The Apulia-African Margin of the Liguro-piemontais Ocean

CHAPTER TEN

The Apulia-African Margin of the Liguro-piemontais Ocean: The Transition from Continent to Ocean Contents Summary 1. Southern Alps Transect: Northern Italy to Southern Switzerland 1.1 Palaeogeographic domains 1.2 Extension in the proximal parts of the Apulia margin in the Southern Alps:

the Lombardy rift and the Tethyan break 1.3 The Lugano fault in the Lombardy rift system 1.4 Extension in the distal part of the Apulia margin: Zones of ductile shear in the

basement west of the Southern Alps 2. The Grisons (= Graubünden) transect in eastern Switzerland 2.1 Palaeogeographic domain 2.2 Stratigraphy 2.3 Extension in the proximal parts of the Apulia margin as seen in the

Graubünden region: normal faults, listric faults and tilted blocks 2.4 The presence and ages of distinct phases of rifting 2.5 Deformation during the start of extension on the distal parts of the Apulia

margin in the Graubünden region 2.6 Deformation during the final phase of rifting and continental break-up: low-

angle detachment faults and ‘extensional allocthons’; the Platta, Err and

Tasna nappes 3. Summary and Conclusion

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Summary Before examining the development of the Liguro-piemontais Ocean, the evo­ lution of the Apulia-African margin is briefly reviewed using examples from the Southern Alps of northern Italy and southern Switzerland, and from the Austro­ alpine nappes of southeastern Switzerland. Since the greater part of the Apulia margin escaped subduction, Tethyan (pre-Alpine) structures are there particu­ larly well preserved, especially those of the former continent–ocean boundary in the Graubünden region of the Central Alps. In these areas, the age of pre-, syn-and post-rift sediments are comparable to those of the European margin. The Western Alps, from Rift to Passive Margin to Orogenic Belt, Volume 14 ISSN 0928-2025, DOI 10.1016/S0928-2025(11)14010-9

� 2011 Elsevier B.V.

All rights reserved.

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The Western Alps, from Rift to Passive Margin to Orogenic Belt

One of the most remarkable structures in the Southern Alps is the Lugano fault. This fault is one of the first to have been described and compared with similar structures observed on seismic profiles across pre­ sent-day margins. The fault bounds a major half-graben infilled with syn-rift sediments. It can be followed laterally for 30 km. Towards its base, the fault penetrates the basement as far as 15 km palaeodepth where it flattens progressively. Its trace is marked by the presence of mylonites and ultra­ mylonites developed in green schist metamorphic facies. In the proximal parts of the Apulia-african margin in the Graubünden area, the structures responsible for thinning the brittle upper crust are highangle listric faults that define tilted blocks and typify rifting. The thinning of the lower crust results from ductile stretching that follows the model described for the Iberian margin and the exhumed margin in the Southern Alps. In contrast, thinning of the crust on the distal part of the margin resulted in a different structural style. A system of low-angle detachment faults that cuts the crust and mantle dismembered the upper crust and exhumed the sub-continental mantle but without the formation of the classical listric faults of rifts. For this reason, the emplacement of the initial axis of the rift defining the morphology of the top of the pre-rift and that of the line of continental break-up are shifted in space. Inheritance from earlier structures may be the reason for the shift. The characteristics of the Apulia-african margin will be presented below by means of two sub-parallel transects: one across the Southern Alps between northern Italy and southern Switzerland (Fig.10.1) and a second in eastern Switzerland in the Graubünden region (Fig.10.2). Some com­ parative points will be made with the Western Alps transect, particularly the evidence of episodic rifting, whose phases are broadly contemporaneous with those known from the European margin.

