The coalification pattern in the Northern Apennines and its palaeogeothermic and tectonic significance

The Coalification Pattern in the Northern Apennines and its Palaeogeothermic and Tectonic Significance

By KLAIJs-J. REUTTER, Berlin, MARLIES TEICHMOLLER and ROLF TEICHMULLER, Krefeld, and GIoRc, IO ZANZUCCHI, Parma *)

With 8 figures and 1 table

Zusammenfassung

Der Nordapennin wird zu einem grol3en Teil aus nicht metamorphen Flyschen und Molassen aufgebaut, die in bezug auf die Deckentektonik (Miozan) teils pra-, teils syn-, tells posttektonisehen Alters sind. Da in diesen Gesteinen h~ufig Pflanzenreste auftreten, kSnnen Inkohlungsuntersuchungen durch Messung des ReflexionsvermSgens yon Vitri- niten (als ~ Rm = mittleres Reflexionsverm6gen) zur Kl~rung der Zusammenh~inge zwischen Orogenese und pal~iogeothermischer Entwicklung beitragen. Es wurden mehr als 180 Proben aus Oberfliichenaufschliissen und drei Tiefbohrungen bearbeitet, deren Ergebnsse einige wichtige Feststellungen gestatten. Im Deckenstapel von Liguriden und Toskaniden nimmt der Inkohlungsgrad yon den hSheren zu den tieferen Deeken zu bis hin zur niedriggradigen Metamorphose der unteren Toskaniden. Innerhalb der Decken nimmt der Inkohlungsgrad vonder ligurischen Kilste (Internzone) in Richtung auf den Rand der Po-Ebene (Externzone) ab. In der hSchsten Liguriden-Deeke konnte 5rtlich eine pr~i-oligoz~ine (d. h. prii-apenninisehe) Inkohlung des Oberkreideflysehes naehge- wiesen werden. Posttektonische Aufheizungen stehen im Zusammenhang mit dem jungen (Obermioziin-Pleistoziin) Magmatismus der Toskana. Von diesen ~ilteren und jfingeren pal/iogeothermischen Ereignissen abgesehen, ist der wesentliche Teil der Inkohlung auf eine thermisehe Beansprnchung naeh der 13bereinanderstapelung der Decken in der Apenninorogenese (Mioz~in), jedoch vor den letzten Deckenschfiben und vor der jiingeren Dehnungstektonik der Internzone des Gebirges, zuriickzufiihren. Deshalb ist die entschei- dende regionale Erhitzung als synorogen oder genauer als sp~itsynorogen zu bezeiehnen.

Abstract

The rocks of the Northern Apennines predominantly consist of non-metamorphic ter- rigeneous deposits (flysches and molasses) some of which are preorogenic, some synorogenie and others postorogenic with respect to the nappe tectonics (Miocene). As plant fragments frequently occur in these sediments, a study of coal rank based on reflectance measurements on vitrinites (~ Rm = mean value of the random reflectance in non polarized light) contributes to the clarification of the relation between the orogenic and the palaeogeothermal development. The determination of the Rm values of more than 180 samples from outcrops and three deep drillings revealed some important features. Within the pile of Liguride and Tuscanide nappes, the coal rank increases from the uppermost nappe to the lower nappes until Iowgrade metamorphism is readaed in the Lower Tuscanides. In the single nappes the rank decreases from the Tyrrhenian coast (internal zone) towards the Po Plain (external zone). This regional trend is disturbed only locally by young post-coa|ification tectonics. In the uppermost Liguride nappe (M. Antola Unit) a pre-Oligocene (i. e. pre-Apenninic) thermal event was detected.

*) Addresses of the authors: Prof. Dr. K.-J. REUTTER, Institut ffir Geologie, Freie Uni- versit~it Berlin, Altensteinstr. 84 A, D-1000 Berlin 38, M. and R, TEmHMOLLER, Geolo- gisches Landesamt Nordrhein-Westfalen, De-Greiff-Str. 195, D-4150 Krefeld, and G. ZANZt~CCm, Istituto di Geologia deU'Unlversitfi, Via Kennedy 4, 1-48100 Parma.

Geologische Rundschau, 72, 8, 861--894, Stuttgart 1988 861

K.-J. I~EUTTER et al.

