Geodynamic-evolution-of-the-atlas-system-morocco-in-postpalaeozo.pdf

GEODYNAMIC EVOLUTION OF THE ATLAS SYSTEM (MOROCCO) IN POST-PALAEOZOIC TIMES Volker +

Jacobshaqen + Freie

Institut

++Institut

Konrad G B r l e r + and P e t e r

Universit~t fur fur

Berlin,

Geologie,

D-IO00 B e r l i n

Altensteinstr.

Giese ++ 33

34 A

Geophysikalische Wissenschaften, R h e i n b a b e n a l l e e 49

ABSTRACT The A t l a s system o f Morocco c o m p r i s e s the M e d i t e r r a n e a n R i f orogen and three foreland intracontinental r a n g e s : M i d d l e , High and A n t i - A t l a s , w h i c h behave as m o b i l e zones w i t h i n the s t a b l e p l a t f o r m . For t h i s r e g i o n , a r e v i e w i s g i v e n on the m a j o r s t r u c t u r a l e l e m e n t s , the s e d i m e n t a r y and v o l c a n i c e v o l u t i o n from T r i a s s i c to r e c e n t , the c r u s t a l and upper m a n t l e s t r u c t u r e and the s e i s m i c i t y . Based on t h e s e data the followinq qeodynamic model has been d e r i v e d : The b r e a k - u p o f Pangaea and the s u b s e q u e n t f o r m a t i o n o f the N o r t h A t l a n t i c and the w e s t e r n T e t h y s a f f e c t e d to some e x t e n t the n o r t h w e s t e r n m a r g i n o f the A f r i c a n continent. D u r i n g the T r i a s s i c - D o g g e r interval, rift grabens, pulla p a r t b a s i n s and accompanying v o l c a n i s m are f e a t u r e s o f t h e s e moderate mobilizations. From Upper J u r a s s i c onward the r i f t i n g p r o c e s s e s slowed down and ceased d u r i n g the Eocene. The accompanying magmatism changed from b a s a l t i c e f f u s i o n s to a l c a l i n e intrusions, thus indicating a change from a t e n s i o n a l to a c o m p r e s s i o n a l s t r e s s - f i e l d . The t h i r d p e r i o d o f the e v o l u t i o n , from O l i g o c e n e to r e c e n t , was governed by c o m p r e s s i o n a l movements between A f r i c a and E u r o p e , c u l m i n a t i n g i n collisional structures o f the B e t i c C o r d i l l e r a s and the R i f . Compress i o n and u p l i f t o f the R i f A t l a s and o f the t h r e e i n t r a c o n t i n e n t a l b e l t s took p l a c e w i t h i n the same t i m e i n t e r v a l s , t h u s i m p l y i n g a common r e l a t i o n s h i p to c e r t a i n p l a t e t e c t o n i c e v e n t s . As e v i d e n c e by magnetotelluric and s e i s m i c s t u d i e s i n t r a c r u s t a l s h e a r i n g and d e t a c h ment have been f a c i l i t a t e d by the e x i s t e n c e o f i n t r a c r u s t a l high conductivity and low v e l o c i t y l a y e r s , caused by Mesozoic and T e r t i a r y thermal activities. A mechanism o f t r a n s f e r r i n g compressional stress from the a c t i v e m a r g i n to i t s f o r e l a n d i s p r o v i d e d by the BALLY model of thin-skinned thrusting. The c o m b i n a t i o n o f t h e s e p r i n c i p l e s with the s t r u c t u r a l situation in n o r t h e r n A f r i c a s u g g e s t s the f o r m a t i o n o f an i n c l i n e d thrust plain structure, w h i c h has p e n e t r a t e d the sediment a r y c o v e r and deeper c r u s t a l c o m p l e x e s . A s t r o n g e r c o m p r e s s i o n had t a k e n p l a c e w i t h i n the f r a c t u r e d and weakened f o r m e r r i f t z o n e s , c a u s i n q the o b s e r v e d young u p l i f t o f the r a n g e s . A final stage of collision i s marked by the NE/SW s t r i k i n g Transalboran f a u l t z o n e , e x t e n d i n g o v e r a l e n g t h o f 1100 km from s o u t h e a s t ern Spain a c r o s s the A l b o r a n Sea and a l o n g the M i d d l e A t l a s to A g a d i r .

482 I.

Introduction

The A t l a s

system o f Morocco c o n s i s t s

Middle Atlas,

High A t l a s

structural

evolution

siderable

differences.

since

the m i d d l e

that

there

of

But a l l

of

connections of

parts

of

bides,

of geophysical

Atlas

structure

oroqen o f

displays

the

tinental

fraqment

facing

of

BOUILLIN e t a l .

position

involved within

the nappes:

Rif

differs

The u p p e r nappes,

the M e d i t e r r a n e a n

coasts,

the

the e x t e r n a l

l o w e r ones o f

The n o r t h e r n

margin

(1986)

of

the Maghre-

the s o u t h .

The

and by DURAND-DELGA were o f the o p i n i o n

Rif

this

originated

now o c c u p y i n g

to the

plate

is

far

to

from the O l i g o The age o f

stacking

an i n t e r n a l

during

zones d e v e l o p e d

the A f r i c a n

situated

collisions.

according

were o v e r t h r u s t

on a con-

f r a g m e n t was t r a n s p o r t e d

movements and,

i n West M e d i t e r r a n e a n

overthrusts

are

have been summarized by

o f the

by t r a n s c u r r e n t

the

due to the

Atlas,

ALKAPECA w h i c h was o r i g i n a t e d

to

cene onward,

due

CHOUBERT and FAURE-MURET ( 1 9 7 4 ) ,

D u r i n g the Lower T e r t i a r y

present

Geodyn-

widely

type called towards

the R i f

the n o r t h e a s t . its

the T e l l

the M e d i t e r r a n e a n

and M e s o z o i c r o c k s

called

believe

history.

even more,

prolongation,

evolution

vol.).

