1

The Periadriatic Lineament and the Role of Strike-Slip

Faulting in Alpine Tectonics

Bernadette Bastian 319990

E-Mail: bernadette.bastian@rwth-aachen.de

Abstract

The Periadriatic lineament stretches from the Po-Basin in Italy to northern Slovenia. This

fault system consists of many different smaller parts. The overall movement follows a dextral

strike-slip movement, which can be reconstructed by close observation of plutons in the area.

There are two main theories on the origin of the PL. One produced by Pomella which is the

only theory attempting to explain the occurrence of a partial sinistral movement in the larger

dextral strike-slip fault. It is based mainly on the uplifting of a Bathiloth. Laubscher puts

forward the idea of retrotranslation whose uncertainties increase exponentially after the third

phase.

1. Introduction

The Periadriatic lineament is a prominent

large-scale fault system (700 km long and

around 10 km deep) of the Alps. A

lineament will comprise a fault-aligned

valley, a series of fault or fold-aligned hills,

a straight coastline or indeed a

combination of these features. Other

features are fracture zones, shear zones

and igneous intrusions, which give rise to

lineaments [1]. The Periadriatic lineament

(PL) or Periadriatic fault system (PAF)

represents the tectonic boundary between

the Southern Alps, which have a weak

alpine structural and metamorphic

overprint, and the strongly affected

Northern Alps [4]; the lineament strikes

nearly E-W, from NW Italy to northern

Slovenia (Fig.1). The Northern Alps is a

general term for Western, Central and

Eastern Alps. The Periadriatic lineament

represents the contact zone between the

Eurasian plate and the Adriatic microplate,

which was part of the African plate but

some analysis show that the Adriatic

microplate dives further under the Alps up

to the Tauern window [9]. Near to the PL

lies the Tauern window, this geological

window shows older nappe units within

younger nappes.

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Fig.1: Overview of the Periadriatic lineament (http://de.wikipedia.org/wiki/Periadriatische_Naht 23.08.2013)

2. Trend of the Periadriatic lineament

The Periadriatic lineament is marked

through different kinematics, tectonic

settings and lateral valleys. The lineament

is subdivided into different parts of faults;

the generic term is Periadriatic lineament.

Segments, of this generic term, are from

west to east: Cremosina Line (Cr),

Canavese Line (Ca), Tonale Line (To),

Giudicarie Line (Ju), Pustertal Line (Pu),

Gailtal Line (Ga) and Karawanken Line

(Ka) (Fig.2) [9]. The Insubric line is also one

of the subunits, it dips down near to Turin

into the Po basin in the outside western

part. “The width of the main fault system

(PL) can vary locally up to several hundred

meters” [4]. The Periadriatic fault system is

a dextral shearing system with a

displacement of approximately 300 km [8],

other references evaluate around 120 km

[4] or maximum 200 km [9]. New

interpretations mean that the total

displacement is around 30 km (one theory)

or 100 km (second theory) [6].

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Fig.2: The prominent subunits of the Periadriatic lineament [9]

The plutons along the Periadriatic

lineament are another prominent feature.

These plutons (Granite- and Tonalite-

pluton) developed in a plurality during the

Oligocene. An intense magmatic activity

was recorded along the whole PL at this

time[5]. The molten material rose up at

weak zones along the Periadriatic fault

system and developed the plutons. These

intrusions are closely related to the

Periadriatic fault appear to have intruded

at a similar time (around 30-32 Ma [4]).

These Oligocene plutons follow the whole

length of the PL like pearls on a string.

These intrusive structures are very

prominent in the central part of the PAF.

Some plutons, like the Adamello intrusive

from the Oligocene, are thin, elongated

and foliated. The intrusion age of this

pluton varies from around 42 Ma in the

south to around 30 Ma in the north [4].

