and European plate. Development of thrust faults during this
period was complex, as southwestern, western, northern and northeastern
vergences are documented. The variable vergence of thrusts may be the result
of collision along an irregular front and rotation of the Apulian fragment.
Both the Apulian fragment and European plate were narrowed by at least
50-100 km and the upper part of the Apulian continental crust overrode
the European plate by more than 100 km. Lower parts of the continental
crust are missing in the Eastern Alps (although they are locally present in the
Western Alps) indicating substantial A-type subduction. In fact, the Eastern
Alps is the area where the concept of subduction was developed by
Ampferer (1906), and Ampferer and Hammer (1911) which they called
“verschluckung” and describes only what is called here A-type subduction.
A few small plutons along the southern part of the Eastern Alps define a
poorly developed magmatic belt. In general these plutons are post-thrusting
and intruded the southern parts of the thrusts. Metamorphism of about the
same age affected the emplaced thrust faults, and was accompanied by
folding during Eocene-Oligocene
time. Deep-seated magmatism was the
probable cause of metamorphism, and the exposed plutons represent plutons
intruded to higher levels or uplifted by later faulting. Because of the late
52
development
of the magmatic belt and the superposition
of it and metamorphism
on the thrust faults, a northward
migration,
by lower plate
imbrication,
of south-dipping
A-type
subduction
zones is suggested.
Although the igneous rocks of the Eastern Alps appear to be a continuation
of the Dinaric magmatic belt, they are the result of southward subduction
not northeast subduction as in the Dinarides.
East of the Eastern Alps evidence for Eocene---Oligocene
events is poorly
developed, largely because rocks of the proper age are not present in the
areas of deformation.
Minor closing of the oceanic terrane in the Carpathians
is suggested during this time interval but cannot be demonstrated.
In summary, large-scale northeast-dipping
A-type subduction took place in
the Dinaric-Hellenic
belt during Eocene--Oligocene
time following diachronous collision of the Apulian and Rhodopian fragments in Late Cretaceous
to Early Tertiary time. A-type subduction caused narrowing and disruption
of the eastern part of the Apulian fragment. Stress generated through the
combined
fragments was sufficient to cause deformation
within the combined Rhodopian-Moesian
fragment. As the magnitude of A-type subduction decreased
northward
in the Dinarides, the convergent
motion of
Apulian fragment with respect to Europe increased from east to west along
the north margin of the Apulian fragment and led to collision with the
European plate. These opposed directions of subduction suggest considerable
internal strain that must be accommodated
within the Apulian fragment. Unfortunately,
evidence .for structures
related to t,his strain may be largely
covered in the western part of the Pannonian basin.
MIOCENE EVENTS
Thrust faults vergent toward the European plate and affecting only the
external flysch zones of the Eastern Alps and Carpathians were the most
prominent
structures
developed during Miocene time (Fig. 9). Southward
subduction
is suggested by vergence of thrust faults and a well-developed
Miocene volcanic belt which lies south of the thrusts in the Carpathians. In
the Eastern Alps the subduction was A-type, however, in the Carpathians the
subduction
began as B-type and ended as A-type following collision of the
continental
crust of Europe and Russia with a disrupted Apulian fragment.
Magnitude of subduction was probably greater in the Carpathians than in the
Eastern Alps which favored the development
of a well-defined volcanic arc
in the Carpathians.
In the Eastern Alps, thrust faults along the northern
margin of the orogen formed as a result of A-type subduction which caused
imbrication
of the European plate, but these structures are mostly hidden
beneath the allochthonous
mass of the Northern Calcareous Alps. At least
part of this zone of imbrication is exposed along strike in the Urseren zone of
the Western Alps. In the Carpathians,
the European-Russian
plate underthrust a combined Apulian-Rhodopian
fragment such that the original crust
(oceanic internally and continental externally) below the flysch thrust sheets
has been largely subducted.
53
Miocene
’
B
Pig. 9. Structures developed during Miocene and Early Pliocene time. Double shanked
arrows indicate areas of spreading in the Pannonian and Transylvanian basins. Triangles
indicate areas of Miocene volcanism. For explanation of symbols used see Fig. 4.
One of the most difficult aspects of Miocene subduction to explain is the
nearly radial pattern of thrusting in the Carpathians. The magnitude of subduction between Europe and a combined, but disrupted, Apulian-Rhodopian fragment appears greatest in the Carpathians where the external flysch
belt was probably underlain in its inner parts by oceanic crust and is backed
up by the Miocene volcanic belt. Subduction probably died out rapidly at
the Southern Carpathian bend where the external part of the flysch belt and
major belt of Tertiary thrust faults end. The volcanic belt ends before
reaching the Southern Carpathian bend. Together with the fact that A-type
subduction of Miocene age continues westward around the convex westward arc of the Western Alps the following model is proposed to explain the
radial pattern of subduction. Motion of a combined Apulian-RhodopianMoesian fragment relative to the European plate was north or northwest.