1. SOUTHERN ALPS TRANSECT: NORTHERN ITALY TO SOUTHERN SWITZERLAND The Southern Alps lie between the Peri-adriatic fault array to the north and the Tertiary foreland basin of the Po plain to the south (Fig. 10.1: Insubric line; Fig. 10.2A; Fig. 2.4: PC, PG, PT, S, E, M lines). In this area, mild, southerly verging Alpine deformation has had relatively little effect on the rift fabric formed in Liassic to Middle Jurassic times. The main faults and tilted blocks strike nearly north–south, i.e. perpendicular to Alpine struc­ tures. In this region, reconstruction of the Tethyan margin involved in

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The Apulia-African Margin of the Liguro-piemontais Ocean

200 km N

RA

JU

LPS

AL A

TR CEN

2

Insu

SOU TH

line

ERN ALPS

WES TER N

3

AL

bric

PS

1

Mediterranean Sea 1: Southern Alps transect European continental margin units

Adriatic Sea APENNINES

2: Graubünden, Central Alps transect Oceanic units and ophioltes

3: Dauphine-Brianconnais, Western Alps transect Apulia-african continental margin units

Molasse basin

Figure 10.1 Location of cross sections in the Southern Alps, Graubünden (= Grisons) and the Western Alps in the context of the main Alpine structures. Refer to Fig. 2.2. Modified from Lemoine et al. 2000, Fig. 5.2, p. 64.

Alpine folding was first made from a comparison with the present-day western margin of the Central North Atlantic (Bernoulli and Jenkyns 1974).

1.1 Palaeogeographic domains The Apulia palaeo-margin (Fig. 10.2B) is bounded to the east by the shallow marine carbonate platform of Friuli deposited from Triassic to Cretaceous time, i.e. during the pre-, syn- and post-rift phases. The area is somewhat symmetric with the Causses–Jura platform of the inner Eur­ opean margin. To the west of the Friuli platform, Early and Middle Jurassic rifting differentiated four major palaeogeographic domains. These are: – The Belluno Trough – infilled with calcareous turbidites, which were derived from both Friuli to the east and Trentino to the west. – The Trentino, or Venice, submarine platform – covered with shallow marine limestones. A system of faults or syn-sedimentary flexures oriented N10oE to N20oE marks the boundary between the subsiding basin and adjacent platform. The system was active from the Permian to the Jurassic with a 2 km

192

The Western Alps, from Rift to Passive Margin to Orogenic Belt

F. P. G. J. P.

NO

I FRIOUL

Zone of Lakes (upper crust)

Alpine granites (Oligocene) Bergell Adamello

LU

L BE

T.

F. P.

N

MITES DOLO

F.

Fig. 10-3

INO

DY

NT

LOMBAR

E TR

P.

C.

ALPS

F.

A

segments of the Peri-adriatic fault array: F.P.C.: Canavese; F.P.T.: Tonale; F.P.J.: Judicaria; F.P.G.: Gail

Ivrea zone (lower crust)

LIG.PIEM. CANAVESE OCEAN - BIELLESE Lake Pogollo W maggiore fault fault

LOMBARDY BASINS Lugano fault

Po valley molasse basin

Mesozoic and tertiary sediment

Basement and Permian cover

TRENTINO

BELLUNO FRIULI BASIN

Lake Garda fault

Fig. 9.3

E

Ivre

a

CANAVESE Gozzano high

Monte Lugano fault Generoso basin 157Ma

B LOMBARDY BASINS

?

Ivrea Upper mantle 30 Km

Upper continent Po gal lo f ? aul t

crust

?

?

? Underplated gabbro

Lower continental crust

C

Figure 10.2 Map and cross section of the Jurassic margin in the Southern Alps. (A) The map shows in bold lines the Peri-adriatic fault system. FPG: Gail segment; FPJ: Judicaria segment; FPT: Tonale segment; FPC: Canavese segment (the latter should not be confused with the palaeogeographic domain with the same name) and the main syn-rift faults. South of the Peri-adriatic fault system, crosses indicate basement and Palaeozoic cover; dark grey: Mesozoic and Tertiary sediments (except molasse); light grey: Molasse Basins of the Po valley. (B) The corresponding cross section is a time slice at the end of rifting and onset of post-rift sedimentation. Post-rift sediments are not shown. In the Lombardy Basin and further west, radiolarites mark the onset of post-rift deposition, to the east, in Trento and around Belluno, nodular, Ammonitico Rosso limestones are the first post-rift sediments. LM: Lake Maggiore fault; Lu: Lugano-Monte Grona fault (Fig. 10.3); Ga: Lake Garda faults. See also Figs 2.9 and 4.7. (C) Tentative reconstruction of the Lombardy rift. In part after Winterer and Bosellini 1981; Bertotti et al. 1993; Manatschal 2004; Manatschall et al. 2007, Fig. 6, p. 300. Modified and completed from Lemoine et al. 2000, Fig. 11.6, p. 125.