Palaeozoic

show c o n -

however,

DURAND-DELGA and FONTBOTE (1980)

and OLIVIER ( t h i s that

Rif,

to h i g h m o u n t a i n s

most g e o l o g i s t s

knowledge and,

FAURE-MURET and CHOUBERT ( 1 9 7 1 ) , MICHARD ( 1 9 7 6 ) ,

rocks,

structural

system d i f f e r ,

nappe s t r u c t u r e s

and g e o l o g i c a l

belts:

situation,

investigations.

and i t s - e a s t e r n

a normal

that

Therefore in their

the A t l a s

to many gaps i n our g e o l o g i c a l

The R i f

mountain

them have d e v e l o p e d

the T e r t i a r y .

are c l o s e

state

four

The g e o t e c t o n i c

and the sequences o f deformed

amic i n t e r p r e t a t i o n s present

of

and A n t i - A t l a s .

the

position

the of

along

Oligocene,

during

buried

order

whereas

Miocene phases.

beneath

the

Rif

oro-

gen. Middle tory.

and High A t l a s

Recent o v e r v i e w s

and FAURE-MURET ( 1 9 6 2 ) , (1976). vol.)o

and p u l l - a p a r t African sidence vity,

history

Atlas,

DRESNAY p r e s e n t s

in a mobile

zone a t

a first

and MICHARD

review

from M e s o z o i c r i f t the n o r t h e r n

his-

by CHOUBERT

and JACOBSHAGEN e t a l .

seem to have d e v e l o p e d

basins

and g e o l o g i c a l

have been g i v e n

has been summarized e . g .

JACOBSHAGEN (1986)

For the M i d d l e Both ranges

in structure

FAURE-MURET and CHOUBERT (1971)

The p o s t - P a l a e o z o i c

and WURSTER [ 1 9 8 1 ) , volume.

are s i m i l a r

on the High A t l a s

margin

by STETS (this in this grabens of

the

p l a t e , w h i c h was the f o r e l a n d o f the R i f . F r a c t u r a t i o n and subo f t h e s e grabens and b a s i n s was accompanied by magmatic a c t i -

mainly

by t h e e f f u s i o n

of basaltic

rocks,

up to

the M i d d l e J u r a s -

483 sic.

A second p e r i o d

lowed by i n v e r s i o n ceous took

place vol.).

literature

Precambrian

MICHARD 1976, is

poorly

sence o f Further in

the

place

dykes

we can c o n c l u d e

during

zoic

rocks

tiary

of

elevated

explain et

1981,

1975,

2000 m N.N. several

COURBOULEIX e t

has n o t

al.

been i n c l u d e d

based on the i d e a

that

in

Palaeozoic

Most o f

these authors

for that

these

orientation

in

agree,

Thus, M e s o z o i c o p e n i n g o f formation

of

rift

grabens

of rocks

Anti-Atlas

the m o u n t a i n

is

range o r i g i -

and Lower P a l a e o -

o f a Neogene b a s i n

al.

now b e i n g

1987 and t h i s

1977,

The u p l i f t

vol.).

considerations.

fractures

in

field

development of of major

within

lateral

to e x p l a i n

the o r i g i n

MATTAUER e t a l . o f NW A f r i c a

Ocean

and p u l l - a p a r t

pre-

the basement. displacement

(1977)

of have

had changed i t s

o f M e s o z o i c and C e n o z o i c p l a t e the A t l a n t i c

however,

the M e s o z o i c and C e n o z o i c e v o -

model

system,

(MICHARD

h y p o t h e s e s are

by r e a c t i v a t i o n

geotectonic stress

All

structural

that

to

STETS and WURSTER

o f the A n t i - A t l a s ,

furthermore,

the A t l a s

the c o u r s e

took

the High and the M i d d l e A t l a s

influenced

the r e g i o n a l

thickness

with-

happened from the u p p e r m o s t T e r -

or even P r o t e r o z o i c

the w h o l e ensemble o f envisaged

of

the p o s t - P a l a e o z o i c

a general

periods, uplift

(G~RLER e t a l .

a l o n g t h e s e f a u l t s p l a y e d a key r o l e l u t i o n o f M i d d l e and High A t l a s . Looking

detritus

the a d j a c e n t

the P r e c a m b r i a n

This

LEBLANC 1 9 7 3 ) .

and w i t h i n

The t o t a l

height

by the p r e -

by HEINITZ ( 1 9 8 4 ) .

domes o f the C e n t r a l

MATTAUER e t

1982).

t h e s e ranges was s t r o n g l y existing

of

on

evolution

testified

geodynamic models have been proposed

and o r i g i n

MICHARD 1976,

focussed

of Anti-Atlas

important

as documented by r e l i c s

structure

al.

influxe

several

km. The p r e s e n t

years,

is

has been g i v e n

by s i m p l e updoming o f

more than

In r e c e n t

that

(GURLER e t a l . ,

post-Palaeozoic

the High A t l a s

the Sahara c r a t o n .

onward,

but generally

grand dyke de Foum Z g u i d , events

the c r y s t a l l i n e

to 8-10

the Upper C r e t a -

vol.),

i s more or l e s s

Its

the M e s o z o i c and T e r t i a r y .

however,

in

was f o l -

(CHOUBERT and FAURE-MURET 1971,

vol.).

from the

cover at

basins,

eroded on top o f nated,

(e.g.

on f r a c t u r a l

sedimentary

estimated

history

Early Mesozoic fracturation

basaltic

Upper C e n o z o i c

this

on the A n t i - A t l a s

WALLBRECHER, t h i s

evidence

Finally,

locally

times

or even Lower Miocene onward

and P a l a e o z o i c

known.

