Download TOPIC:EVOLUTION OF THE SOUTHERN ALPS AND DOLOMITES

Paper related to the Alps Excursion
Lecturers: Prof. Dr. Janos Urai & Prof. Dr. Ralf Littke
By Jan Gronewald (322018) – Summer term 2013
TOPIC: EVOLUTION OF THE SOUTHERN ALPS AND DOLOMITES
Content:
1. Introduction
2. Pre-Alpine geodynamic evolution of the Southern Alps
3. Orogeny of the Southern Alps & Dolomites
4. Conclusion
References
1
Abstract
This paper will illustrate the essentials concerning the evolution of the Southern Alps and
Dolomites since the post-Variscan conditions up to their present setting. Late and postHercynian, Lower Permian extensive magmatism (e. g. Bolzano quartz porphyry with a
thickness up to 2 km) combined with those of Middle Triassic age, well exposed in the area
of the Dolomites, determined the development of the future Southern Alps during its
subsequent structural & sedimentary evolution in Mesozoic and Tertiary times by
strengthening the upper crust. At the beginning of the convergence evolution (Late
Cretaceous) of the Alps only the central and western parts of the Southern Alps were involved
producing pure compression in the area of the present Bergamasc Alps. The following
mesoalpine deformation phase (Eocene) generated thin-skinned tectonics in the eastern
Southern Alps by forming NW-SE trending overthrusts, which were supported by ramp-flat
systems (principle of a décollement). This formation is considered to be the front of the
Dinarids which were deformed at the same time. During the neoalpine deformation phase
(Late Oligocene-Neogene) the Giudicarie fault or rather Periadriatic fault was formed,
accompanied of the NNW-movement of the Southalpine indenter and creating primarily
south-vergent thrusts involving the basement throughout the Southern Alps. The Southern
Alps, representing the retrowedge of the Alpine orogeny, are generally not that deformed as
in the nappe stack in the north of the Periadriatic seam.
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1. Introduction
The Southern Alps of which the Dolomites are an integral part, are located southwards of the
Periadriatic fault system (PAF) which represents a separation to the Western-, Central- and
Eastern Alps in the north (Figure 2). The striking PAF is approximately 700 km long and
reaches from north-western Italy to Slovenia (POMELLA
ET
AL. 2012). Smaller and bigger
granitic-tonalitic plutons got towards the surface inside of the fault plane and in its near
(BÖGEL 1976). Although rock formations are similar north- and southwards of the PAF, the
tectonic conditions are very different. Large-scale fold and nappe structures like in the north
as well as young metamorphism like in the Eastern Alps cannot be recognized in the Southern
Alps (BÖGEL 1976). The Southern Alps represent the retrowedge of the upper plate of the
Alpine subduction (BERNOULLI 2007). During the alpine evolution south-vergent thrusts were
created parallel to the strike of the older, Mesozoic continental margin units which are still
in their original palaeotectonic position relative to each other (BERNOULLI 2007). These
compressed passive margin sequences were deposited on top of a hercynian basement
before (DOGLIONI 2007).
The Southern Alps belonged to the Adriatic microcontinent and have a lot of, during
Mesozoic created, common features with the external Apennines, Dinarides and their
Adriatic foreland (BERNOULLI 2007).
2. Pre-Alpine geodynamic evolution of the Southern Alps
Within the Southern Alps Permo-Mesozoic, non- or locally anchimetamorphic sediments
were deposited on top of the partially varying Variscan basement. Between the basement and
the overlying sediments is an unconformity of the Late Carboniferous (Westphalian). The
Palaeozoic sediments are characterized by an increasing grade of metamorphism from east
to west. The metamorphism increases from deep burial diagenesis and anchimetamorphism
in the external Variscan zone in the east (Carnic Alps) to amphibolite-grade in the internal
zone (Strona-Ceneri zone) in the west. Deeper parts of the Variscan basement contain
radiometric signatures of a possibly Proterozoic (“Cadomian”) orogeny and of Ordovician
(“Caledonian”) metamorphism and granite intrusion (BERNOULLI 2007).
3
Figure 1: Palinspastic east-west cross-section of the Early Permian crust of the Southern Alps with U-Pb zircon (Zr) and allanite (All) age data;
undifferentiated upper crustal metamorphic basement (grey area) and little to non-metamorphic pre-Moscovian successions of Carnia (light grey)
are indicated schematically. Red colours mark plutonic and volcanic rocks; non-volcanic sedimentary basin fills comprise mainly fluviolacustrine
clastic rocks (light blue) coarse alluvial deposits (yellow) and in Carnia, marine carbonates and clastics (dark blue); (SCHALTEGGER & BRACK 2007)
After the Variscan orogeny extension
occurred and led to the development of
sedimentary basins and silicic volcanism
during the Early Permian (Figure 1). The
basin formation and magmatic activity in the
South Alpine upper crust took place
between 285 and 275 million years ago
(SCHALTEGGER & BRACK 2007).