The Apulia-African Margin of the Liguro-piemontais Ocean

193

thickness of Permo-Triassic on one side of the fault array and 4–5 km on the other. In the Dolomites, this family of extensional faults is associated with another array of transfer faults oriented N70o to N90oE (type: Valsugana line; Fig. 5.3B), which are both transtensional and transpressional, and associated with the regional extension. – The Lombardy basin, a wide and complex domain bounded by horsts and grabens with well-preserved syn-rift faults. – The Biellese and Canavese zones. These areas are poorly known because of poor, sparse outcrop in low-lying areas. In addition, Alpine deformation is intense due to their original proximity to the former continent–ocean boundary of the major Canavese (Peri-adriatic) fault trend. The Liguro-piemontais Ocean was present to the west of the area as documented by some ophiolite outcrops within western Canavese units.

1.2 Extension in the proximal parts of the Apulia margin in the Southern Alps: the Lombardy rift and the Tethyan break The Lombardy rift (Fig. 10.2B and C), which developed between Late Triassic and Late Liassic times, was located between the Lugano and Lake Maggiore faults, and the Lake Garda fault. The Lugano and Maggiore faults downthrow eastward towards the Apulia–Africa continent while the Lake Garda fault downthrows westward towards the nascent Tethys. The Lom­ bardy rift does not predate the future Tethyan break-up line which was located west of the Canavese as shown below.

1.3 The Lugano fault in the Lombardy rift system The north–south Lugano fault is a major palaeofault largely unaffected by Alpine deformation that can be followed for 30 km (Fig. 10.3). It has great scientific importance because it is a rare example of an exposed fault in the Alps that allows study at outcrop of the continuation in depth of a major listric fault located at the boundary or transition between the ductile lower crust and brittle upper crust. In the Monte Generoso basin, immediately adjacent to the Lugano fault, thousands of metres of hemipelagic sediments of Rhaetian to early-Middle Liassic age are interbedded with sedimentary breccias derived from the adjacent fault scarp. In contrast on the footwall, west of the fault, Rhaetic and Liassic beds are only a few tens of metres in thickness. These differences in thickness are also present in the Norian. The Lugano fault is sealed by

194

The Western Alps, from Rift to Passive Margin to Orogenic Belt

LUGANO SWELL

MONTE GENEROSO BASIN (SOUTHERN ALPS)

0

Toarcian (ammonitico-rosso)

Stabiello Gneiss

10 Km A

5 Km

0

Hettangian to Pliensbachian (synrift) breccia an er m i Rhaetia to P n n Noria rnia n Ca M.Muggio Gneiss mylonites(greenschist

facies) and ultramylonites

JEANNE D'ARC BASIN (NEWFOUNDLAND)

W

E

post-rift syn-rift B 10 Km

5

time

10 sec

MOHO

MOHO

Mantle

Figure 10.3 (A) Reconstruction of the Lugano - Monte Grona syn-rift fault. The thickness of Rhaetian and Liassic sediments exceeds several kilometres. The Lugano fault became active during deposition of Norian dolomites which vary in thickness across the half graben. Above the syn-rift sediments, the nodular, “Ammonitico Rosso” limestones of Late Liassic (Toarcian) age are not displaced by the fault. After Bertotti in Bernouilli and others, 1990; modified from Lemoine et al., 2000, fig. 11.7, p. 127. (B) Comparison with the Jeanne d' Arc Basin (Newfoundland margin) Line drawing of Lithoprobe deep seismic line 85-4. The Murre fault soles at about 22km (7sec.). Conspicuous features include a slight rise in an otherwise flat Moho, intracrustal detachment, rollover of crust and Mesozoic stratigraphy into listric extensional faults and a relatively thin post rift megasequence. After Tankard and Welsink, 1989; Manatschal and Bernoulli, 1998, fig. 4b, p. 376.