These plutons and their submagmatic and

solid-state ductile fabrics prove the

Oligocene age for dextral ductile

deformation [4]. They are shifted to the

dextral side along the PL and are an

evidence for the strike slip movement

along the Periadriatic fault system. There

are also other plutons which are

developed during the Permian and Tertiary

time, but the most important once are from

the Oligocene time.

The collective appearance of the

Periadriatic plutons denotes that they

aren’t “directly contolled by older

(Mesozoic) discontinuities, but rather by

the synmagmatic activity of” [6] the PL. The

Plutons along the Periadriatic lineament

and mylonites, which were formed in the

PAF, offered an overprinting relationship.

This item encourages, that deformation

processes were active “during and after

magma crystallization” [6].

The Periadriactic lineament is a very

complex fault system, which is difficult to

explain. In the following part comes a

rough overview about the evolution of the

Periadriatic fault system. The focus lies on

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the central fault system and the western

evolution of the Periadriatic system.

3. Evolution of the Periadriatic fault

system

3.1. Central and eastern part of the

Periadriatic lineament

The central part of the Periadriatic fault

system is built up of different fault

systems. Batches of shear zones ray out

from the PL, these shear zones separate

the crystalline in the southern part of the

Tauern window in big blocks. The blocks

have the same metamorphism and cooling

temperature. These fault systems, like the

DAV (Defereggen-Antholz-Vals fault),

branch out from the PL and have different

kinematics [2]. Big differences in alpine

metamorphic conditions are found along

the intra-Austroalpine Peio, Jaufen and

DAV (Defereggen-Antholz-Vals fault)

faults, which show these higher

metamorphic units on their N(W) side [4]

(Fig.3). The Periadriatic line, in this part

also known as Pustertal line (Pu), is

localized in this central part as a boundary

between Austroalpine Gneiss (crystalline)

and the undeformed Permian Grandiorite

[4].

Fig.3: Trend of the DAV line [4]

The PL developed through the collision of

the Adriactic microplate and the European

plate, because of that other faults

developed next to the PL, like the DAV.

The DAV fault is “taken as the

southernmost ductil alpine overprint of

Quarz-rich rocks” [4] and shows an

increasing of alpine overprint to the

Tauern window. The movement direction

of the DAV fault shows a sinistral

movement, which is different to the moving

direction of the Periadriatic fault system.

Some analysis of greenschist-facies

Mylonites at the DAV line shows a sinistral

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strike-slip movement in combination with a

subordinate component of uplift north of

the DAV [8]. There are also Biotites and

deformed dykes in the DAV line, which are

associated with Oligocene intrusives. The

movement of the DAV line has to be at the

same time like the Periadriatic fault

structure on the basis of these structures

and minerals (which are dated of around

late Oligocene) [8]. Radioactive dating was

the way to identify the age of the dykes

and minerals in the DAV line. The sinistral

shear sense of this and other faults around

the PL was identified with aid of

porphyroclasts (Fig.4) and microstrucutres

in the dykes. Other authors are of the

opinion that the DAV line was active the

first time during the Cretaceous [8].

Fig.4: Andalusite porphyroclast with a sinistral

shearing from the DAV line. Growing of Biotite and

Muscovite in the pressure shadow of the Andalusite

[4]

As previously stated in the upper part

there are some complex structures in the

central part of the PL, like the DAV line. In

the eastern part, the Karawanken line

(Ka), also follows a dextral strike-slip

movement. Kinematic studies are

important for the eastern part of the

nappes of the Alps. This texture-analytic

study for kinematic is best explained with a

transpression model: This model

represents lateral movements, horizontal

compression and vertically extension.

These factors overlay each other in a non-

coaxial deformation process [9]. This multi-

phase dextral transpression leads to a

lateral movement of continental crust

along the Periadriatic fault system [9].