Postorogenic heating is connected with the magmatic activity of Late Miocene to Pleistocene age in Tuscany. Except for these preorogenic and postorogenic thermal events, the main coalification is generally younger than the emplacement of the nappes in the nappe pile during the Apenninic orogeny in the Miocene, but it is older than the last thrust movements and the final tensional tectonics in the internal zones of the chain. For these reasons, the main regional thermal event has to be considered as synorogenic or, more precisely, as late-synorogenic.

R6sum6

L'Apennin septentrional est const/tu6 surtout par des flyschs et molasses, qui sont en partie pr6-orog6niques, syn-orog6niques et post-ol'og6niques par rapport ~ la tectonique de nappes. Comme des fragments v6g6taux sont fr6quents dans ces s6diments, l '6tude du degr6 de leur carbonification bas6e sur des mesures de r6fleetance des vitrinites (~/0 Rm = valeur moyenne des pourcentages de la lumi6re r6fl6chie non po- laris6e) peut contribuer h 6claircir la relation entre le d6veloppement orog6nique d'une part, et pal6og6othermique d'autre part. La d6termination des valeurs Em de plus de 180 6chantillons d'affleurements et de trois sondages profonds a r6v616 des traits ira- pod• Dens la pile de nappes Iigures et toscanes le degr6 de carbonification aug- mente dans le sens vertical. La r6flectance s'accrolt allant des nappes sup6rieures celles inf6rieures aboutissant an m6tamorphisme de faible degr6 des roches de la nappe toscane inf6rieure. Dans chacune des nappes, le degr6 de carbonification diminue partir de la c6te figure (zone interne) en direction de la bordure de la plaine padane (zone externe). Localement cette tendance est interrompue par une tectonique post- carbonification. Dans la nappe sup6rleure (Unit6 du M. Antola) un 6v6nement ther- mique pr6-Oligoc6ne (pr@-Apenninique) rut constat6. Un r@chauffement post-orog6nique en Toscane est Ii6 ~ l'activit6 magmatique allant du Mioc6ne sup6rieur au Pl6istoc6ne. Mis ~ part ces 6v6nements thermiques pr6-orog6niques et post-orog6niques, la carbonification principale est plus r6cente que l 'empilement des nappes durant ]'orog6n6se Apen- ninique au cours du Mioc@ne; mais elle pr6c@de les derni@res pouss6es orog6niques et la tectonique distensive finale qui affecte la zone interne de l'Apennin. Pour ces raisons ce r6chauffement principal est de nature syn-, voire tardi-orog6nique.

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862

The Coalifieation Pattern in the Northern Apennines

Introduction

The entire Northern Apennines are built up on the western rim of the continental Adriatic mieroplate. They consist of Mesozoic to~Tertiary sediments whieh belong to two different depositional areas. The aut6chthonous sediments and those of the lowermost nappes (Tuseanides and Umbro-Marehean sequenee) were deposited upon the continental crust of the~idriatie plate. Because of its trend of continuous sinking since the Triassie, the neritie carbonate sediments of

�9 / , / i :

late Triassic and early Jurassie age passed over pelagic to deep-water facies until, at the beginning of the Apenninie orogeny (Late Oligoeene) thick graywaeke sequenees were deposited (until the Late Miocene). The overlying alloehthonous units (Ligurides) consist of eugeosynelinal sediments of Late Jurassic to Eocene age, mainly flyseh formations, and some ophiolites (Jurassic)., They were deposited in the oceanic Penninie-Ligurian eugeosyneline which sinee the Jurassic bordered the Adriatic plate to the west, separating it from the pre-Alpine European con- tinent.

It is generally considered that the oceanic material was abducted upon the western edge of the Adriatic plate at the Oligocene-Miocene boundary and that from then into the early Pliocene it was distributed by gravity tectonics (gravity nappes and olistostromes) to the northeast towards the interior of this plate. Orogeny was a eomplex migratory process which produced first crustal downwarping, then eompressional teetonies and crustal updoming which gave rise to gravitational sliding, and finally tensional tectonics accompanied by some magmatism during the Late Miocene and the Plio-Pleistocene in the internal zones (Tuscany). The geometric relationships between the tectonic units and nappes are represented in Fig. 1.