post-rift pelagic and hemipelagic sediments that include the Ammonitico Rosso nodular limestone of Toarcian age (190–180 Ma). The characteristics of this fault were first documented as early as 1964 by Bernoulli prior to the recognition of tilted blocks on present-day passive margins. However, its geodynamic significance as an eastward-dipping fault separating the footwall from the half-graben to the east was not understood

The Apulia-African Margin of the Liguro-piemontais Ocean

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until some years later. The Lugano fault is a basement-involved listric fault as shown by more recent work which has defined the deeper part of the fault trace in Triassic sediments as well as in the underlying crystalline basement (Fig. 10.3). Within the basement, the fault trace is characterized by narrow mylonitic bands affected by retrograde green schist metamorph­ ism. At about 10 km below the base of the Upper Liassic, the fault curves to become almost parallel to the sedimentary strata, notably to the Jurassic– Triassic boundary and the contact between the crystalline basement and overlying Triassic sediments. The lower trace of the fault was therefore approximately horizontal at the time of formation. An elegant comparison can be made between the observed and restored structure of the Monte Generoso half-graben with the deep structure of the half-graben underlying the Jeanne d’Arc basin off Newfoundland inter­ preted from seismic profiles. In the two examples, the bounding major listric fault approaches the horizontal towards 10–15 km depth, i.e. towards the middle of the crust. The seismic profile also shows elevation of the Moho above the stretched zone (Fig. 10.3A and B). Comparable structures are known on the European margin in the Sub-alpine domain. However, the level reached by erosion is insufficient to expose the deeper traces of the normal faults. Their qualification as listric normal faults is based only on geometrical reconstructions constrained by the structure observed at out­ crop (see Fig. 6.2, Ornon and Fig. 13.1).

1.4 Extension in the distal part of the Apulia margin: Zones of ductile shear in the basement west of the Southern Alps Within the basement, two rock assemblages are present representing the upper and lower continental crust (Fig. 10.2C). The upper continental crust consists of pre-Permian gneisses and micaschists known as the ‘Zone of Lakes’ unit on the Central Alps transect (Chapter 2). The lower crust is composed of granulite-facies-grade gneisses and, towards the base, i.e. close to the palaeo-Moho, of gabbros and peridotites. This lower crust corre­ sponds to the Ivrea zone of geologists, also called the First Dioritic-kinzigite zone that outcrops south of the Peri-adriatic fault line (Fig. 2.7). Zones of ductile shear intersect the continental crust at several places of the Lombardy, Biellese and Canavese Alps (Fig. 10.2C). The most important shear zone on the distal margin is the Pogallo (Fig. 10.2A and C), which transects the upper crust to curve at greater depth and then merge with the upper to lower crust boundary. It plunges eastward, i.e. beneath the Apulia margin itself.

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The Western Alps, from Rift to Passive Margin to Orogenic Belt

Ductile shear is accompanied by a local, weak metamorphism which typically post-dates the (Hercynian) main metamorphism and deforma­ tion. Formation of the post-Hercynian shears at depth is shown by their association with green schist facies metamorphism and locally by amphi­ bolite facies, as in part of the Pogallo shear. Thermo-barometric results show the fault was active during Liassic time. Likewise, Rb/Sr and K/Ar radiometric dating of other early Mesozoic shear zones within the Ivrea have yielded some Permian ages but mainly Triassic and Liassic ages, i.e. from 220 to 170 Ma. The structural, metamorphic and geochronological signatures of rifting are therefore recorded within the lower continental crust. The effect of the Pogallo fault has been to reduce the thickness of the continental crust by about 10 km solely by stretching of the lower con­ tinental crust without apparently affecting the upper crust. Even if the ages are the same, the mechanism of stretching is clearly different for the proximal and distal parts of the margin.

2. THE GRISONS (= GRAUBÜNDEN) TRANSECT IN EASTERN SWITZERLAND The Austro-alpine nappes outcrop mainly to the east in the Grau­ bünden (=Grisons) within the Eastern Alps. This area, situated close to the Swiss–Austria border, lies at the western erosional limit of the Austro-alpine nappes.

