The Middle Triassic magmatic-tectonic arc development in the Southern Alps

Tecronophwx,

79

146 (1988) 79-89

Elsevier Science

Publishers

B.V., Amsterdam

- Printed

in The Netherlands

The Middle Triassic magmatic-tectonic arc development in the Southern Alps A. CASTELLARIN ’ Dipartimento

I, F. LUCCHINI

2, P.L. ROSS1 2, L. SELL1 ’ and G. SIMBOLI 2

do Scienre Geologiche, Via Zamboni

.’ Dipartirnento di Scienre Mineralogiche, (Received

67, Uniuersitci do Bolognrr-I. 40127, Bologna IIt&)

Piazza dr porta S. Donate I, Uniuemrd di Bolognu-I, 40127 Bologna (II&) March

30, 1987: accepted

May 15, 1987)

Abstract Castellarin

A., Lucchini

development

F., Rossi

in the Southern

P.L.. Selli L. and Simholi,

G., 1988. The Middle

Alps. In: F.-C. Wezel (Editor),

The Origin

Triassic

and Evolution

magmatic-tectonic

arc

of Arcs. Tectonophysics.

146: 79989. The geodynamic uncertain,

The main

Dolomites. assumed

meaning

of the Middle

problem

is the significance

In fact these structures by several authors.

or not seriously more modem

taken into account,

a Middle Triassic

thrust

lends more credence

The magmatic talc-alkaline character

trends

recognized uncommon

setting.

petrochemical Southern

to the compressional

is rather

geodynamic

or considered

system located

Furthermore

affinity,

Alps, owing

tectonic transcurrent

events

tectonic

in the Southern

structures

geodynamic

elements

interpretation

transpressive

Middle Triassic

in the crustal

Triassic

the time-space ensialic

rifting) ignored

tectonics.

distribution volcanic character,

rocks display

an “erogenic”

by rifting

but

is more

of the South Alpine arcs.

Nevertheless

character,

and Hellenic

in accord magmatic

the general

does not fit the oceanic

with

Other of

Dolomites)

The geodynamic meaning of the Middle Triassic magmatic and tectonic event in the Southern Alps is still uncertain. The main problem deals with the significance of the compressional structures recognized in the central Dolomites (Pisa et al., 1980). These are incompatible with the extension (crustal rifting) admitted by several authors

with typical

domains.

Such a

a compressional

rocks, coupled geodynamic

with their

context

of the

model.

The aim of this work

is to review

the more

recent geochemical, petrological and geological data in order to attempt a geodynamic interpretation of Middle

Triassic

magmatic-tectonic

events.

1. Magmatism Magmatism of Middle Triassic age is widespread in the Alpine chain as well as in the Dinarides and in the Hellenides. In the Southern Alps (Fig. l), it started during Late Anisian-Early Ladinian times first with acidic volcanics (rhyolites, rhyodacites) in the eastern (Tarvisio-Lucchini et al., 1980) and southern (Recoaro) areas, and then with basalts and basaltic andesites in the

to explain the coheval magmatism. So far, the compressional elements of the Dolomites have either been mostly ignored or overlooked, or considered as local phenomena reflecting a volcanictectonic interplay (“ vulcano-tettonica”-Leonardi, 1965). G 1988 Elsevier Science Publishers

interplay.

Recent discovery

east of the central

Alps as well as in the Dinaric

evolution

are close to the modern

(crustal

event.

Middle

across the whole Southern

is still

in the central

have been mostly

due to the volcanic-tectonic

and/or

Alps

recognized

of the Dolomites

in the Piave River area (some tens of kilometres

Introduction

0040-1951/88/$03.50

tectonic

as local phenomena

on regional

and plutonic

to their pronounced

and

with the extensional

the compressional

are founded

series of volcanic

magmatic

of the compressional

are incompatible

Therefore

interpretations

Triassic

B.V

MIDDLE Early

TRIASSIC Late

MAGMATlC

ROCKS -

c

Modern boundary rhyoltte -andeslte and basalts

(a) basalt (shoshonite)

rhyoliteandeslte

occupied

volcanics

northern

mainly

magmatic

by uppermost

occurrences Ladinian

with an early start of the volcanism

(Dolomitic)

of “monzonitic”