The eastern part of the combined fragments, however, were welded to the
European-Russian plate, and had been since the latest Cretaceous or Early
Tertiary. Continued relative motion was accomplished by disruption of the
54
combined fragments and only their western part moved north or northwest.
The boundary along which relative motion occurred was a broad zone of
faults that trend north or northwest through eastern Greece and Yugoslavia,
western Bulgaria and southwest
Romania cutting across former fragment
boundaries. Collision had already occurred along the northwest and part of
the northern margin of the former Apulian fragment, however convergence
continued
disrupting the new fragment into several second-order
units by
fracture zones along the Insubric-Pusteria
line, and perhaps other zones now
hidden beneath the Pannonian basin. The crowding of the northern part of
the fragment against the possibly uneven edge of the European plate, combined with a small oceanic terrane in the area of the Carpathians, caused the
northern part of the plate to move to the east relative to the southern part.
This dextral shear thus produced a northeasterly
motion of the northern part
of the fragment relative to Europe. The dextral motion in the area of the
South Carpathian
bend did not manifest itself by the development
of
through-going
fractures but was accomplished by distributed faulting and by
considerable
dextral bending and rotation within the Eastern and Southern
Carpathians.
The Miocene motions hypothesized
here are similar to those
indicated by the focal mechanism studies of Mackenzie (1972) in the eastern
Mediterranean
region. The present hypothesis and the study of Mackenzie’s
suggest that fragment boundaries within continental
areas are much more
diffuse and less continuous
than those in oceanic areas; in fact it can be
questioned
whether they are really boundaries at all, or simply the most
prominent faults in a continuously
deforming crust.
Further distortion of the northern part of the combined Apulian-Rhodopian fragment was caused by extension and high-angle faulting in the Vienna,
Pannonian and Transylvanian
basins. The extension may be caused by penetrative fracturing
and graben formation
as this part of the fragment was
driven northeastward
into the last remaining recess in the European-Russian
plate. Part of the extension may also have been the result of an intracratonic
back arc spreading associated with the Miocene volcanic arc. Extension was
great enough to thin the crust beneath the Pannonian and Transylvanian
basins to less than 20 km locally. Extension was probably of the order of
50-100
km and probably resulted in additional override of flysch nappes
over the European-Russian
plate and accentuated
the northward and eastward bulges in the Western and Eastern Carpathians, respectively.
Tectonic activity in other parts of the Eastern European Alpine system
was relatively minor during Miocene time. Folding of the Dinaric-Hellenic
foreland is partly of Miocene age and probably represents the final stage of
Early Tertiary
intercratonic
convergence.
However,
in the southern
Hellenides thrust faulting in the external part of the thrust belt is of Miocene
age and appears to increase in magnitude eastward into Crete and Turkey.
The thrust faulting was within the disrupted Apulian fragment and represented the end phase of A-type subduction inherited from Eocene-Oligocene
time.
55
In summ~,
during Miocene time continued convergence disrupted a
combined Apu~~-Rhodopi~-Moes~
fragment that had become partly
welded to the European plate. Convergence was such that diachronous collision broke the combined fragment into smaller structural units which caused
its western part to move north or northwest and its northern part to move
eastward relative to its southern part. Dextral shear yielded a northwest
movement of the northern part of the fragment relative to Europe. It was
during Miocene time that the main development of the convex-east loop of
the Eastern Carpathian orocline was completed.
PLIOCENE AND RECENT EVENTS
Near the end of Miocene time, all former oceanic terranes were subducts,
and collision of a combined but disrupted Apul~n-Rhodopian-Moesi~
fragment with the European plate was completed. Convergence between
Africa and Europe continued, but the locus of plate boundary activity
shifted or is in its final stages of shifting south of the Eastern European
Alpine system (Fig. 10). Only Pliocene-Early
Pleistocene(?) folding in the
Southern Carpathian bend along the former boundary between the Moesian
and Rhodopian fragments represented the final stages of formation of the
convex-east Carpathian loop of the orocline. These folds represent the final
eastward movement of northern part of the disrupted Apulian-Rhodopian
fragment. Below the Southern Carpathian bend is the only area of deep
earthquake foci in the Eastern European Alpine system (Roman, 1970).
These foci probably mark a fragment of lithosphere, sinking after the termination of Pliocene-Early Pleistocene(?) underthrusting.