Silicic
magmatism
and
mafic
intrusions in the lower crust of the Ivrea
Zone were coeval. High heat flows generated
by granulite-facies metamorphism, as well
as magmatic underplating by basaltic melts
enhanced the generation of silicic melts in
the lower crust and migmatization. These
melts rose towards the surface creating
large caldera complexes (Bolzano and
Lugano “porphyries”) and shallow plutonic
bodies (Baveno Granite) (BERNOULLI 2007).
Another
effect
of
the
Permian
magmatism was the further reconstitution
of the crust and reequilibration of the
petrological
crust-mantle
boundary
at
approximately 30 km after the Variscan
orogeny (HANDY
ET AL.
1999). The Early
Permian extension and magmatism might
represent the first rifting phases which
induced the opening of the Alpine Tethys
(BERNOULLI 2007).
Permian
Furthermore Lower
magmatism
and
structural
characteristics of the Southern Alps seem to
4
have a linkage to a phase of crustal-scale extension. The short time span of magmatism and
the hiatus in sediment accumulation are compatible with the generation of magma during
the initial transtensive phase of continental-scale strike-slip (SCHALTEGGER & BRACK 2007).
Within the mid-Permian an unconformity exists with 10 to 15 million years of nondeposition, so that erosion played the major role by levelling volcanic edifices of
Borgomanero, Lugano and Bolzano. Due to the transgression of the Palaeo-Tethys from the
east a sedimentary wedge developed on top of the Lower Permian deposits during the Late
Permian. The ingression from the east reached central Lombardy in the late Early Triassic
and the western Southern Alps in early Anisian times (BERNOULLI 2007).
This was followed by varying subsidence rates and local uplift during early-middle
Anisian. Subsequently, from late Anisian-early Ladinian times, there was massive subsidence
creating carbonate buildups of the Dolomites (and the Bergamasc Alps). Estimates about the
maximum accumulation rates declare ~ 650 m/106 years to ~ 850 m/106 years during Late
Ladinian times (EMMERICH ET AL. 2005). Deep basins between these atolls were infilled by
pelagic, mass flow and volcanic deposits (BERNOULLI 2007). In addition the basins were locally
inverted by sinistral transpressive movements along the N70°E axis forming flower
structures within the basement in the Late Ladinian (DOGLIONI 1987).
Due to a distinctive sea-level-fall and the input of siliciclastic and volcanic detritus the
activity of carbonate platforms decreased and led to accumulations of first terrigenous,
evaporitic and lesser carbonate sediments (Raibl Group) and subsequently peritidal
dolomites (Hauptdolomit). These sediments belonged to a larger continental margin wedge
on top of local basement highs and represented an interlocking with the Germanic facies
realm to the north and west bordering the Maliac-Meliata Ocean to the east. The continental
crust of the western Southern Alps might may have been close to isostatic equilibrium at the
end of the Triassic, whereby thermal subsidence of the western Maliac-Meliata margin may
have been the reason for a continuing subsidence (BERNOULLI 2007).
During the late Norian up to approximately 10 km wide basins developed which
contained thick-bedded, fine-grained, organic carbon-rich limestones and dolomites.
Extension was still present in the Rhaetian because of a varying sedimentation process, but
the activity of extension began to decrease, because during the Early Liassic strain was still
concentrated along a few major crustal faults. Normal faults were sealed by the prograding
5
Hauptdolomit platform. While cooling and embrittlement of the thinned crust, extension
began to concentrate in the western parts of the Southern Alps. Lower crustal (Canavese
zone) and mantle rocks (Piemont zone) were exhumed to the seafloor within a system of lowangle detachment faults (BERNOULLI 2007; BERTOTTI ET AL. 1993).
The post-rift evolution of the South-Alpine margin was influenced by prolonged
thermal subsidence contemporaneous with the opening of the Liguria-Piemonte segment of
the Alpine Tethys. The margin reached bathyal depth due to the progression of deep oceanic
facies on top of the distal margin in the west (Canavese zone, Lombardian basin). The Trento
High, which is part of the recent Dolomites and remained as a carbonate platform into the
early Middle Jurassic, was flooded creating an isolated, current-swept plateau. The Friuli area
persisted as Bahamian-type carbonate platform throughout the Cretaceous. The increasing
water depth from the Middle Jurassic to the Early Cretaceous enabled the deposition of
Middle-Upper Jurassic radiolarites (BERNOULLI 2007).