Presumed locatron of the ltmlt (a) before Tertiary compresslow

( data AGIP 1977.1981 )

Fig. 1. Main Middle Triassic sector,

. -

plutonite

Subsurface

1x1

_

between

(Middle

sector. Here, intrusive

composition

outcrop

in the Southern

basaltic

Alps and adjoining

flows (shoshonites),

Ladinian

bodies

in Fiemme

Valley (Predazzo) and Fassa Valley (Monzoni). The intrusive and volcanic rocks are comagmatic and strontium isotopic ratios (0.7040-0.7050) indicate a deep magmatic source with very low crustal contamination of the parent magma (Pisa et al., 1980). Wide-spectrum geochemical studies were carried out at the University of Bologna in the last ten years on the Dolomitic products. Analyses of the well known “immobile” elements suggested an “erogenic character” with talc-alkaline and shoshonitic trends. These were further confirmed by the Rare Earth Elements (REE) patterns and by the main phases chemistry (Bosellini et al., 1982 and references therein; Lucchini et al., 1982).

plains.

Note the limit between

and the SSE sector

including

the northern

only rhyolitic-andesitic

or earlier).

The erogenic

affinity

in coheval products ern Alpine

chain

is also clearly

recognizable

found across the whole South(e.g. Lombardy-Cassinis

and

Zezza, 1982, and Recoaro-De Vecchi and Sedea, 1983), as demonstrated by Castellarin et al. (in press), and is well documented for the Dinarides and Hellenides (Bebien et al., 1978; Pe-Piper, 1982). A synthetic picture of the erogenic character of the Middle Triassic igneous rocks of the Southern Alps is given in Fig. 2 by the Ti-Zr covariation diagram (Pearce, 1982), and in Fig. 3 by plotting in the F,-F, diagram (Nisbet and Pearce, 1977) some selected clinopyroxenes from the basic lavas of the Dolomites. New analyses (available on requests) of REE, Th and some High Field Strength (HFS) elements

81

on selected

basic

the Dolomites

and intermediate

permit

now to approach

lem of their geodynamic most recent chemical

studies.

methods

re-appraisal

/

led many

,’ , , 1 ‘_

features

1979; ARC

LAVAS

’

\

that \

n’ /

authors

first

to identify

geotectonic

the main trace-ele-

environment

(Wood

1982, 1983; Saunders

\

HFS

immobile Tarney,

to a

then to

by which a lava may be assigned

Pearce, the

the probof the geo-

elements

during

and

basalt

Th

et al.,

confirmed

are essentially

alteration

1984) and stressed

to

and Tarney,

1984; Pearce et al., 1984). Such studies

/

from

in the light of the

of the use of trace elements,

a different

/ I

context

The development

focus their attention ment

samples

(Saunders

the importance

and of the

1000

Large

10

Fig. 2. Ti-Zr

covariation

from the main Middle Southern and

Alps.

close

basalts;

Note

diagram Triassic

volcanic

the distribution

to the arc

lavas

WPB: within-plate

1980 (Tarvisio);

(Pearce,

De Vecchi

districts

of the whole

of Ti-Zr

field.

MORB:

basalts.

values

within

mid-ocean

ridge

Data from: Lucchini

et al.,

and Sedea,

chini et al., 1982 (Dolomites);

1982) of samples

1983 (Recoaro);

Luc-

Crisci et al., 1984 (Lombardy).

.9

.?