2.1 Palaeogeographic domain Within the Engadine window in the Inn valley region, the following Austro-alpine nappes are distinguished from base to top (Fig. 10.4B): the Err (mainly basement with cover remnants), the Samedan (cover), the Julier and Bernina nappes (basement), the Ela and Ortler and some Engadine dolomites (mainly cover with minor basement) and the Silvretta and Otztal (mainly basement). The entire nappe pile overthrusts the Platta ophiolitic nappe that was derived from the Liguro-piemontais Ocean. The Err and Platta nappes provide a unique and important record of the early phases of rifting on the distal parts of the margin and involve wellpreserved structures resulting from the exhumation of sub-continental mantle.

197

The Apulia-African Margin of the Liguro-piemontais Ocean

palaeokarst

breccias and turbidites CRETACEOUS

POST -RIFT

A

UPPER JURASSIC and UPPER MIDDLE JURASSIC

SYN SYN-RIFT AND -RIFT PRE-RIFT(?)

MIDDLE JURASSIC and UPPER MIDDLE LIASSIC LOWER LIASSIC RIAN

RHAETIAN

NO

NORIAN

NIAN

CAR

IC

LE

MIDD

SS TRIA

SIC RIAS IAN ER T LOW D PERM AN

limestones radiolarites dolomites

100 m approx.

B W

Platta

Err

0 5 5 15 oceanic crust 20 Km

Malenco

Permian gabbro

Err 0

L

Ortler Fig.9.5 E 0

Bernina-Ela

MJ

Bernina Tr

continental crust 30 Km

Margna fault

Ela L

C MJ

Tr

Tr

10 Km

Figure 10.4 (A) Synthetic and theoretical Triassic to Cretaceous stratigraphy of the Austro-alpine nappes of the Grisons in relation to a syn-rift palaeofault. Along the arrays of palaeofaults, syn-rift deposits vary in thickness and facies; shallow marine carbonates deposited on fault block highs commonly rest on an eroded surface cut by tectonic fissures and perhaps karstified. Hemipelagic sediments interbedded with breccias derived from fault escarpments infill the half-grabens. Fault throws are towards the west and the ocean in the Err nappe, which originated close to the Liguro-piemontais Ocean. Modified from Lemoine et al. 2000, Fig. 11.1, p. 121. (B) and (C) Cross section of the Jurassic margin in the Grisons (Central Alps) simplified from Manatschal and Nievergelt (1997); Manatschal and Bernoulli 1999, Fig. 4b, p. 1105.

2.2 Stratigraphy Pre-rift: basement and Triassic. Above the crystalline basement and Palaeozoic sediments, the pre-rift succession consists of: – Early Triassic sandstones. – Middle Triassic platform carbonates deposited in very shallow marine environments.

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The Western Alps, from Rift to Passive Margin to Orogenic Belt

– Late Triassic beds composed of Carnian evaporites, overlain by Norian dolomites (Hauptdolomit or Dolomia Principale facies) which were deposited over a broad, tidally dominated area. The overlying Rhaetian beds (latermost Norian) were deposited in an open but shallow sea. However, there is no evidence of active Late Triassic faulting in the Grisons. Syn-rift: Middle to Late Liassic and Middle Jurassic in part. The succession consists of sediments deposited either on the raised edge of tilted blocks or in the adjacent half-graben. Sediments infilling the half-grabens include olistoliths, breccias and turbidites derived from the fault scarps of the tilted blocks. Successive phases of rifting are dated Hettangian, Sinemurian, and probably Early Pliensbachian (?) and Late Liassic (?) (Fig.10.7). The age of these episodes compare well with those recorded on the European margin (Fig. 6.19). Post-rift: Late Middle Jurassic and Cretaceous. The lithostratigraphy compares well to that of the European margin: – radiolarites and light-coloured pure Aptychus-bearing limestone dated as Late Middle Jurassic and Late Jurassic, – alternating shales and limestones dated as Early Cretaceous that are closely comparable to the Replatte formation of the Western Alps and the Palombini of the Apennines, and – pelagic calcareous shales dated as Late Cretaceous.