The zone of maximum crustal thickness

and biggest MOHO-depth lies in the

northern border of the South Alps and

along the Periadriatic Lineament. This

knowledge is derived from refracting-

seismic analysis. This phenomenon (zone

of maximum crustal thickness) is explained

by a crustal doubling as a result of a low

crustal reverse fault. The upper crust

segment is from the southern part (Adriatic

microplate) and the bottom crust comes

from the east Alpin continental border

area. The dextral shearing sense of the PL

comes from the Adriatic microplate, which

was rotating against the clockwise

direction. This was discovered through

paleomagnetic analysis, though the time

for the key movement phase isn’t certain

for the PL. It could be an eoalpine

(Cretaceous) movement component

because of the connection between the

shearing-off of the alpine nappes and the

rotation of the Adriatic microplate [9].

Nevertheless, there are different opinions

about the initiation of the Periadriatic

lineament. Some scientist say that the

initiation of the PL itself was in the mid-

Oligocene at the same time with magmatic

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intrusions in the northern part, parallel to

the Periadriatic fault system [4]. The

Periadriatic lineament is marked by (back)

thrusting and dextral strike-slip; around 20

km [8] of vertical displacement of the

northern units has occurred by back

thrusting but the main displacement is

performed by dextral transpression. This

dextral strike slip deformation is

recognizable through Riedel shears,

slickensides (subhorizontal [9]) and

subvertical small scale fold axis at the

PAF. These structural indicators represent

only the final moving direction; there is no

inference to prior deformations. Some rock

analysis done by the Rb-Sr method [5]

proposed for the different faults that

comprise the PAF, a long history from Late

Cretaceous to Middle Miocene. Mylonites

along the Guidicarie line (Ju) (fault in the

central part of the PL with sinistral

shearing) demonstrate that top-to SE

thrusting, which developed simultaneous

with the dextral strike-slip movement in the

Oligocene (29-32 Ma) [5]. This thrusting at

the Guidicarie fault (Ju) cannot be

produced by later rotation of an existing

dextral strike-slip fault, the conclusion

being that there cannot exist any

combination of known kinematics and

direct age control (Rb-Sr ages from Biotite

(Oligocene)). The Biotite comes from thin

Tonalite “lamellae” along the Guidicarie

line (Ju), “which record magmatic flow and

solid-state deformation processes” [5]. This

indicates on different Mesozoic

sedimentary facies on both sides of the

Guidicarie line (Ju), that this fault

represents an inherited structure

developed by inversion of a

“synsedimentary normal fault” [5] of

Mesozoic age. That could be the reason

for the sinistral strike-slip movement of the

Guidicarie line.

The Guidicarie line can be subdivided into

the northern Guidicarie and the Meran-

Mauls fault. These two faults get

summarized under the term tonalitic

lamellae or tonalitic lenses [6]. “Along the

boundaries of the tonalitic lenses, ductile

deformation can be observed” [6]. These

faults were analyzed by magnetic fabric

and structural field data. The evaluation

offered a dextral strike slip deformation at

the northern part of the Guidicarie fault

and an overprint (younger down-dip

stretching) at the Meran-Mauls fault [6]. The

Meran-Mauls fault shows a change in

deformation from dextral strike slip to top-

SE thrusting, “this may be caused by a

rotation or bending of the fault after the

intrusion of tonalities” [6] or the

emplacement of the lamellae. The tonalitic

lamellae (Guidicarie fault and Meran-

Mauls fault) could be interpreted as

lenses, which were sheared off from the

Adamello batholith. This thesis is based on

zircon ages from the lamellae and the

batholith pluton. Some of the lamellae

indicate Oligocene or late Eocene age.

The Adamello pluton intruded in the

Eocene which connects Oligocene and

Eocene through the formation of tonalitic

lamellae [6].

Zircon (northern side) and Apatite

(southern side) took out from the Periadric

fault system and the age was analyzed by

radiometric dating. Both minerals have the

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same age of 24 Ma and were taken from

the same elevation [5]. The vertical

displacement since 24 Ma can be

appreciated by the geothermal gradient

and the differential in closure temperature

for the two minerals. The solution gives an

uplift of around 4 to 5 km of the

Austroalpine in contrast to the Southern

Alps since 24 Ma [5]. Linked to the

Periadriatic fault are Riedel faults (both

synthetic as well as antithetic) especially

synthetic Riedels and dykes with

phenocrysts inside, which consist of

Feldspar, Muscovite, Biotite and other

minerals. The dykes are variable

deformed, but the phenocrysts into the

dykes demonstrate the shear sense like

shown above at the DAV (the phenocrysts

have a dextral shearing component in this

part).