Cretaceous

FROITZHEIM e t a l . ,

from O l i g o c e n e

Geological

in

w h i c h may have s t a r t e d

(FROITZHEIM 1984,

this

its

o f calm s u b s i d e n c e

movements.

w o u l d have caused the

basins

in

the m o b i l e

zone

484

north

of

the Sahara c r a t o n .

lifted

in

times.

C o n t r a r y to

referred

connection with

for

Recently,

transpression implies

the

evolution

o p e n i n G , but n o t to M e d i t e r r a n e a n i n f l u e n c e s . o f the High A t l a s

the same a u t h o r s

(FROITZHEIM e t a l . ,

paper i s ,

o f the A t l a s

situation

this

of

the A t l a s

to p r e s e n t a new model o f

system's

of

palaeotectonic

point

of view,

n o t be d i s c u s s e d

here)

can be s u b d i v i d e d

the r e c e n t

including

Morocco south o f into

parts

of

consist

of

A m a j o r zone, easterly

faults

the A n t i - A t l a s

striking

slightly

direction

"massif ancien"

as i t s

(which

stable

regimes

updomed n o r t h e r n

and the w e s t e r n

and a l o n g the c e n t r a l

of recent

is

Morocco i n

a north-

the c e n t r a l in

the

High A t l a s

and

the v i c i n i t y

direction

of

from J e b i l e t

In the e a s t e r n High A t l a s

it

branches

the High P l a t e a u o f Oran.

where t e c t o n i c

are c o n c e r n e d , active.

of

in an e a s t e r l y

Hiqh A t l a s .

A l t h o u g h one has t o ,

still

crosses

ENE, r e s p e c t i v e l y .

south o f A g a d i r t h r o u g h

to the M e d i l ; e r r a n e a n c o a s t

to the NE and ends w i t h i n

heterogeneous, it

parts

margin.

zones which p r e d o m i n a n t -

the NE and to the

sigmoidal-shaped,

The second zone runs

faults

to

from the Souss b a s i n

a l o n g the M i d d l e A t l a s Melilla.

whole,

the R i f

three

the w e s t e r n High A t l a s ;

These r e g i m e s are s e p a r a t e d by two f r a c t u r a l

2,

the

Oran;

the Sahara c r a t o n w i t h

of

geodynamic

development since

1):

the High P l a t e a u o f

ly

the geodynamic

situation

From a g e o t e c t o n i c

The Moroccan Meseta,

-

which n e c e s s a r i l y

system o f Morocco i n Mesozoic and C e n o z o i c t i m e s .

Recent g e o t e c t o n i c

(fiq.

-

vol.)

Mesozoic.

2.

-

has n o t been o f -

have a l s o c o n s i d e r e d a

This model has to be based upon a d i s c u s s i o n

will

Cenozoic

Mediterranean influence.

The aim o f t h i s

early

in

the model o f STETS and WURSTER (1981)

inversion,

however,

model

were compressed and up-

West M e d i t e r r a n e a n c o l l i s i o n s

view,

o n l y to A t l a n t i c

An e x p l a n a t i o n fered.

this

These s t r u c t u r e s

character

and a c t i v i t y

e n v i s a g e these f r a c t u r a l

evident

that

This

due to the f a c t

is

the NE s t r i k i n g

earthquake-hypocenters coincides

see a l s o SAADI 1982, map 21,

that

with

periods

zones as somewhat zone i s ,

on the

a linear

arrangement

this

fault

SCHWARZ and WIGGER, t h i s

zone ( f i g .

vol.).

Several

485

1

\,6,

8o

10 °

2o, .......i

MEDITERRANEAN

Tanaer.,,~q

3

S E A

/4

o RIF • ...,.,

Rabai

:+

Casabh

+

.

PLATEAU5

/4t¢¢-. J z.

/

".

...-..~!+~.~

~

rrachidia--

~

3~

"..*..'.

' ~

;LU :; "

.,..~ .'.;.~ ,

-I

,

Ouarzaza t /

"t

/

~

,

Cenozoic

ANTI -

~lh

/

•, ~

/

ir,'~

basins

Cenozoic v o l c a n o e s Southern

border of

the Maghrebides

0

__~,,lnn~m

L

Fiq.

L

1:

J

however~ r e c e n t

High A t l a s ,

t

~ - -

major faults

G e o t e c t o n i c s k e t c h - m a p o f the A t l a s s y s t e m , based upon MICHARD ( 1 9 7 6 ) , JENNY ( 1 9 8 3 ) , DRESNAY ( t h i s v o l . ) and JACOBSHAGEN e t a l . ( t h i s v o l . ) .

them were l o c a t e d

zone,

Highand Middle Atlas, Jebilet

s

ATL AS

of

~

_ - "

in a depth of earthquakes

near the c r o s s i n q

Another region

of

of

the

km. A l o n g the

happened o n l y w i t h i n both

high seismic

c o m p r i s i n q the A l b o r a n Sea, b a s i n s o u t h o f the R i f .

100-200

the c e n t r a l

zones.

activity

Rif

E/W-striking

is

situated

in

the n o r t h ,

and t h e Upper C e n o z o i c molasse

486

#'"0 '>

0

8
0 0

oOjo 0

0

f

3~,o

0 /

/ 11/'/0 / 0 .I"

/

.,

[]

/ ~

j 0

-%

0

~X7

0

0

O

0

z<:

50

[]

50 <: z <

?5

'~

75 < z <~00

~;~ 100 < z 0 L

Fig.

100Km ~

_J

2: D i s t r i b u t i o n of earthquake hypocenters in relation to the f r a c t u r a l p a t t e r n of the A t l a s system ( d a t a from HATZFELD and FROGNEU× 1981).