While Early Cretaceous times the accumulation of sediments in the Southern Alps was
influenced by the orogeny of the Eastern Alps.
3. Orogeny of the Southern Alps & Dolomites
“At the end of the Early Cretaceous the Eurasian and African plates inverted their motion and
the continental margin convergence controlled the subsequent evolution of the Alps up to
their present setting (CASTELLARIN ET AL. 2006).” The convergence evolution of the Alps can be
separated into the Late Cretaceous pre-collisional, the Eocene collisional and the Late
Oligocene-Neogene post-collisional (neoalpine) compressional events (TRÜMPY 1973).
The onset of the Cretaceous convergence took place in Aptian-Albian age and can be
indicated by the clastic mostly pelitic input presented throughout the Southern Alps, as well
as coeval by the Early Flysch in the northern Dolomites (CASTELLARIN ET AL. 2006).
During the eoalpine deformation (Figure 2 & 3) only the central and western parts of
the Southern Alps seemed to be influenced. Sigma 1 (N0-30°W) produced pure compression
in the western part (Bergamasc Alps) and sinistral transpression in the central sector
(Giudicarie Belt), so that elongated horst and graben structures (flower structures)
6
7
Figure 2: Simplified map of the Southern Alps indicating the outstanding number of south-vergent thrusts and the result of the different compression
phases; (CASTELLARIN ET AL. 2006)
were created (DOGLIONI & BOSELLINI 1987). The paleo-Giudicarie lineament, which can be seen
as a regional and wide sinistral transfer structure, led to a displacement of some 50 km
(CASTELLARIN ET AL. 2006). The frontal zone of the south-vergent Alps (retrowedge part of the
orogen) was only dozed to the south in the western Southern Alps in the west of the
Giudicarie lineament during the Late Cretaceous-Early Eocene, because the magmatic events
in the area of the Dolomites consolidated the upper crust making this sector robust
(CASTELLARIN ET AL. 2006).
Figure 3: Sinistral transpression along the Giudicarie lineament (GT)
during Late Cretaceous-Early Eocene; pure compression within the
Bergamasc Alps (PA: Periadriatic belt, G: Gallinera thrust);
(CASTELLARIN ET AL. 2006)
The model of CASTELLARIN ET AL. (2006) takes the assumption that the Periadriatic fault system
(PAF), whereof the Giudicarie lineament or rather the Giudicarie fault system (GFS) is only a
small part, was originally curved. Whereby the Neogene sinistral transpression is triggered
by the Early Permian to Lower Liassic NE-SW trending horst and graben structures.
8
Therefore the sinistral offset on the GFS in the
Miocene gets reduced and limits the dextral strike
slip displacement on the whole PAF to nearby 30 km
(POMELLA 2012).
The model of POMELLA (2012) supports
another assumption with Miocene zircon fission
tracks ages within a sector along the Northern
Giudicarie fault (NGF). This gives an evidence for a
polyphase deformation along the GFS beginning
with an originally straight dextral strike slip PAF
(Figure 4). The PAF was curved along a NE-SW
orientation to the east of the Adamello batholith
while the NNW-ward movement of the Southalpine
Indenter. This bent sector of the PAF is today
represented by the Meran-Mauls fault (MMF). The
MMF emerged from the E-W striking Tonale fault
(TF)
with
the
assistance
of
the
sinistral
transpressive NGF during the Miocene.
Within the Middle Eocene-Lutetian the
mesoalpine deformation phase or rather “Dinaric
compression”
became
apparent
and
was
responsible for NW–SE (sigma 1 N50-90°E)
trending thrusts (overthrusts & folds) in the eastern
Southern Alps. This led to thin-skinned tectonics in
the area of the Dolomites, because only the
sedimentary cover seems to have been deformed.
The basement might have been involved east of
Friuli, in the Dinaric chain (DOGLIONI & BOSELLINI
1987). The sedimentary cover was shortened by 1015 km and ramps developed at the rims of Triassic
Figure 4: Evolution of the GFS; (a)
Oligocene: intrusion of the NE-units of the
Adamello batholith & dextral strike-slip
PAF; (b) Late Oligocene/Early Miocene:
NNW-ward movement of the Southalpine
indenter (large arrow); (c) situation
nowadays; (POMELLA 2012)
carbonate platforms (DOGLIONI 2007).
9
The ramps occurred along the coral reef slopes made of clinoforms, which were built during
the progradation of the Triassic carbonate platforms. These ramp features and the presence
of evaporitic or marly-shaly horizons (Bellerophon, Raibl Group) have influenced the
tectonics of the Dolomites. The sedimentary characteristics of the evaporitic & marly-shaly
horizons supported the process of thin-skinned tectonics during the “Dinaric” compression
and acted as “flats”. These ramp-flat systems (Figure 5) were responsible for a great number
of “summit” overthrusts. Due to subsequent erosion the summit overthrusts were formed to
tectonic “klippen” (Figure 5), not because of gravitational phenomena (DOGLIONI 1985).