In the southeastern part of the Eastern European Alpine system, subduction shifted to the southern boundary of the former Apulian fragment where
eastern Mediterranean lithosphere is being consumed. Modern seismicity
indicates that complex fracturing of the combined fragments of the Eastern
European Alpine system is in progress. Mackenzie (1972) has suggested that
modem seismicity defines several microplates whose boundaries cut across
the former Rhodopian, Apulian and Moesian fragments (Fig. 10). However,
several boundaries defined by MacKenzie have no obvious surface expression,
and continued convergence niay be absorbed by diffuse zones of deformation or semicont~uous deformation similar to that described for older structural zones above. In fact it is questioned that meaningful boundaries can be
defined.
Modem seismicity indicates subduction is active only south and west of
Greece, and subducted lithosphere has reached a depth of 200 km, deep
enough to produce igneous rocks and extension in the Aegean Sea. The present tectonic and magmatic events in the Aegean area are probably analogous
to the early stages of Tertiary development in the Pannonian and Transylvanian basins.
56
/
_ -
-.
Piiocene
Pig. 10. Location
Pliocene time.
of zones of deformation
and intra-cratonic
A-type subduction
during
CONCLUSIONS
The picture of fragment interaction in the Eastern European Alpine system presented here is very preliminary, however a number of conclusions
concerning collisional events in general and the Alpine system in particular
can be deduced. Although the two belts of the Eastern European Alpine system form two bifurcating but continuous mountain chains their internal
structural development is conspicuously diachronous and discontinuous.
Examination of geologic data indicate the initial structural development of
the belts was dependent upon the relative motions of at least three fragments, and that convergence first took place diachronously along fragment
boundaries until collision of continental crust occurred at which time the
fragments became disrupted into new fragment arrays. During most of
Jurassic time pieces of continental crust were rifted from the European and
African plates and oceanic crust developed between them. The original posi-
57
tions and genesis of the fragments is poorly understood and were not discussed here, however by Late Jurassic time the fragments consisted of both
oceanic and continental crust. Early convergent motion was taken up by
subduction within oceanic terranes as B-type subduction, sometimes associated with antithetic or synthetic A-type subduction in adjacent continental
crust. It is evident that following collision, most of the structures which
remain preserved in the mountain belts are those developed by A-type subduction or rarely those developed by transform and extensional faulting
either along the boundaries or within the fragments. Thus, the record of
similar structures in oceanic terrane has been largely lost by subduction, or
altered beyond recognition by changes in plate boundary motion or collision
of continental fragments. Only fragmentary evidence of plate motion in the
oceanic terranes remains in abducted ophioli~s, melange, blueschist belts and
volcanic belts. Therefore, within a collisional belt, most of the critical evidence for deciphering fragment motion is lost and must be deduced by other
evidence. Even though, as suggested here, some volume of the lower continental crust was subducted, most of the upper lo-15
km was not and preserves the record of tectonic events. The time periods chosen to describe the
convergent evolution in the Eastern European Alpine system reflect the
emphasis on “significant” events recorded in continental crust.
From the timing of events at different fragment boundaries, changes in
fragment motion were not generally controlled by collision of continental
crust. For example, following collision of the Apulian and Rhodopian continental fragments in the Late Cretaceous, considerable A-type subduction, at
times associated with antithetic thrusting to the east, continued to Miocene
time, whereas large tracts of oceanic crust remained to be subducted in the
Carpathians, by the presumably mechanically easier process of B-type subduction.
Comparing the finite motion path of Africa relative to Europe as viewed
from a point on the European plate in the position of the present Western
Carpathians (from Dewey et al., 1973) with fragment motions deduced from
geology alone (shown in Figs. 3-10) only minor co~espondence is noted.
Jurassic extension, the onset of Late Jurassic convergence and general convergence from Late Jurassic to Recent time are reflected in the fragment
motions. However, most of the events cannot be read in more detail, and
some events such as the ~oun~r~lock~se
rotation of the Apulian fragment
in Late Cretaceous through Oligocene time, are opposite to the suggested
dextral movement of Africa relative to Europe during that time. Clearly the
evidence suggests that complex triple-junctions were present throughout the
region, and that the motion of the fragments was only loosely connected or
sometimes unconnected to that of the two major plates.
Many of the fragment boundaries have functioned as different types of
boundaries at different times. Structures along some former fragment
boundaries are exceedingly complex. For example, the Vardar zone probably
contains structures related to Jurassic extension, Jurassic and Early Cretace-
58
---.
/-/
/
\
\
\
Recent
Fig. 11. Recent microplate
boundaries
as defined by McKenzie
(1972) from fault
solutions.
Barbed lines = subduction
zones; double lines = spreading zones;single
fracture zones. Triangles locate recent volcanoes and crosses locate recent thermal
ity. The nature of the boundaries
along coastal Jugoslavia and through southern
slavia and Romania could not be resolved by McKenzie with available data.
plane
lines =
activJugo-
146
M. Kimmsriqion
Fig. 12. The finite motion path of Africa relative to Europe viewed from a point on the
European
plate located
approximately
in the Western Carpathian
(from Dewey et al.,
1973). Numbers are time in millione of years before the present and co rreaponding
series
or stage names are shown.