2.3 Extension in the proximal parts of the Apulia margin as seen in the Graubünden region: normal faults, listric faults and tilted blocks Extension in the proximal parts of the margin is mainly due to normal fault movements in the brittle upper part of the crust and ductile stretching in the lower part following the classical model mentioned above for the Southern Alps (Fig. 10.3) and Western Alps (Fig. 6.2A), and developed from seismic profiles across present-day passive margins (Fig. 1.8). In these areas, the majority of the syn-rift faults have either been reactivated or sheared by thrusts (Fig. 14.1: Ortler and Campo Nappes). However, several have been preserved within nappes such as the Ortler (Figs. 10.5 and 10.6: Livigno area). Intercalated clastic beds, breccias and then turbidites further basinward in half-graben that also include megabreccias at the foot of palaeofaults provide additional evidence for syn-rift faulting (Figs. 10.4A and 10.6). The distribution of these syn-rift sediments allows reconstruction of the orientation and displacement of the syn-rift

The Apulia-African Margin of the Liguro-piemontais Ocean

Tr

199

L

Livigno Lake

Figure 10.5 The palaeo half-graben of Il-Motto near Livigno (Grisons, Central Alps). The very well-preserved palaeofault (F), virtually unaffected by post-Liassic deformation, dips S or SE, i.e. towards the right. Displacement took place during the Hettangian (1, 2) and Early Sinemurian (3) and marks the first phase of rifting known on the European margin. Emphasized by a narrow couloir of fallen rocks, the throw of the normal fault is more than 300 m. At the southern or southeastern foot of the fault escarpment, breccias and mega-breccias are intercalated within ammonite and belemnite-bearing limestones and marls (L) of the half-graben. These breccias and mega-breccias are due to collapse of the fault scarp. T: Triassic (Norian) dolomites; L: syn-rift Liassic limestones and marls with intercalated breccias, dated by ammonites. 1, 2: Hettangian; 3: Early Sinemurian; 4: Late Sinemurian. Stratigraphic results after Eberli 1988. Adapted from Lemoine et al. 2000, Fig. 11.2, p. 123.

faults: the throws of syn-rift faults, from direct observation or reconstruc­ tion, are eastward and towards the Apulia–Africa continent in the Bernina, Ela and Ortler nappes (Fig. 10.5). In addition, major listric normal faults have been documented from field evidence.

2.4 The presence and ages of distinct phases of rifting The timing of syn-rift faulting is documented mainly from the age of turbidites derived from submarine fault escarpments (Figs. 10.4, 10.5 and 10.7). Each phase of fault movement triggered deposition of a sequence of coarse, thinning upward turbidites that is also widely observed on the European

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The Western Alps, from Rift to Passive Margin to Orogenic Belt

Figure 10.6 Syn-rift palaeofault in the Western Ortler nappe: Piz Chaschauna sector near Livigno. This well-preserved palaeofault dips NE (towards the right). To the left, pre-rift Triassic carbonate (T); to the right, dark coloured marls and limestones, intercalated with breccias, are syn-rift Liassic (L) deposits. After Froitzheim and Eberli 1990. Photo N. Froitzheim. Adapted from Lemoine et al. 2000, Fig. 11.3, p. 123.

margin (Figs. 6.5, 6.6 and 6.19). The first phase of Early Liassic age (Hettangian–Sinemurian, about 200 Ma: 1a and 1b in Fig. 10.7) is well dated by ammonites. The later phases are of Middle Liassic (Early Pliens­ bachian: 1c) and Late Liassic–Middle Jurassic (2) age but are poorly dated, especially the latter. Again, these ages are similar to those documented on the European margin (Chapter 6).