sw i ~ 4 : i : ? ~ / n t;,ot a:i:!:i:!:;!:!:i:i:i: (HOnqhd~ oi'O-):~ ~ ~ _ ~ - _ _ ~ ~ " ~ % ~ ,

i!?ii!iiiiiiiii0t"iic"C~i":"Ciii"ii?!?!?iiiii?iiii?i~ ~ N:!:!:;??:i:?!:i:!:i;:i:?~ - ~ -~ . : . ~ ' % ~

i:i:i:~:i:i:i:i:i:i:i:i:?:?:il, O n e t 0 t 0 ::::::::::::::::::::: ~ r . . . . . ~ , ~ - - - ' ~ - ~ ' - ~ - - ~ ~ r 4 o a l n o - / z L Tuscanidesl l ~ " - - , ~ C e r v a r i | t ~ Po Plain,

Umbra - Harc l ies

Hasso Z conid + Autocl, thonous Basement + + + * y ~ 2 ; ~ + + + + ~ ~ + + + + + + + + + +

4 - ' t - 4- 4 - 4 -

Precursory slides from Antolo- Cassio nappe

Fig. 1. Geometric arrangement of the nappes and tectonic units of the Northern Apen- nines (from R~VTTEa, 1980). Shaded: Liguride units; blank: units belonging to the

Adriatic Platform.

863

K.-J. REUTTER et al.

It is especially noteworthy that the whole orogenetie development was con- comitant with sedimentation. The graywacke flysehes of the Tuseanides (Macigno Fm.), of the Modino Cervarola Unit (Cervarola Fm.), and of the Umbro-Marches (Marnoso-Arenacea Fm.) reflect the migration of the initial foredeep towards the Po Plain. At the top of the uppermost Liguride nappe (Antola-Cassio Unit) molasse sediments were deposited during the nappe movement (Ranzano-Bis- mantova sequence; Fig. 1). While in the Po Plain bordering the Apennines (final foredeep) sedimentation was more or less continuous, in the internal zones Late Miocene, Pliocene and Pleistocene sediments accumulated in locally subsiding basins (intradeeps) after nappe tectonics had come to an end. With the exception of the lowermost internal units, which suffered some epimetamorphism (low grade metamorphism; e. g. Apuan Alps), all sediments are affected solely by diagenesis and anchimetamorphism (very low grade metamorphism). As orogenic development and thermal history are intimately linked, a study of the diagenesis and metamorphism of the sedimentary rocks of the Apennines should reveal new in- sights not only into the palaeogeothermie history of the rocks, but also into the orogenic processes.

Most of the sediments of the Northern Apennines are clastie rocks (flyseh and molasse sequences) which frequently contain plant debris ranging in size from microscopic up to entire trunks. In the turbidite formations and moreso in the molasse sediments, the organic material is drifted in. It can occur in the form of isolated particles (large or small fragments of leaves, twigs, and trunks), but usually, in the turbidites as well as in shallow water elastic sediments, it is en- riched in certain layers and may even form "seams" of impure coal some eenti- metres or even tens of centimetres thick. Therefore, the best applicable method of studying the thermal history of these sediments appared to be determination of coal rank by reflectance measurements (TEmItMt~nnER & TEmHMOLLE~, I981). It is well known that plant matter is very sensitive to temperature increase. The degree of coaliflcation (rank of coal) rises with increasing rock temperature and depends also on the duration of heating. Hence, rank of coal allows estimation of the depth and history of subsidence, and of palaeogeothermics.

M e t h o d s

The rank of coal was determined by microscopic reflectance measurements on humie plant remains (huminite/vitrinite) embedded in the various rocks. Most samples were rich in measurable coaly inclusions. In many eases the reflectance could be measured on large pieces of drift wood.

In the lignite stage the reflectance of many coniferous woods is relatively low due to impregnations with cellulose and/or resinous matter. As far as possible, measurements of this type of huminite (called "ulminite A") were avoided. In- stead, the gelified "ulminite B" type was measured or - - if this was not present the "eorpohuminite" cell fillings. In problematic eases, fluorescence microscopy was usecl to back up the reflectance measurement. The visible fluorescence of lip- tinites (hydrogen-rich coal macerals) extinguishes in the fat coal stage (1.8-- 1.6 ~ tlm).

Except for borehole cores and some surface samples, most rocks were more or

864

Table 1. Following the order of the sample numbers, information is given about laboratory numbers, location, formation and rock type, age, results of reflectance measurements,

number of individual measurements, degree of oxidation, and special remarks.