59
ous convergence, early Late Cretaceous transform movement, Late Cretaceous-Early Tertiary subduction and Late Tertiary transform movement and
extension. Furthermore, structures formed during a particular event were
probably not formed in their present relative positions. For example, the
position of Apulian fragment during pre-Albian ophiolite abduction, was
probably not directly west of Rhodopian fragment, but may have been far to
the south of it. Structures formed during different events and in relative positions different from today, may be juxtaposed during later events. For
example, in the Apuseni Mountains and the Southern Carpathians, north and
south directed thrust faults carrying pre-Mesozoic crystalline rocks formed at
two different times and now apparently root in a zone occupied by Late
Mesozoic oceanic rocks. These thrusts developed in early Late and Late
Cretaceous time, respectively, and were juxtaposed by latest Cretaceous
transform motion (Figs. 1,6, 7 and 9).
Belts of igneous rocks developed during convergent events are related to
both B- and A-type subduction. Volcanic rocks present among the oceanic
rocks along the Vardar zone suture were probably developed above B-type
subduction zones as were the Late Cretaceous and Miocene volcanic arcs
of the Balkan Mountains-Southern
Carpathians and Western and Eastern
Carpathians respectively. The Eocene--Oligocene age volcanic arc of the
Dinaric-Hellenic belt, however, may have developed from A-type subduction and may be geochemically different. These belts of igneous rocks added
heat to the crust, increased ductility at shallow depth and controlled the
location and detachment of thrust faults either antithetic or synthetic to the
main subduction.
Fragments of continental crust in the Eastern European Alpine system
were not rigid pieces of lithosphere, and during collisions they were
shortened, reduced in areal extent by A-type subduction, broken by the
transform and extensional faults and differentially rotated. During and following collision, the fragments were cut by new boundaries which broke
through older boundaries dividing combined fragments into new arrays.
Many of the boundaries are broad diffuse zones of faults, and it can be questioned if they represent boundaries at all, or just major faults in a continuously deforming anisotropic continental crust.
Present fragment size and shape cannot be used for paleogeographic
reconst~ctions
because of their earlier history of deformation. During their
formation, their margins were extended by crustal attenuation and during
convergence their margins and interiors were extensively shortened by
thrusting which involved not only the detachment of sedimentary cover
rocks but often the detachment of the upper part of the crystalline continental crust. Magnitude of thrusting is so large in some cases it does not appear
as if all the continental crust originally below the thrust sheets can be accom-
modated in a thickened sialic root. Some continental material may have been
subducted with the lithosphere.
Distinct phases of deformation within the Eastern European Alpine sys-
60
tern may represent
changes in the relative motion of the fragments and
shifting of deformation
from one boundary to another. The overall convergent system may be in continuous motion.
The Carpathian orocline is formed by the diachronous
moulding of the
Rhodopian
fragment around an irregular European-Russian
plate margin.
The southern convex-west loop formed from Albian to Late Cretaceous time
by collision of the Rhodopi~
and Moesian fragments. The northern convexeast loop formed from latest Cretaceous to Piio-Pleistocene
time by moulding of northern part of the Rhodopian fragment against the European-Russian plate which also included the Moesian and North Dobrogean fragments
at that time. The disrupted Apulian fragment to the west of the orocline and
the European-Russian
plate to the east are not oroclinally bent, The Rhodopian fragment which forms the orocline has been detached by thrusting from
the lower crust and mantfe and the entire orocline may be only 60 km wide
and 15-20 km thick.
The geologically relevant problem in the Alpine system is to work backwards from the geology alone and attempt to deduce the large plate motions.
Unfortunately,
geologic structures that remain preserved provide only minimum values for the magnitude of convergent, transform and extensional displacements. Because most of the relevant data from the oceanic areas is lost,
and those data which are preserved are a result of nonrigid behavior it seems
very unlikely that we may be able to deduce the large plate motions in the
near future. This remains, however, one of the significant challenges of the
Alpine system, for it is the only mountain system which has developed over
the period of time for which the spreading history of an ocean (the Atlantic)
permits us to check the geologically derived motions of Africa and Europe
against those derived from the Atlantic spreading history.
ACKNOWLEDGMENTS
This work has developed through study largely supported by the Eastern
European
Exchange Program of the National Academy of Science of the
United States and the National Academies of Yugoslavia and Romania. Additional support was supplied by the Walter B. Sharp Fund, Rice University.
The manuscript was reviewed by Dr. A.W. Bally whose help is here acknowledged.
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