.8

I

Lithofile

(LIL)

relative

to the HFS

ones. Among the various patterns, the geochemical characteristics of lavas from destructive plate boundaries are those of greatest interest for the Southern Alps magmatic products. This is underlined by the triangular diagram Th-Hf-Ta of Fig. 4 (Wood et al., 1979) where the Dolomite volcanics

Fl 1.0

Ion

have

been

plotted

in the “destructive

plate margin” field, towards the Th corner. Using the multi-elements geochemical pattern of Fig. 5

2.3

VA0 Hfls 2.4

2.6

WPA WPT

/

\

Th 2.7

Fig. 3. Plot of discriminant enes analyses and Pearce,

function

from basic lavas of different 1977). VAB-volcanic

floor

basalts;

WPT-within-plate

plate

alkalic

basalts.

Dolomites

F, against

volcanics

The (striped

magma

arc basalts; tholeiites;

clinopyroxenes

Fig.

Ta 4. Th-Hf-Ta

F2 for pyrox-

(1979)

types (Nisbet

different

OFB-oceanWPA-withinfrom

the

area) plot in the VAB field.

basic

MORB;

with

the

discrimination fields

tectonic

settings:

(A)

(D)

destructive

(C) WPB;

intermediate

selected samples

rocks of the Dolomites plate margins”

diagram

of magma

field.

of Wood

compositions

N-type plate

MORB; margins.

et al.

erupted (B)

in

E-type

Basic and

from the Middle Triassic volcanic

have been plotted

in the “destructive

10 , (Pearce,1982)

t 50

>

t

CONTlNENTAL OCEANIC

I

1

1

100

10

o WPB ALKALIC A WPB THOLEIITIC . “AB H-K CAB , _44;I;fTSCONT MARGINS

ARC ARC

IO

Zr (ppm)

Fig. 7. Zr/Y position

versus

Zr diagram

of the Middle Triassic

continental-arc

(Pearce,

basalts

1983) showing

of :he Dolomites

the

in the

field

1

tl ‘“sr

I I I I 11 K Rb Ba Th Ta Nb Ce

Fig. 5. Multi-element

geochemical

average

N-type

basalts

of the Dolomites.

patterns

for some typical

WPB tholeiitic) and

“active

(Pearce.

MORB,

Pearce,

and related continental

P

Zr

’

Hf SmTi

patterns

are pointed

unrelated

(VAB high-K margins”

Y

(normalized

1982) for the Middle

In the inset basalts

i/

1’7

11

to an Triassic out

(WPB alkalic talc-alkaline

basalts)

to

the and

basalts

subduction

1982. 1983).

-i 5

’

St-

’

’

’

Fig. 8. Geochemical basalt

’

’

’

’

K Rb Ba Th Ta Nb Ce

from

subduction (shaded

pattern

the Dolomites components

(Pearce,

’

’

’

1

1

Zr Hf

Sm TI

Y

for an average

Middle

Triassic

showing

(striped

area) according

’

P

the contribution

area)

and

Yb

of both

within-plate

ones

to Pearce (1983).

1982), the contribution

of both

subduc-

tion (Sr, K, Rb, Ba, Th, Ce, Sm and P) and within-plate (Ta, Nb, Zr, Hf, Ti, Y and Yb) components is apparent. Such a distribution is very different from both tholeiitic mid-ocean ridge ~~

~

Fig.

6. Plot

Dolomites agram .I

TalYb

’