The ranges o f not exactly

Km

with

the High and the M i d d l e A t l a s the fractura]

coincide

zones. Modern a u t h o r s

roughly,

agree t h a t

but these

m o u n t a i n ranges d e v e l o p e d from Mesozoic grabens and b a s i n s by i n v e r sion. a hint

Both are accompanied by Upper Cenozoic b a s i n s , to the age o f ' i n v e r s i o n

To the major

features

- but not to i t s

and t h i s

may g i v e

cause.

o f the younq geodynamic e v o l u t i o n

o f Morocco

487 belongs a wide-spread publications ly

for

volcanic

have p r e s e n t e d

the n o r t h e a s t e r n

and BELLON 1985, Rif

occurred

from the

do n o t a l l o w

the northern

14-8 m . y . latter

M i d d l e Atl~Ls two p e r i o d s (14-10 m.y. volcanic are

and 1 , 8 - 0 , 5

centers

still

Ch. G r o t h e geochemical

will

character

eruptions

in

The r e g i o n a l strictly

bound

adjacent

to

with

the

(1987), with

be p u b l i s h e d

activation

the n o r t h

of

to d i s t i n c t

Transalboran

Spain across ioe.

Structure

of

crust

Where the with

1).

vol.),

seismic may i t

to

for

be v a l i d

also

the

about

not

within fits

or

exactly

to be c o n n e c t e d from

Rif.

This

coast of

Africa

1100 km.

is

increase Rif

review

concerned

two e x t e n d e d n e g a t i v e

have been o b s e r v e d to

the

Rif

High A t l a s

measurements for

an e x c e l l e n t

between R i f

-150 mgal

be d e r i v e d

the M i d d l e A t l a s

This

is

structure

one b e i n g c o r r e l a t e d

and c r u s t a l

magmatism.

centers

the e a s t e r n

have p r e s e n t e d

the M i d d l e and the c e n t r a l

a b o u t 30 km c o u l d

the c a l c a l -

and u p p e r m a n t l e

Bouguer g r a v i t y

system,

volcanism which only

and HERNANDEZ e t a l .

now, to the A t l a n t i c of

Kouss)

and

From the

b u t t h e y are s i t u a t e d

the A l b o r a n Sea to

and u p p e r m a n t l e

v a l u e s down to

Foum el

to a s u b d u c t i o n - r e l a t e d

zone ( f i g .

The

Schermerhorn

s h e a r zone t h e y c o u l d f o l l o w

to an e x t e n s i o n

SCHWARZ and WIGGER ( t h i s

over,

Sarrho,

J.

the u p p e r m a n t l e :

fault

o f the

character.

the near f u t u r e .

i d e a s o f TORRES-ROLDANI e t a i . ( 1 9 8 6 )

of the crustal

vity

J.

of

who have f o u n d t h e Upper C e n o z o i c v o l c a n i s m

at Agadir,

one to

Sirwa, in

and a l k a l i part

can be d i s t i n g u i s h e d

t h e young v o l c a n i c

faults,

the N E / S W - s t r i k i n g

southeastern

Atlas

of

point

s h e a r zone can be p r o l o n g a t e d ,

lie3

activity

In the

eruptions

the southern

being of alcaline (J.

1987).

interpretation.

by s h o s h o n i t e s In

publications

distribution

a SW s t r i k i n g

3.

HERNANDEZ e t a l .

we may deduce an i n t r a c o n t i n e n t a l

was caused by t h e r m a l caline

1986,

a general

of volcanic

o f the A n t i - A t l a s

(Berlin)

data m a i n -

1984,

Middle Atlas, calcalcaline

m.y.),both

Recent

HERNANDEZ

up to 2 m.y.

under investigation;

14 m.y.

(BELLON e t a l .

They were f o l l o w e d

reaching

the l a s t

and g e o c h r o n o l o g i c a l

o f Morocco

HARMAND and MOUKADIRI

and i n

basalts,

within

geochemical

part

Up to now, the r e s u l t s eastern

activity

and A n t i - A t l a s .

beneath

mountains

and the o t h e r

(BOSCH 1 9 7 1 ) .

an a v e r a g e c r u s t a l

Morocco.

From g r a -

thickness

of

Only b e n e a t h the High and

to a b o u t 35 km, and such v a l u e s

(DURAND-DELGA and FONTBOTE 1 9 8 0 ) .

WIGGER and HARDER (1986)

anoma-

the

have d e t e c t e d

a low v e l o c i t y

seem

More-

zone i n

488

the

lower crust

also

of

the M i d d l e and the High A t l a s .

have found reduced v e l o c i t i e s

which p o i n t s

a g a i n to h i g h

Magnetotelluric detection

of 7.5-8.0

thermal

km/s f o r

the upper m a n t l e

activity.

and g e o m a q n e t i c depth

o f a zone o f

The same a u t h o r s

high e l e c t r i c

s o u n d i n g s have l e d to the

conductivity

depths

from the High A t l a s

1986).

Comparable a n o m a l i e s have been found a t the s o u t h e r n

the High A t l a s der o f

the

towards

a t mid c r u s t a l

the M i d d l e A t l a s

(MENVIELLE and LE MOUEL 1985) and a t

Rif,

where i t

dips

to

sions.

the o r i g i n

o f the low v e l o c i t y

of

zone is

the s o u t h e r n

these anomalies could

which m i g h t have been a c t i v a t e d

Therefore,

border of bor-

the N (MENVIELLE and ROSSIGNOL 1982).

SCHWARZ and WIGGER have s p e c u l a t e d t h a t by f l u i d s

(SCHWARZ e t a l .

the

by d e e p - s e a t e d b a s i c

induction

explained

be caused intru-

a n o m a l i e s as w e l l

by s m a l l

amounts o f m e l t

as in

the

m i d d l e and l o w e r c r u s t .

4.