Figure 5: Schematic cross section of a ramp-flat system showing the evolution of
a summit overthrust, or a klippe (K) within the Dolomite region; the interrupted
lines indicate the parts that have been eroded; (DOGLIONI 1985)
The whole Southern Alps including the basement were affected with sigma 1 (N0-30°W)
during the neoalpine compression within the Late Oligocene-Neogene. South-vergent thrusts
(e.g. Val Tompia, Orobie, Valsugana) were the result (DOGLIONI & BOSELLINI 1987).
Most of the recent elevation of the Dolomites was produced during the Neogene
deformation, i. e. within the last 10 million years. The compressive stress during the
neoalpine compression is perpendicular compared to the E-W or ENE-WSW compressive
stress of mesoalpine age. Consequently the approximately west-vergent mesoalpine
overthrusts in the eastern Southern Alps have been faulted and folded by later south-vergent
overthrusts. This led to a superposition of two Alpine tectonic phases and the corresponding
recent elevation (DOGLIONI 1987).
The Dolomites and Venetian Alps experienced a minimum total shortening of 40-50
km. The Dolomites themselves are located within an approximately 60 km wide pop-upsynclinorium, influenced by the south-verging Valsugana thrust and its conjugate north10
verging thrusts (Funes-Line). The push-up of the Dolomites is due to the wide syncline
including the sedimentary cover of the Dolomites, which aligned itself by flexural slip and
flexural shear to the folding of the subjacent basement (DOGLIONI 2007). The basement has
been moved southwards for at least 8-10 km, uplifting the Dolomites by 3-5 km (DOGLIONI
1987).
A more detailed and by zircon fission track dated neoalpine structural feature is the Permian
Ifinger Granodiorite which is overthrusted onto the Southalpine basement (Figure 6),
provided with a Variscan metamorphic overprint and a Permomesozoic cover. The Ifinger
pluton is orientated between the NW dipping Meran Mauls fault (MMF) (Figure 4 & 6), which
is bordering to the Texel complex of the Eastern Alps in the north, and the parallel trending
Naif fault (NF; mean orientation 320/45) (POMELLA 2012). In the contact zone between the
Ifinger Granodiorite and the Permian volcanic rocks (Bolzano quartz porphyry) or rather
volcanoclastic sediments near the surface, cataclasites representing the brittle fault (NF) can
be seen. The fission track data of the Naif fault is a sign for top to SE thrusting with an age of
Oligocene to Miocene for its activity (POMELLA 2012).
Figure 6: Cross-section of the S-vergent overthrusting Ifinger pluton in contact with the South Alpine
basement in the border zone of Southern and Eastern Alps along the Periadriatic fault system; (MMF) Meran
Mauls fault; (NF) Naif fault; (POMELLA 2012)
4. Conclusion
The future Southern Alps experienced three different phases of extension after the Variscan
orogeny during its Permo-Mesozoic evolution:
(1) Post-Variscan extension in the Early Permian with high-temperature, granulite facies
metamorphism, as well as magmatic underplating triggered by basaltic melts in the
11
deep crust, and basin development, shallow intrusion of granitoids and volcanic
extrusions (e. g. Bolzano quartz porphyry; Figure 6);
(2) Extension, which might be combined with transtension and transpression, and
shoshonitic igneous activity during the Middle Triassic in the eastern Southern Alps,
and might be connected with the genesis of the young Maliac-Meliata Ocean to the
east;
(3) Late Triassic to Middle Jurassic extension that was responsible for the subsequent
development of a new passive continental margin, which was characterized by basins
and plateaus and the opening of the Alpine Tethys to the west.
The magmatic events consolidated the upper crust, so that it is less deformed and shortened
in the whole Southern Alps.
During the convergence evolution three main compression phases influenced the
development of the Southern Alps:
(1) Eoalpine
deformation
(Late
Creataceous-Palaeogene
p.p.)
producing
pure
compression in the north-western parts of the Southern Alps (Figure 2);
(2) Mesoalpine deformation or “Dinaric compression” (Middle Eocene-Lutetian)
generating NW-SE trending overthrusts or rather summit overthrusts (ramp-flat
systems, Figure 5) in the region of the Dolomites which were formed into tectonic
klippen after erosion;
(3) Neoalpine deformation (Late Oligocene-Neogene) which affected the whole Southern
Alps involving the basement, as well as the NNW-movement of the Southalpine
indenter (Figure 4) and the development of the Giudicarie fault as part of the
Periadriatic fault.
12
References
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