.~~~~~ ~~~~ _~~ of the

Middle

in the Th/Yb

for basalts

~~~ _~ Triassic

versus

from oceanic

margins

and “alkalic”

settings

(Pearce,

1983).

oceanic

.~

volcanic

Ta/Yb

-rocks

of the

discrimination

arcs, from active

di-

continental

arcs, and from non-subduction

x3

basalts

(MORB)

It does not

and within-plate

fit that

of intra-oceanic

matches

that of some active

(Central

Chile basalts).

the

bivariate

Zr/Y-Zr

(WPB).

arcs, but

continental

diagrams

Th/Yb-Ta/Yb 7 (Pearce,

from the Dolomites

it

them from both the oceanic arcs basalts

In conclusion, explain

ing

magmatic

rocks

of the

magma

involving

that the characterization is totally

in agreement

subduction

contribution

processes,

is evident

arcs, of the

with meltwhose

geo-

in the diagram

of

Fig. 8.

field, clearly dis2. Middle Triassic geological setting

ones.

the new data allow us to better

the differences

Triassic

Alps and those of the true island

chemical

an area in

tinguishing

oceanic

original

and

continental

and the “alkali?

Southern

in

the “active

Middle

and to underline

1983) where

define

margins”

of the

margins

This is also confirmed

of Figs. 6 and

the samples

basalts

between

Extensional

the geochemistry

tectonism,

faults, controlled

by vertical

the Middle

Cal

and normal

Triassic

sedimentary

Rodella 2484

Dis

N 39” E

G CAMARINOS

Fig. 9. Section

across

slab of Ladinian

the Co1 Rodella

carbonates

(C. della

Marmolada

(Scytian);

3 = sandstones

ucts;

6 = Sciliar

Ladinian); Camian

Fm.)

the underlying

and conglomerates

the imbricate formations.

(Anisian);

a = Sciliar

Ladinian-Lower

gravitational

sliding,

mostly

tectonic products

structure

I = mainly

tidalic

4 = shelf carbonates (Ladinian);

of the Col Rodella

and the unconformity carbonates

(Anisian):

7 = volcanic

zone with the apical

between (Upper

the volcanic Permian);

5 = “agglomerati”,

conglomerates

exotic

conglomerates

2 = Werfen

i.e.: melange-like

(C. della Marmolada)

Fm. prod-

(Uppermost

contact.

scheme of the Col Rodella

units.

(Uppermost al., 1980.)

and

peak showing

Fm.). Note the melange-like

Fm. (shelf edge and slope carbonates)

8 = mechanical

A. Tectonic

(Sciliar

Fm.

area showing

(Ladinian); Camian);

on the western

the Middle Triassic

b = Conglomerato d = mainly

basaltic

part of the area);

della

deformations Marmolada

pillow-lavas /=

transform

and fault;

underneath Fm.;

the Uppermost

c = Wengen

pillow-breccias;

p.p.

e = thrust

R = trace of the geological

and

Ladinian-Lower S. Cassiano

front section,

(partly (From

Fms. due

to

Pisa et

84

and magmatic recognized

evolution

1980; Bosellini

and

authors

a

Middle

very probably

predating

(Castellarin, central

(e.g.,

system

is crossed

striking

transform

et al., 1982 and references

Moreover, phase,

as

Pisa et al.,

of the Southern

by several

postdating

lower 1983)

Dolomites

Triassic

Carnian has

and

been Cadore

upper

therein).

recognized

in

zone

neous this

the

1982;

1982, 1984a,

1984; Picotti

has

NNE-SSW

by volcanic

con-

della Marmolada

Fm.)

age. Since the transversal

system

to be referred event

of the Rodella

to the (Pisa

Middle

et

al.,

Peak, Triassic

1980,

pp.

1099-1100).

and Vai,

Other

198413; Blendinger,

1984, 1985; Sarti and Ardizzoni,

sealed

or, more likely, penecontempora-

to the thrust

compressional

(Pisa et al..

a subvertical

Ladinian

fault is younger

events

by fault,

(Conglomerato

of uppermost

Ladinian

extensional

et al., 1980; Castellarin

Prosser,

glomerates

compressional

1980; Castellarin Doglioni,

Alps,

and

field

evidences

marized

below.

Angular

unconformity

of this event

are sum-

in press).

The

Middle

Triassic

compressional

structures

were first recognized in the Dolomites at the Permo-Scytian south-verging thrusts of the Rodella

Inside the uppermost Ladinian-lower Carnian succession, an angular unconformity occurs at the