Tectonic

and magmatic e v o l u t i o n

o f the p o s t - P a l a e o z o i c

As m e n t i o n e d a b o v e , the p r e - O l i q o c e n e ted f a r

from t h e i r

present

ment o f the A n t i - A t l a s deciphering

is

position, poorly

documented.

history

i n Mesozoic and Cenozoic t i m e s .

plate

therefore,

was s t r o n g l y

tral

rift

LORENZ, t h i s

were c a l l e d

by some a u t h o r s Triassic, regional

basins vol.).

Atlantic

by two g e o t e c t o n i c

to

these

and magmatic

events:

African the f o r m a t i o n

the T e t h y s

to cen-

Both e v e n t s caused a c t i v e

in c e n t r a l

the

fault

and e a s t e r n Morocco (MANSPEI-

C o n s e q u e n t l y , the p r e v a i l i n g

fault

direc-

( a b o u t NE/SW) or M e d i t e r r a n e a n ( a b o u t E/W)

(KANES e t a l .

localized

develop-

key areas i n

the l a t e r

zone and the e x p a n s i o n o f

and w e s t e r n M e d i t e r r a n e a n a r e a s .

ZER 1982;

the

tectonic

the n o r t h w e s t e r n edae o f

zones and f a u l t - b o u n d e d tions

now examine t h e i r

influenced

the M i d - A t l a n t i c

Thus,

and the Neogene b a s i n s a d j a c e n t

We s h a l l ,

of

had o r i g i n a -

are the s e d i m e n t a r y c o v e r o f

ranges.

D u r i n q the T r i a s s i c ,

o f the R i f

and the p o s t - P a l a e o z o i c

the geodynamic e v o l u t i o n

High and the M i d d l e A t l a s

rocks

history

1974, MICHARD 1976).

s u b s i d e n c e o f the b a s i n s

s u b s i d e n c e began c a u s i n g

During

decreased,

transgressions

the Upper

but a more

from the e x p a n d i n g

Tethys ocean and from the j u s t

originatinq

In e a r l y

a r e a s were a l r e a d y f l o o d e d by seas.

Fig.

Jurassic

times,

3 shows a l a r q e

Middle Atlas

with

large

Atlantic

Tethys embayment ( " A t l a s

the n o r t h e a s t e r n

Meseta,

Gulf")

(LORENZ,

this

vol.).

which c o v e r e d the

the c e n t r a l

and e a s t e r n

489

longer

_

Orcm

EI13,

Me~,i~J.Q

RIF 115 RobQt

£QSQb(QnCQ

"

j

. ..- .:'l

HiGH ATLAS - ~.'. ~:

~

:.

i

• ErrGchidiQ ' . . . . '. , ~

,

Mnrrakech

";'"

"',';i:

O •

Fig.

Fig.

4:

,

3:

i."

200kin

|

i

.

j

Major features of Middle Liassic palaeogeography of Morocco (simplified a c c o r d i n g t o DRESNAY 1 9 7 1 ) . I c o n t i n e n t a l , 2 shallow marine, 3 deeper basins, 4 southern border of the Maghrebides, 5 southern border of the High Atlas.

Mesozoic transq r e s s i o n s and regressions a) o f t h e w o r l d (according to HAQ e t a l , 1 9 8 6 ) , b) o f t h e E s s a o u i r a basin (data from HOSSNER 1 9 8 5 ) and c) o f t h e A t l a s Gulf (Middle A t l a s and c e n tral High Atlas). The two M o r o c c a n r e g i o n s show a different evolution from the Dogqer o n w a r d s .

b

G

~ ~

c

H,. L. Malm ---

~

Doqger

~

-L~s

~ ~

u.i M

t

r

m

c

m

c

490

Hiqh A t l a s without

and the High P l a t e a u s .

sedimentation

the T r i a s s i c . of

reflect,

The r e l a t i v e l y

the w e s t e r n High A t l a s ,

majority

of recent

land bridge the

Essaouira basin,

vol.).

This

opinion

o f the m a r i n e J u r a s s i c graphic arquments.

Gulf"

also

(STETS and WURSTER 1981,

in

High A t l a s

and WARME, t h i s in a d d i t i o n ,

as w e l l

consider

in a c c o r d a n c e w i t h

to

Lower C r e t a c e o u s t r a n s g r e s s i o n ,

1981,

influence

o f the

Essaouira basin

the w e s t e r n It

vol.,

to the r e c e n t

may a l s o e x p l a i n

is

the v e r y

and JACOBSHAGEN e t a l . ,

geodynamic p a t t e r n today part

s u b s i d e n c e and s e d i m e n t a t i o n and by v e r t i c a l

of

vol.).

Compressional structures

in

in

DRES~!AY, t h i s

vol.,

this

vol.). Perhaps,

this in

block

rotations

within

the A t l a s

G u l f were accompanied

displacement along inherited

o f Upper L i a s s i c

or

tectonic

p e r i o d ceased in

rinq

Upper Doqger, and was f o l l o w e d Upper J u r a s s i c

this

Dogger age are

be e x p l a i n e d

a regime o f d i s t e n t i o n

However, t h i s

subsidence throuqhout

Lower and M i d d l e Ju-

and JACOBSHAGEN e t a l . ,

sidence.

by

and g e n e r a l

the A t l a s

sub-

G u l f du-

by w i d e - s p a n n e d , but m o d e r a t e

and C r e t a c e o u s t i m e s

up to the

Eocene.

Mesozoic and Lower T e r t i a r y ted w i t h

that

to n a r r o w zones and may, c o n s e q u e n t l y ,

end o f the

Gulf

d e s c r i b e d a b o v e , most

the Meseta b l o c k .

and l a t e r a l

(reviews

the

the A t l a s

low d e g r e e o f c o m p r e s s i o n a l d e f o r m a t i o n

faults

local

4):

an Upper

High A t l a s .

by f r a c t u r i n g

confined

the VAIL c u r v e ,

whereas in

has been d e m o n s t r a t e d by many a u t h o r s

rassic

tectonic

onward ( f i g .

i n c r e a s e d from the B a t h o n i a n upward (JENNY e t

DRESNAY, t h i s

In r e f e r e n c e

BRECHBOHLER

as b i o g e o -

the d i f f e r e n t

The E s s a o u i r a b a s i n continental

distribution

(see e . g .

vol.)

a

throughout

FROITZHEIM e t a l . ,

from the M i d d l e J u r a s s i c

shows,

which c l a i m s

h o w e v e r , the f r a c i e s

the c e n t r a l

We may,

to the

in the r e g i o n o f

exists

d e v e l o p m e n t o f both b a s i n s

al.