The relevant building process for the

strike-slip faulting at the Periadriatic fault

system was the movement of the alpine

belt, which swings into the N-S oriented

strike of the western Alps, the westward

motion of the Adriatic microplate was

considered responsible for EW directed

shortening in the western Alps [8].

There are two new theories for the dextral

and sinistral shearing in the central part

(especially the Guidicarie line) of the

Periadriatic lineament from Pomella:

First model: This model supposes that the

PAF was originally curved, where the

sinistral transpression (Neogene) was

“triggered by an inherited Early Permian to

Lower Liassic NE-SW trending horst and

graben structure” [7]. This theory sees to it

that the sinistral adjustment gets reduced

at the Guidicarie line during the Miocene.

Through this process, the complete PAF

dextral strike slip movement is restricted to

around 30 km [7].

Second model: this theory assumes that

the origin of the PAF was straight and the

“sinistrally offset” [7] on the Guidicarie line

during the Miocene “due to the NNW-ward

advancement of the Southalpine indenter.”

[7]. This model permits for the total PAF to

have a larger dextral strike slip component

of around 100 km. This model can be

subdivided into three parts (Fig.5):

The first part starts (A) in the Eocene to

Oligocene age. The Batholith pluton

(northeastern unit) begins to intrude into

“the south of the straight, dextral strike-slip

PAF. The northern rim of the batholith was

becoming dextrally sheared.” [7].

The second part (B) begins in the late

Oligocene to earliest Miocene [7]. The

Southern Alps start to move to the NNW-

ward direction. This movement leads to a

bending of the fault and a NE trending of

the northeastern Adamello tail.

The third part (C) developed during the

early Miocene. The Meran-Mauls

basement was uplifted through the

thrusting of the Meran-Mauls fault. The

Tauern window developed during the

same time. It is only in this part of the PL,

probably due to the priorly mentioned

uplift, that sinistral strike-slip movement

occurs.

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Fig.5: Sketch of the evolution of the Guidicarie

line [7]

MMF= Meran-Mauls fault; PGF= Pustertal-

Gailtal fault; SGF= Southern Guidicarie fault;

NGF= Northern Guidicarie fault; PF=

Passier fault; JF= Jaufen fault; BF= Brenner

fault.

In the next step we focus on the western

part of the Periadriatic fault system and

are looking for a different evolution model

for the western part of the PL.

3.2. Western part of the Periadriatic

lineament [3]; [7]

The western part of the PL is explained by

a retrotranslation thesis from Laubscher.

Laubscher firms up his model of earlier

analysis from Argand. The new model

uses a step-by-step retrodeformation of

the western Alps; it starts with the latest

movement and goes to the oldest motions:

The youngest part in the western PL is the

belt of dextral ordered crustal folds from

the Belledonne to the Aar massif (Fig.6).

Their total shortening is too small (around

30 km) in comparison with the total alpine

kinematics, so there was little to no

attention paid to this. At the southwestern

part of the Arc of the western Alps clear

movement took place during the Miocene

to Quartenary. This northward movement

of thrusts shows a displacement of around

tens of kilometers.

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Fig.6: First stage movement of the western Periadriatic lineament [3]

The second step is in the middle Miocene.

During this time, there was a movement in

the southern Alps and especially in the

subsurface of the Po basin (Fig.7).

In middle until late Miocene was a

shortening on the order of 100 km. This

process was developed by the Lombardic

thrust system. Addionally the Lombardic

thrust system swings towards north into a

transpressive belt and its western end

wedges out close to the Western Alps.