Peak (Canazei) (Fig. 9). These structures were previously attributed to Tertiary events. The thrust

COSTABELLA

(Fig. 10)

contact

MAERINS

between

COL RODELLA

the volcanic

conglomerates

r

(C.

7

I”

S NICOLO

CONTRIN

VERNEL FEDAIA

SOURA SASS

VARDA

ALTIF? DI CHERZ

II”

Km

1-0.5

9~lOpjg-j

Fig. 10. Possible tectonic data

(open

(Anisian); mainly

circles).

4 = Anisian

sliding

deposits

most Ladinian-Lower

setting of the central

1 = evaporites dolostones;

(Permian);

during

2 = bioclastic

5 = Buchenstein

(“agglomerati”); Carnian);

Dolomites

Fm. (Ladinian);

9 = epiclastic

II = subvolcanic

the Early Carnian,

limestones

volcanics;

bodies;

6 = carbonate 10 = volcanic

12 = Triassic

restored

(Permian);

faults;

11

by means of stratigraphic

3 = Werfen shelf deposits

conglomerates 13 = outcrop

12pj7

Fm. (Scytian) (Ladinian);

13(]

and structural

and elastic

7 = basal@

(C. della Marmolada of Middle Triassic

deposits

8 = chaotic

Fm.) (Upper-

structures.

85

della Marmolada) discontinuity because rocks. and

and the underlying

is not visible

it is located

or evident

mostly

within

The conglomerates the

overlying

Cassiano gently

Fm.,

folded

underlying

Cassiana faulted,

conglomerate

deformed

and affected

irregular.

interfingering

tures generated

everywhere

Fm.

p.p.,

Fm.)

are more

in the underlying upper

(Castellarin

et al., 1982b;

elements the

are locally

activated

by

1985; Pisa et al., 1980).

later

Alpine

S.

Late and post-tectonic

,

re-

compressions

Many

strongly

Doglioni,

upper

layer

by basaltic

Permian

(faults

dykes. Good anticline,

evaporitic

nucleus,

and exam-

with a wide

and the associ-

ated minor folds at the S. Nicolo’ Pass (Castellarin

eyaporites 1984a).

and meso-structures

ples are the pseudodiapiric

struc-

decollement

macro-

folds) are crossed

These are often

Permian

intrusive bodies

et al., 1982b;

All

Doglioni,

1984a), and the folds and

reverse faults along the southern

1 \a

strongly

are only

with pseudodiapirc

by thick

tectonic

(Doglioni,

whilst the units

by thrusts.

represented

these

Marmolada)

(Wengen

Dolomia

The

dark volcanic

(C. della

units

and slightly the

strata.

side of the Mount

5

2 Ii :: Psso d, CAMPOLONGO

09”

29

P

_

Gran Verne1

n Marmolada

\

MARMOLADA .if”.e

Km

Fig. 11, Main Middle Triassic faults.

reverse

anticline

faults

at St. Nicolo’

4 = detached

carbonate

Ladinian-Lower 8 = thrust

conglomerates Soura

Sass;

deformations

reverse

front

buried by tabular

the Co1 OmberttCostabella

tectonic

9 = vertical h-thrust buried

in the central

and the chaotic

Note the correspondence

sliding

assemblages

between

(“agglomerati”)

the tectonic

(black).

lines (thrust

I = pseudodiapiric

2, S = chaotic assemblages (“agglomerati”) with major carbonatic olistolithes; conglomerates (5) (C. della Marmolada Fm.) and elastic deposits (7) (Uppermost

compressional and

transform

deformations: fault:

elastic deposits

volcanic

(Camian)

6 = basaltic

volcanics

IO = slide detachment

system at Co1 Rodella;

by discordant

reverse faults,

Dolomites.

mainly

area;

5, 7-volcanic

sealing

fault;

at Mt. Bustaccio; d-thrust

structures faults)

Pass and adjoining slab;

Carnian).

and

tectonic

and transform

w

c-thrust

conglomerates

at Ronchi;

surface.

(Upper

front sealed by chaotic (C. della

f. g-tectonic

Ladinian-Lower

a-unconformity

Marmolada

assemblages Fm.)

Carnian);

of the volcanic (“Aggfomerati”)

at Livine;

front of the Marmolada-Collaccio

at

e-tectonic thrust

and of

86

Marmolada, dinger,

Cima

Ombretta

1984; Doglioni,

discordant, The

1985). All are crossed

undeformed

largest

discordant

Group.

pressive

downsequence porphyry

and to

pluton

volcanics

body

Lower

(Castellarin

is the

of the Monzoni

body

cuts the com-

meso-structures

the

character

recorded

Permian

quartz-

et al.. 1982a).

(see section

The igneous ring

magmatic