According

and the E s s a o u i r a b a s i n

contradicts,

JACOBSHAGEN e t a l .

of part

both b a s i n s were s e p a r a t e d by a small

But an i n t e r p r e t a t i o n

the whole Mesozoic p e r i o d

Jurassic

structures

was open to the A t l a n t i c .

"Atlas

highs

which now is

between the Meseta and the Sahara c r a t o n

"massif ancien"

et a l . ,

and l o c a l

h o w e v e r , the i n h e r i t e d small

publications,

c o n n e c t i o n between the this

Facies d i s t r i b u t i o n

magmatic a c t i v i t y ,

knowledge o f

the number,

magmatic e v e n t s . vol.)

and s u b s i d e n c e were a s s o c i a have u n c o m p l e t e

the age and the ~ e o c h e m i c a l c h a r a c t e r

A Late T r i a s s i c

the whole r e g i o n south o f LORENZ, t h i s

distension

but up to now we s t i l l dolerite

the R i f

and f u r t h e r

formation

(MANSPEIZER 1981,

basaltic

eruptions

is

of

the

known from

SALVAN 1984,

or d y k e s ,

probably

491

b e i n g o f Upper L i a s s i c , from the High A t l a s cy o f

(see JACOBSHAGEN e t a l . ,

these eruptions

were r e l i c s

Dogger and Lower C r e t a c e o u s age, were r e p o r t e d decreased with

petrolonical

Atlas.

They show an a l c a l i n e

et al.

1976),

thus

At p r e s e n t ,

connected with

geochronological

however,

these

ages be-

and e a s t e r n High

CHEVREMONT 1972, TISSERANT

intrusions

geotectonic

they

intrusions

and w i t h

a change to a more c o n t i n e n t a l

of

develop-

can not be d e f i n i t e l y

events,

due to

the p o s t - P a l a e o z o i c

o f the m o u n t a i n ranges o f of

Besides t h a t ,

the c e n t r a l

(e.g.

The f r e q u e n agree t h a t

the l a c k

of reliable

and q e o c h e m i c a l d a t a .

The second p e r i o d uplift

trend

indicating

certain

vol.).

authors

and g e o c h e m i c a l c h a r a c t e r

tween Dogger and Eocene are known in

version

this All

o f an a b o r t e d Mesozoic r i f t i n q .

of different

ment.

time.

the p r e v i o u s

the A t l a s

graben s t r u c t u r e s

evolution system.

for

is

marked by t h e

This means an i n -

the M i d d l e and the High

Atlas. According

to most a u t h o r s ,

connected with

compressional

ranean c o l l i s i o n s .

these processes were, deformation,

At p r e s e n t ,

there

age and the geodynamic c o n d i t i o n s On a c c o u n t o f

In the c e n t r a l did

by a g r e a t (this

kech,

not s t a r t number o f

vol.),

this

a b o u t the

and i n v e r s i o n . consider

the

the M i d d l e A t l a s ,

in-

again.

earlier

and i n

than the O l i g o c e n e .

sections

that

Therefore,

This

show c o n t i n u o u s

b e q i n n i n g of

is

we do not a g r e e w i t h inversion

seem to be o f a l i m i t e d

in

testified

sedimentation

who, due to an a n g u l a r d i s c o n t i n u i t y

have p l a c e d the

These o u t c r o p s

discussion

led to u p l i f t

and e a s t e r n High A t l a s

the Cenomanian upwards. al.

which

degree,

p r o b a b l y caused by M e d i t e r -

a vivid

the r e a s o n s m e n t i o n e d a b o v e , we p r e f e r a b l y

High and l he M i d d l e A t l a s ,

version

is

to a c e r t a i n

from

FROITZHEIM e t

south o f M a r r a -

the Upper C r e t a c e o u s .

importance

(JACOBSHAGEN e t a l . ,

vol.).

The h i s t o r y reflected

of uplift

of

by the s e d i m e n t a r y f i l l i n g

which l a r g e l y

consist

G~RLER e t a l .

1987 and t h i s

LER e t the A i t

the Hiqh and the M i d d l e A t l a s

al.

of psephitic

succeeded i n

Kandoula b a s i n s

o f the a d j a c e n t deposits

vol.,

clearly

Cenozoic b a s i n s ,

(GURLER and ZUCHT 1986,

FRAISSINET e t a l . ,

separating

is

microfaunas

in

this

vol.).

the O u a r z a z a t e and

south o f the High A t l a s which d e l i m i t a t e

age o f

several

tritus

was o b s e r v e d w i t h i n

phases o f u p l i f t :

The f i r s t

fanglomerates at

influxe

GUR-

o f High A t l a s

the base o f the A i t

the deKan-

492 doula very

formation thick

conglomerates

o f Hiah A t l a s strated force

(Lower Miocene - ? P l e i s t o c e n e ) . rocks.

by Mesozoic of gravity.

which

Strong

limestone

slabs

The l a r g e s t

of

300 km 2 i n s i z e .

least

of

10 m.y.

Atlas

relative

Upper P l i o c e n e . went on w i t h

formation

diminished

intercalation

of megabreccias

marks a t h i r d

stage

four

tigraphic

data o n l y .

stages

are d o c u m e n t a t e d (see ST~BLEIN, Gravity up to a t the

least

estimated rate

the

High A t l a s .

for

are not

the c e n t r a l is

sure w h e t h e r

this

phase o f its

s i o n w h i c h produced together scribed

with

the

an u p t h r u s t

Pleistocene

beds.

slope

rate

has c l i m b e d

the O u a r z a z a t e

basin

the Eocene.