2.5 Deformation during the start of extension on the distal parts of the Apulia margin in the Graubünden region Those structures resulting from extension on the distal parts of the margin (Fig. 10.4) are very different from those on the proximal parts (Figs. 10.3 and 10.4B). They are due to displacement along low-angle detachment

201

The Apulia-African Margin of the Liguro-piemontais Ocean

post-rift sédiments

Ma: 160 Callovian

Bathonian Bajocian

180 Aalenian Toarcian

2

ophiolites

2

2

2

1c 1b 1a Chaschauna (Fig. 9.6)

BERNINA

ELA

1a II Motto (Fig. 9.5)

ORTLER Middle Jurassic

Rifting phase 2 Rifting phase 1a to 1c SE

10 km

0,5 km

Pliensbachian Sinemurian 200 Hettangian NW ERR PLATTA Mid-Juras. ocean

2

Liassic Triassic (carbonates) −basement

Figure 10.7 Reconstruction of the African margin in the Austro-alpine of Graubünden at the syn-rift–post-rift boundary and ages of the principal phases of rifting. The successive phases are: 1a: Hettangian; 1b: Sinemurian; 1c: Early Pliensbachian (?); and 2: Late Liassic (?). Those correspond in the half-graben to megasequences composed of turbidites whose bed thickness and grain size decrease upward (compare also Figs. 6.3 and 6.5). Initiation of each megasequence is considered to mark the start of a rifting phase. See the migration with time of the onset of rifting from the proximal to the distal part of the margin, as shown by Manatschal et al. (2007). After Eberli 1988; Lemoine and Trumpy 1987. Adapted from Lemoine et al. 2000, Fig. 11.4, p. 124.

faults leading to stretching of the lower crust but without any notable stretching of the brittle upper part of the crust during the start of extension. The Margna fault (Figs. 10.4B and 10.8B) is one of the main detachment faults situated in a distal position. It plunges eastward beneath the conti­ nental Apulia block. It separates the brittle upper crust from the ductile lower crust. Extension along the fault has led to significant thinning of the continental crust. Thermo-barometric results from the footwall and hanging wall show a difference of 0.3–0.4 GPa indicating about 10 km of crustal thinning during the Liassic. Comparable results have been obtained from the Pogallo fault in the Southern Alps (see Fig.10.2C). The constant thickness of syn-rift sediments, though small due to starvation, suggests that there was no extensional fault with a notable throw in the upper continental crust. A half-graben appears to be absent in a distal position, in contrast to what is clearly observed on the proximal part of the margin. The transition from platformal sediments to pelagic sediments with cherts from the start of rifting shows, however, that the subsidence of the whole margin began early at the start of rift-driven subsidence.

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The Western Alps, from Rift to Passive Margin to Orogenic Belt

Upper crust

A

Lower crust Permian gabbro Upper mantle

AREA OF FUTURE BREAK-UP FUTURE DISTAL MARGIN

FUTURE PROXIMAL MARGIN differential subsidence, controlled by normal faulting

synrift dediment

B

Lower crust

underplated gabbro

Upper mantle

mantle exhumation crust thinning to 10 km

C – Oceanic Crust

Zone of Exhumed Continental Mantle

Continental Crust

extensional allochtons

0

10 km

5 km

tectono-sedimentary breccia serpentinized peridotite

pillow basalts

continental

crust

gabbros

Figure 10.8 Tentative reconstruction of the development of the Apulia-African passive margin on both the Southern Alps and Grisons transects (Northern Italy and SE Switzerland). Simplified from Manatschal 2004, Fig. 4, p. 439 and Fig. 8, p. 447.

2.6 Deformation during the final phase of rifting and continental break-up: low-angle detachment faults and ‘extensional allocthons’; the Platta, Err and Tasna nappes The succession of syn-rift deposits in the Err nappe (Fig. 10.4) comprises from base to top: (1) hemipelagic limestones with cherts of Middle Liassic age, (2) a submarine hardground of Pliensbachian age, (3) poorly dated deep marine hemipelagic sediments interbedded with clastic horizons and (4) radiolarian cherts of Middle Jurassic age. These sediments rest horizontally in places on the surfaces of the detachment faults, which are characterized by a fringe of crushed granite topped by a well-defined ‘black gouge horizon’ which outcropped on the sea floor. These relationships show that the youngest syn-rift sediments were deposited during or just after the detachment and exhumation of the