The igneous

macro-

by

dykes.

huge mafic and ultramafic Mounts

and Mt. Fop (Blen-

complex

monzonitic

1).

suite includes of

pluton

are the classic

area.

The lavas include basalts,

intrusions and

rocks

(Agglomerati

Auct.)

(Figs.

10

Chaotic sedimentary

admixtures including magmatic and elastics are geometrically and geneti-

cally linked to the Middle sional

southern

structures.

These

Triassic

major compres-

masses

recall

olis-

rhyolites,

rhyodacite

to andesite)

border

of this belt includes

andesites

located

in the Brescia

and

zone

(Fig.

1). Since

area

coheval

the southward

curvature

by detachment relieves created

be more pronounced

structures River).

from

the Dolomites

to Cadore

Dolomites,

Middle

Triassic

igneous

rocks

are irregularly

distributed in various parts of the Southern Alps (Pisa et al., 1980) but reach their maximum extent and volume in the Dolomites and Lessini Mts. and within the nearby PO Plain (data AGIP 1977, 1981; Brusca et al., 1982). In the past, they were considered as an alkaline association related to a crustal rifting episode that preceded the Jurassic opening of the Thetyan Ocean (Ferrara and Innocenti, 1974; Marinelli, 1975). Recently, however, they have been shown to comprise a calc-alkaline and shoshonitic association of erogenic

are

basalts

(Ricci

are

and Serri.

of this belt could

to the north, which is basalts and extends from di Non-to the central

and continues

with sporadic

outcrops

in the eastern Southern Alps (Fig. 1). These belts formed progressively in time: in the acidic southern belt the volcanic activity started during the Early to Middle Ladinian (or earlier), whereas activity in the northern end of the Ladinian. The onset of the rhyolitic, furthermore, is well recorded dle Ladinian

of the Triassic magmatic rocks

which

than in Fig. 1. (2) A broad,

subparallel zone farther dominated by shoshonitic the Giudicarie area-Val

bedded 3. Distribution

from

in the Recoaro

alkaline

1975)

major (Piave

character

extending

the PO Plain near Mantova, through the Vicenza region, to the Tarvisio area: the northwestern

and Liguria

the thrusting surfaces. All these rocks were considered in the past as products of the explosive volcanic activity (Leonardi, 1965). The “agglomerati” are absent in the succession overlying the volcanic conglomerates: on the contrary they always mark the Triassic compressional

in fairly

(Fig. 1): (1) An arcuate

in Tuscany

evolve into cataclastic rocks (tectonic melanges) owing to the friction mechanism along and under

rare andesites,

are distributed

present

several places (Canazei, Collaccio, Soura Sass), these chaotic products seal the thrust fronts and

The

of the Dolomite

tostromes and/or large megabreccia bodies. They represent syntectonic gravitational deposits formed and sliding processes along the by the compressional tectonics. At

mainly

Mounts.

belt of acidic and intermediate

(rhyolite,

and 11)

lavas, the the

regular belts. Taking into account the volcanic rocks recognized in the PO Plain subsurface, two belts can be distinguished

Melange-like

and

of the Monzoni

latter

shoshonitc

dominant

Predazzo

belt began

rhyodacitic activity, by the Lower-Mid-

pelagic sequences.

pyroclastics,

largely

Here acidic thinly

predating

the basaltic flows, form the Verde” member of the Ladinian Inside

the southern

belt

at the

the base of

so-called “Pietra Livinallongo Fm.

the volcanic

activity

could have continued through the Carnian, as observed in eastern Lombardy (Brescia Province: Val Trompia 1982).

and Val Sabbia)

(Cassinis

and Zezza,

4. Conclusions The time-space N-trending polarity of the volcanics is the main Middle Triassic regional feature so far recognized in the Southern Alps. The polarity is clearly documented by the com-

87

position

and distribution

by their general

evolutionary

kaline

(rhyolite/andesite

basalts.

In particular,

volcanics

implies

lithosome