A similar

o f the High A t l a s

between

gravity

the s t a g e s

the c e n t r a l

applies

overlain uplift

along ver-

by the A i t of

Toundoute".

with

High A t l a s ;

uplift.

we

The main

Hadida f o r m a t i o n ,

Kandoula f o r m a t i o n .

was a s s o c i a t e d ,

"nappe o f

coincided

has been dated as Lower

the O l i g o c e n e

and f o l d i n g

Furthermore,

of uplift

High A t l a s

the O u a r z a z a t e

amount o f

and e a s t e r n

to the O l i g o c e n e

include

East

c o u l d n o t be r e c o g n i z e d The t o t a l

and

have

to 2000 m.

tilting in

high differences

this

where l a r g e - s c a l e

sliding

the c e n t r a l

and P l e i s t o c e n e

of

the base o f

there,

that in

structures

Which i s d i s c o n f o r m a b l y Pliocene

uplift

s o u t h o f the High A t l a s

to areas w i t h

High A t l a s .

reduced,

deformation

since

based on l i t h o s t r a -

by FERRANDINI and LE MARREC ( 1 9 8 2 ) .

gravity

good e v i d e n c e

compressional Miocene,

to

the n o r t h e r n

basin,

displacement

There i s

have even

GHRLER & ZUCHT (1986)

and the Haouz b a s i n ,

rim o f

compressional

limestones,

vol.)

o f High A t l a s

In the Toundoute r e q i o n

the c e n t r a l

Ouarzazate

southern

tical

seems to be c o n f i n e d

has been d e s c r i b e d

the

structures,

(this

however,

Kandoula Upper P l i o -

of Jurassic

levels

the High

the A i t

sequence o f

stages

3-4000 m between the bottom

reqion

of

vol.):

seems c o r r e c t

sliding

of glacis

even 5000 m r e f e r r i n g

the T i c h k a of

this

displacement.

summits o f

which are, Quaternary

by a t

the S e r r a v a l l i a n

The top o f

conglomeratic

FRAISSINET e t a l .

uplift

by a s e r i e s

sliding

in vertical

of

from

the u p l i f t

and g l i d e - s l a b s

The f i n a l

demonby the

"nappe o f T o u n d o u t e " o f

quiescence intervall

by

pebbles

the b a s i n s

t h a t , owing to s l u m p i n g

of uplift.

of

moreover,

second phase was f o l l o w e d

velocity.

age,

ist, into

them was the

tectonic

i s made up o f a n o t h e r

discerned

completely

which slid

This

Durinq this

cene or Lower P l e i s t o c e n e

almost

Lower Miocene u p l i f t

approximately to the

consist

They are f o l l o w e d

again,

the A i t

with

Upper

compres-

Kandoula f o r m a t i o n

ST~BLEIN ( t h i s

vol.)

b a s i n w h i c h even t i l t e d

DUTOUR and FERRANDINI (1985)

has deMiddle have con-

493 cluded

from d e f o r m a t i o n

studies

that

kech has been u n d e r c o m p r e s s i o n moreover, of

important

the c e n t r a l

tion

of

the

Oligocene

ern b o r d e r

The Cenozoic The t e c t o n i c

history

flat-lying

the M i d d l e

of

degree

cut

by a few f r a c t u r e s the

of compression

according

to

margin

is

for

Late

not

Eocene,

Late

Bearing

this

processes

of

d e f o r m a t i o n , whereas o n l y b u i l t up o f (DRESNAY,

by l a t e r a l three

Oligocene

shows a ROBIL-

displacement. orogenic

and M i d d l e Miocene the e a r l y

i n mind the l a t e s t

Pleistocene,

stratigraphic

of the High A t l a s ' s o u t h e r n

we presume, the

known in d e t a i l .

o f MARTIN 1 9 8 0 / 8 1 ,

happened i n

formations

vol.)

a change i n c h r o n o l o g y

the d e f o r m a t i o n a l

during

the n o r t h -

known as M o y e n - A t l a s

have d i s t i n g u i s h e d

the T e r t i a r y

(G~RLER e t a l . ,

an a l t e r n a -

the M i d d l e A t l a s

more i n f l u e n c e d

The main u p l i f t

these authors.

determinations Atlas

in

respectively.

is,

than the M i o - P l i o c e n e

strip

(see s e c t i o n s

b u t was a p p a r e n t l y

phases c u l m i n a t i n g

south,

and w r e n c h - f a u l t s

High A t l a s ,

CHOUBERT and FAURE-MURET ( 1 9 6 2 ) times,

Atlas

the s o u t h e a s t e r n

Compared w i t h

LARD 1 9 8 1 ) ,

are y o u n g e r

It

borders

sedimentation

whereas a l o n g

a relatively weak c o m p r e s s i o n a l zone ( M o y e n - A t l a s t a b u l a i r e ) is

strata

vol.).

In the

evident,

Quaternary.

and n o r t h e r n

and o f t h i c k

is

s o u t h of M a r r a -

vol.). of

structure

pliss# indicates the n o r t h w e s t e r n

smaller

interval

this

to L a t e

the s o u t h e r n

folding

the main d e f o r m a t i o n s al.,

Early

that

show d i f f e r e n c e s .

phases of

- Pliocene

(FRAISSINET e t

this

from

to m e n t i o n

High A t l a s

paroxystic

the Haouz b a s i n

tectonic

similarly phases.

o f M i d d l e and High A t l a s

for

the M i d d l e

We are s u r e t h a t were c l o s e l y

con-

nected.

5.