This thrust system implies two sinistral

transfer zones. In the northeastern part

lies a segment of the Insubric line (western

part of the Periadriatic fault system), which

now passes straight into the Pustertal

(Pu)-Gailtal (Ju) segment. In the

Fig.7: Second phase of the retrotranslation of the

western part of the PL during the middle to late

Miocene [3]

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southwestern part lies the only segment in

the Miocene, which separates the pre-

Oligocene Alps from the early Apennine.

The third phase is the Oligocene and early

Miocene phase (Fig.8). It’s also called the

Insubric-Helvetic phase. This phase is

dextrally transpressive at the northern

boundary of the Adriatic microplate. There

is no clear evidence for normal (Fig.9) and

strike slip components (Fig.8), but it is

supposed that there were these

components. These two structural

elements were active at the same time.

Two zones (Ivrea- and Pejo-zone) are

obvious elements, which have a dextral

displacement of around 150 km during the

pre-Mesozoic time. There is a second

type, the Dent Blance and Margna nappes,

which have a dextral displacement of 150

km. These two examples make sure that

the main motion was dextral slipping of the

early Miocene. This dextral strike-slip

movement along the Insubric line followed

the Rhone valley branch. There was a

collapse phase which arose through the

westward transfer of the Adriatic

microplate during the Oligocene. Through

this event, the Piemont-Liguria Tertiary

basin, Sesia basement and the late alpine

intrusions are lining up along the Insubric

fault system.

Fig.8: Partly retrotranslation of the Adria indenter in

the late Oligocene to early Miocene. During this

period of time is a strike slip component

recognizable [3]

Fig.9: Supposed normal component during the

retrotranslation of the western PL [3]

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The Liguria Alps were a part of the

Sardinia-Corsica-Liguria complex of a

continental block that was separated from

the Eurasia and Iberia by the Baleares-

Provence-Ligurian sea collapse (Fig.10).

The Sardinia-Corsica-Liguria complex was

another microplate next to the Adriatic

microplate. The new microplate complex

moved in northern direction like the

Adriatic microplate. In figure 9 is an

overview of the movement directions of

the Liguria complex and the Insubric-

Helvetic-Toscanide motion.

The next phase is the Eocene Penninic

phase. In this oase was the central

Penninic domain covered by higher

nappes after the deposition of the Middle

Eocene Flysch. During this time, the

complete Ultrahelvetic-North Penninic-

Middle Penninic complex of the central

Alps moved to the southern part of the

Helvetic area. It seems in the same time

that the Western Alps appear to demand a

sinistral rotation of the Penninic belt of

around 90°. This is equivalent to around

300 km EW translation. The sinistral

rotation of the Penninic belt is compatible

with the sinistral transpression of the

Adriatic indenter (along the western edge

of the NW-moving). This could lead to the

adoption that the whole Eocene

deformation may have been these two

rotations and an unknown NW translation

of the Adriatic indenter. The late Eocene is

characterized by a triple point where the

European, Iberian and Adriatic plates or

microplates come together (Fig.11).

Fig.10: Overview of the continental blocks and

moving direction during the Oligocene to early

Miocene age [3]

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Fig.11: Retrotranslation in the Eocene phase [3]

The following step is the earliest phase of

alpine kinematics. It’s difficult to

reconstruct this part of the alpine evolution

because the earlier stages have been too

strongly destroyed. Laubscher tried to

reconstruct the early Cretaceous starting

point from a plate tectonic model, which

reconstructs the last situation (mentioned

above). He used the model for Dewey et

al. (1973) to rearrange the plate motions

and alpine kinematics. There are other

models too and all (include Dewey et al.