of the magmatites trends

lateral

northward

magmatic

zones

to shoshonitic

the area1 distribution accretion

can be seen as a surfacial average

from calc-al-

products)

across the Southern

of the

of the volcanic

Alps. This polarity

image

dipping inside

and

of the inclined

surface

the lower

joining crust

the

and

the

(Fig. 1) are correct, follow

Classical

rift models

the more modern

such as those proposed

studies

(Ferrara

and Innocenti,

1974; Marinelli, 1975; Bechstadt et al., Cortesogno et al., 1982; Cros and Zabo, Doglioni,

1983; Bosellini,

Pamic, 1984; Garzanti, 1985; Martini to explain

1978; 1984;

1984; Crisci et al., 1984;

1985; Garzanti

and Jadoul,

et al., 1986) do not appear

the pattern

in

and chemistry

adequate

of the Tri-

pattern

extent)

to the present

active

continental

margins.

tween

the Triassic

magmatic recorded

is not very intense South

Recent

for the

The

arcs of

correlation and

be-

the present

unsatisfactory,

because

oceanic

compressional

crust is tectonism

and does not seem to affect the

Alpine

data available

(except

Alps no Triassic

and the Triassic

volcanics

magmatic

magmatites

arcs is, however,

in the Southern

Triassic

similar

smaller

whole

mantle.

an arcuate

the Middle

area, at least on the base of

so far.

and

still unpublished

analyses

on the

minor tectonic structures and deformations carried out in the Dolomites (between the Adige Valley

and

the Marmolada

Group)

dences of two major compressional NNW-SSE arated

and N-S

directions,

by an interval

of vertical

provide

evi-

episodes (u, in respectively)

sep-

movements

cou-

assic magmatic products of the central Southern Alps and the northern PO Plain. A rifting model

pled with extensive The distribution

implies

displays a general ENE-WSW-trending central pattern (Fig. 1) which represents the main direction of the regional Hercynian metamorphic belts in the Southern Alps (Castellarin and Vai, 1981; Vai and Cocozza, 1986). Furthermore, most of the central-eastern setting of the magmatic arc is nearly normal to the NNW-SSE and N-S maxi-

a bilateral

symmetry

in the timeespace

distribution of the magmatites and an opposite evolutionary trend, possibly from basaltic to andesitic/rhyolitic products. A possible model coherent with the magmatic data could imply subduction-like mechanism. An ensialic lithospheric body with visco-plastic behaviour may have descended beneath the base of the middle crust into the mantle under the Southern Alps. This could account conditions spheric

including contribution

for all the magmatic

the “subcontinental” in the magmatic

section 1). As for the chemistry

of the

Middle

lithomelts

(see

Triassic

magmatites, the clear talc-alkaline and shoshonitic trends should indicate an erogenic event. A compressional stress field pattern is not in contrast with the geological data discussed above (see section 1). As to the structural style, general transpressional deformations have already been proposed in terms of regional patterns for the central Southern Thetys (Rau and Tongiorgi, 1981; Morel and Irvin, 1981; Badham, 1982; Brandner, 1984; Horvat and D’Argenio, 1985; Vai and Cocozza, 1986) and particularly stressed for the Dolomites in the more recent works (Doglioni, 1984a, 1984b). Moreover, if the reconstructions here proposed

mum

basaltic flows (Selli, 1987). of the volcanics, moreover.

compressional

structural

analyses

axis trends mentioned

may imply genetic connections of the Triassic

magmatic

compressional

stress

obtained

above. between arc

and

by the

These

the setting

the

regional

field and a possible

the Middle Triassic evolution Alpine Hercynian history.

facts

link of

with the Southern

Acknowledgements

We are indebted to Prof. R. Sartori (University of Bologna) who reviewed the manuscript and to Prof. F.C. Wezel (University of Urbina) for the stimulating

discussions.

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