Geodynamic model

When l o o k i n g

for

intra-continental tween two p e r i o d s : formed

resulted

African

zones,

this

commenced w i t h

period

the d e v e l o p m e n t o f the

one has to d i s t i n g u i s h

in w h i c h r i f t

and s h e a r i n g

stress-field Crustal

o f the basement t h e r e

geodynamic

situation. Africa

In Upper O l i g o c e n e this

is

ALKAPECA-fragment

related (Alboran

grabens

is

the

future

and s e d i m e n t a r y

filling

in

activated

seen as the p r i m a r y

and M i o c e n e ,

to the d o u b l e c o l l i s i o n with

source

a new p e r i o d

to a c o m p r e s s i o n a l

block)

be-

and b a s i n s

between the N o r t h

thinning

A geothermally

the change from a t e n s i o n a l

In o u r o p i n i o n ,

westernmost

for

system,

arid European p l a t e s .

beneath Northwest

such f e a t u r e s .

field.

A Mesozoic

subsidence

from

upper m a n t l e for

mountain

due to a t e n s i o n a l

American, graben

a geodynamic model

Atlas

Iberia

has

stressof

and the

the

494

z

l ._t

o

x 0

E'

-t-

O

~r~

5 ~ 02g

o

o

z N 4..~

0

m

~r,.O-Q °~ ~ f~

~,IEI 4-~ e-

w

..¢-

--~ ~ *i--°

g,

N

4-~

0 " ~ e--

.° t~

1.1-

495 northwestern

edge o f

and t h r u s t i n g lift

o f the B e t i c

and f o l d i n g As e v i d e n c e d

a zone o f beneath

of

and WIGGER, t h i s ductivity thus

indicating

ferred less in

far

the

of

tectonics

tween the c o n f i n i n g Rif,

the M i d d l e

deformation direction

strike-slip

transpression stage

We have,

however,

and i n

produce u p l i f t , mentioned and the I),

of

a

have been have been t r a n s -

o f the f o r m e r

No t y p i c a l

of

offers

zones as e v i d e n c e

could

filling

rift

remained more or

mountain

root,

like

zone depends on the a n g l e

be-

Though n e a r e r

shows a l o w e r degree o f c o m p r e s s i o n a l Hiqh A t l a s .

faults;

Running d i a g o n a l

strain,

it

(HARLAND 1971, of

to

the

was m a i n l y

deformed

by

and u p t h r u s t s

observed

are

SANDERSON and MARCHINI 1 9 8 4 ) .

The

the f o l d s

to remember 20% i n

t h e High A t l a s

in

the M i d d l e

we may take

of

into

originated

in

shortening

High A t l a s .

Atlas,

but also for

account that inversions

further

volcanic

the High A t l a s .

It

during

does n o t

is,

centers

without

any

i n the w e s t e r n conditions

the t h e r m a l within

v i e w seems to be s u p p o r t e d ,

Upper C e n o z o i c

w h i c h are m i s s i n g

lateral

Looking

may have caused d e n s i t y This

that

the c e n t r a l

the A n t i - A t l a s .

lower crust.

distribution

ranges m i g h t stress

con-

The model

and the graben a x i s .

"back-thrusting"

lower not only

High A t l a s

(SCHWARZ

of compression.

s u r m o u n t a deqree doubt,

Atlas

which

type of tectonics.

the graben

o f maximum c o m p r e s s i o n a l

Upper P l i o c e n e a late

in

Atlas

1984)

detachment

blocks.

t h a n the c e n t r a l

left-lateral due to

mountain

stress-field

Rif

horizons.

by BALLY ( 1 9 8 1 ,

by t h i s

is

shear s t r e n g t h ,

whereas the High P l a t e a u s

develope

Atlas

the zone o f h i g h e l e c t r i c a l l y

The s e d i m e n t a r y

intermediate

could

b e n e a t h the

the compressional

south.

there

i n the c r u s t

and the M i d d l e

of detachment

intracrustal

measurements,

a zone o f reduced

the assumed i n t r a c r u s t a l

uneffeEted,

The t y p e

with

as proposed

results,

to

the up-

from S to N (HAAK and HUTTON

northwards

Especially

has been f r a c t u r e d ,

the A l p s ,

to the

extends

vol.).

how the

by g e o p h y s i c a l grabens

it

the c r u s t

the e x i s t e n c e

Due to

the f o l d i n g

but also

conductivity

the High P l a t e a u s

tectonics

possibility formed.

and the R i f ,

and m a g n e t o t e l l u r i c

may be a s s o c i a t e d

thin-skinned

caused n o t o n l y

and h i g h e l e c t r i c a l

a ramp w i t h i n

Most l i k e l y

process

Cordilleras

by s e i s m i c

low v e l o c i t y like

This

the M i d d l e and High A t l a s .

the High A t l a s ,

descends 1986).

Africa.

to

processes

the upper m a n t l e too,

i n both

by the

ranges

(fig.

496

The o r i g i n

of

the T r a n s a l b o r a n

fault

s u b s e q u e n t to c o l l i s i o n

and o r o g e n y .

Northwest

zone,

African

fault

o f the West M e d i t e r r a n e a n foreland strate yet

o f the that

fully

Rif.

this

orogens

Deep-seated

the p r o c e s s

is

interpreted

lineament as w e l l

crosses along

between A f r i c a

o f an o l d

the whole ensemble

as the m o b i l e

earthquakes

of collision

as an e v e n t

Being a p r o l o n g a t i o n

zone i n

that

the

zone demon-

and I b e r i a

is

not

extinct.

Acknowledgement: fic

discussion

for

technical

the

English.

The a u t h o r s

and to

are o b l i g e d

R. C o u s i n o ,

assistance.

We a l s o

to

Dr.

S. Huhn, S t .

P. Wigger f o r K~hler,

scienti-

and D. Reich

t h a n k G. Lindemann f o r

helping

with

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