1973) have in common a large sinistral

translation in the Mediterranean area until

80 Ma. The biggest difference is in the

kinematics from 80-0 Ma. Laubscher

suggests that the Dewey (1973) model is

close to the retrotranslation of its own

analysis. The plate tectonic data needs a

forward modeling from a supposed late

Jurassic situation “then leads on two

generalized steps to the Eocene

configuration arrived at before by

retrodefromation.”[3]. An important event

between the Jurassic and Eocene

situation is the opening of the North

13

Atlantic (late Cretaceous). This event

leads to a quick change of the

convergence direction between Africa and

Eurasia. Dewey et al. (1973) created a

change from necessary sinistral strike-slip

to dextral transpression. This dextral

transpression leads to a perpendicular

convergence from the late Creataceous

after Dewey et al. (1973).

Laubschers model of retrodefromation

works well for the first and second phase

but off the third phase the uncertainties

increase. The retrotranslation thesis is a

good guideline through the complex

evolution of the western part. But it has to

take care at the later retrotranslation

processes.

4. Conclusion

The Periadriatic fault system is a very

complex system in the alpine history. This

report includes but a rough overview of the

Periadriatic lineament and its subunits.

There are many theses about the

evolution of the different fault systems and

their fault movements and shear senses.

In general the Periadriatic lineament is a

fault system (tectonic boundary) consisting

of many smaller faults, which separates

the northern part of the Alps from the

Southern Alps. The Periadriatic fault

system strikes approximately in E-W

direction and has a dextral strike-slip

component.

The explanation for the Western Alps from

Laubscher is a method in which all phases

are linked together to reconstruct the

complete history of the western part.

The preferred model is the second model

from Pomella for the development of the

central part of the Periadriatic lineament.

Evidence for an originally straight PAF is

given by the lamellae. These can be seen

as magmatic bodies, which were “sheared

and scraped off from the eastern side of

the Adamello batholith during the activity”

[6] of the Guidicarie line. Another proof is

the displacement of the Periadriatic

lineament; the displacement factor is

higher than 30 km (compared to the first

model), making the second model more

realistic.

5. References

[1] Attoh, K. and Brown, L.D. (2008). The

Neoproterozoic Trans-Saharan/Trans-

Brasiliano shear zones: Suggested

Tibetan Analogs. American

Geophysical Union, Retrieved 2011-01-

31

[2] Decker, K.; Peresson, H. and Faupl, P.

(1994). Die miozäne Tektonik der

östlichen Kalkalpen: Kinematik,

Paläospannungen und

Deformationsaufteilung wärend der

“lateralen Extrusion” der Zentralalpen.

Jahrbuch der geologischen

Bundesanstalt, Band 137 Heft 1, p. 5-

18

[3] Laubscher, H. (1991). The arc of the

Western Alps today. Eclogae

Geologicae Helvetiae, Vol. 84, No.3

14

[4] Mancktelow, N.S.; Stöckli, D.F. et al.

(2001). The DAV and Periadriatic fault

system in the Eastern Alps south of the

Tauern window. International Journal

Earth Science, Vol.90, p. 593-622

[5] Müller, W.; Prosser, G. et al. (2001).

Geochronologiacal constrains on the

evolution of the Periadriatic Fault

System (Alps). International Journal

Earth Sciences, Vol.90, p. 623-653

[6] Pomella, H.; Klötzli, U. et al. (2011).

The Northern Guidicarie and the

Meran-Mauls fault (Alps, Northern Italy)

in the light of new paleomagnetic and

geochronological data from boudinaged

Eo-/Oligocene tonalites

[7] Pomella, H.; Stipp, M. and

Fügenschuh, B. (2012).

Thermochronological record of thrusting

and strike-slip faulting along the

Guidicarie fault system (Alps, Northern

Italy)

[8] Schmid, S.M.; Fügenschuh, B. et al.

(2004). Three lithospheric transects

across the Alps and their forelands.

Springer Verlag, The transmed Atlas:

The Mediterranean Region from Crust

to Mantle

[9] Sprenger, W.L. (1996). Das

Periadriatische Lineament südlich der

Lienzer Dolomiten. Abhandlungen der

geologischen Bundesanstalt, Band 52,

p.1-220