The Apulia-African Margin of the Liguro-piemontais Ocean

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sub-continental mantle. Following Manatschal (2009), the absence of ductile deformation in crystals shows that these faults were active at temperatures lower than 300�C, i.e. in the upper 10 km of the crust, with a major role being played by fluids in localizing extensional faults in the ductile part of the crust. It is important to strongly emphasize that the detachment surfaces are very clearly established as being quite distinct and different to Alpine thrust surfaces. The detachment fault of the Err nappe can be followed in the field semicontinuously for 18 km parallel to the direction of transport. The hanging wall comprises block faults, fragments of upper continental crust constitut­ ing extensional allocthons with a basal truncation that corresponds to the detachment surface (Figs. 10.8 and 12.9). It appears that the blocks belong­ ing to the hanging wall were separated or ‘boudinaged’ at the time of detachment and now rest on top of the footwall. Their existence is the signature of low-angle detachment faults. Similarly, in the mainly ophiolitic Platta nappe (Figs. 10.8 and 12.9), a system of detachment faults separates serpentinized peridotite beneath from basement blocks derived from the adjacent continental crust above. The blocks themselves are covered by syn-rift sediments succeeded by those of the post-rift. They are also interpreted as extensional allocthons lying tectonically on exhumed sub-continental mantle. The zone of faults composed of foliated serpentinite, cataclastic serpen­ tinite and serpentinite gouges records a system of deformation and hydration in conditions evolving between amphibolite-grade facies to those at outcrop on the seafloor. It marks the denudation of the upper lithospheric mantle which is accompanied by modest magmatic activity.

3. SUMMARY AND CONCLUSION Each of the Apulia-African and European margins was about 250– 350 km wide. Prior to spreading the whole rift between the western (Causses–Jura) and eastern (Friuli) carbonate platforms was at least 500– 700 km wide. However, the rift was probably asymmetric because of the presence of the elevated, several hundred kilometres wide, SBR (Briancon­ nais) block on only part of the European margin. Neither the Trentino platform, which remained submerged, nor probably the Canavese zone, poorly known due to sparse outcrop, seem to have played a comparable role on the conjugate margin.

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The Western Alps, from Rift to Passive Margin to Orogenic Belt

Stretching began at the same time on the proximal and distal parts of the margin. Indeed, independent results show that in the Southern Alps, the Lugano listric fault and the low-angle Pogallo fault were active approxi­ mately at the same time during the Liassic. In addition, the ages of the rift phases in the Grisons area are broadly contemporaneous with those observed on the European margin (Figs. 6.19 and 6.20). On the proximal part of the two conjugate, European and ApuliaAfrican margins, the fabric of reconstructed or preserved palaeofaults and the associated syn-rift sediments provides a classic image of tilted blocks and associated half-grabens. Ductile shear zones observed in the basement of the proximal part of the margin are the lower parts of curved listric syn-rift faults that bounded the half-grabens (Fig. 10.3). On the distal part of the margin, structures resulting from extension are quite definitely different from those on the proximal part. Movement along low-angle faults achieved the reduction in the thick­ ness of the continental crust on the distal part of the margin by about 10 km. Such faults ensure decoupling between the upper crust and mantle and affect only the lower crust (Fig. 10.8B). At the end of rifting and during the start of continental break-up, movement along detachment faults exhumed the sub-continental mantle and lower crust with the emplacement of extensional allocthons composed of fragments of continental crust covered by thin syn-rift sediments suc­ ceeded by those of the post-rift. The consequence of the difference in styles of extension between the distal and proximal parts of the conjugate margins is that the domain of upper crustal thinning (for example, the Lombardy rift or Rhone valley) and the line of future break-up are shifted relative to each other, To explain the location of the line of break-up, the role of structural inheritance may also be taken into account. Thus, under the Magna fault (Fig. 10.4), gabbros were emplaced at the crust–mantle boundary during the course of one of the episodes of extension or transtension accompanying Permian magmatism. These gabbros cooled slowly, in isobaric conditions over about 50 Ma, but without the thickness of the crust being reduced from more than 30 km. A possible conjecture is therefore (Fig. 10.8) that the presence of this intrusive body might have induced the line of break-up because of the heterogeneity affecting the base of the crust and the top of the mantle.