Sample Number Sample Location Formation and Age Field Laborat. Rock Type

Rm Rmax Rmin n Oxidat. Remarks % % %

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

i 13800 0.5 km NW of Riumaggiore, Macigno, upper part, Oligocene/ 5.07 6.22 4.06 60 - Cinqueterre (SP) graywacke Miocene

2 13801 5 km $6 Porretta Terme Cervarola fm./ l ig- L. Miocene 1.10 65 ++ (BO), M. Calvi nite from grayw.

3 14350 Road along Trebbia River Bobbio (Cervarela) M. Miocene (!.34) 4 ++ S Bobbio(PC), 0,7 km SE fm., graywacke

4 14351 same, lower strat, po- same M. Miocene 1.88 2 .11 1.64 15 si tion

6 14353 same, lower strat, po- same (?) M. Miocene (1.60) 2.12 7 ++ sition i km SSE S. Sal- vatore

7 14365 Aveto r iver, I km WNW Aveto fm, central Oligoc.(?)/ 2 . 5 9 2 .82 1.78 20 § Castagnela (PC) p., graywacke Miocene

8 14366 Passe Bratello, S Bur- Gottero fln., U. Cret./ (1.02) 10 ++ gotaro (PR), 0.6 km 9raywacke L. Paleoc. NW M. Cucco

9 14367 same same same (1.05) 8 ++

10 14359 Gordana valley, WSW Macigno, middle U. Olig./ 1.65 2 .13 1.52 10 (+) Pontremoli (MS), part, graywacke L. Miocene 1.5 km ENE Noce

11 14358 Passe del Cerreto, Cerreto (Cervarola) L. Miocene 2.49 14 0.6 km N Cerrete fm, top, graywacke d'Alpi (RE)

12 14362 1.5 km SW M. La Macigno, lower to U. Olig./ 1.03 15 Nuda near Mommio (MS) middle portion, L. Miocene

graywacke

13 14360 SW Pontremoli (MS) Macigno, middle U. Olig./ 2.11 2.18 1.81 20 ++ part, graywacke L. Miocene

14 14361 5 km NNE Pontremoli same, upper part same (1.33) 14 +++ (MS)

15 14357 0.5 km N Montecreto Cervarola fm. L. Miocene 1.00 40 + (MO)

16 14363 0.6 km SSW V a r a n o Salsomaggiore fm, Helvetian 0.42 11 Marchesi (PR) middle to upper

part, sandst.

17 14364 same same, upper part same D.33 14

18 14354 i km NE Civago ( R E ) Cervarola fm, near L. Miocene 2.07 2 .31 1.85 10 bottom, graywacke

19 14355 1.2 km NE Civago (RE) same, middle part same 1.87 2 .09 1.75 8 §

20 14356 same, 1.5 km NE Civago same, near top same 1.98 1.86 1.77 58 + (RE)

21 14895 Baganza Valley, near Salti del Diavolo Turonian 0.58 13 +§ Chiastre 2.5 km SSW conglomerate Cassio (PR)

23 14897 Ceno-Valley, near Pes- Ranzano fm, Oligocene 0.56 37 + sola, 2.5 km S Serra- graywacke valle (PR)

24 16666 1 km E Monteregio (PC), Ranzano fm gray- same 0.74 52 (+) Nure Valley wacke

25 14709 Nure Valley, 600 m Dosso-Flysch fm, Paleocene 0.70 15 ++ NNE Olmo (PC) carbonatic mudstone

26 14710 500 m E Spettine (PC), Ranzano fm. gray- Oligocene 0.51 22 + Nure Valley wacke

26A 14711 same, strat igr, lower Monteventano mb Olig.(?)/ 0.54 38 position of Luretta fm, U. Eocene

mudstone

27 14712 Nure Valley near Re- sam~ carbonatic M. Eocene O. 61 29 + ciso, strat igr, lower mudstone position

31 16525 0.5 km S Gabbiano, Monghidoro fm , U. Cretaceous 0.62 9 (+) road to Valle (BO) graywacke Paleocene

32 16526 0.8 km E Brigola, same same (0.75) 2 § 2,6 km ESE Rioveggio (BO)

33 16527 0.6 km E exit Rioveggio Loiane fm sand- Oligoeene 0.43 50 of Autostrada (BO) stone

bi-ref lectance variable

much v r i t r i t e , pe t r i f i ed wood

value uncertain

reworked graphite

value uncertain, reworked graphite

l i t t l e v i t r i t e , reworked graphite

measures from corpo col l in i te, reworked graphite

fluorescent v i t r i t e

v i t r i t e with much pyrite

fluorescing v i t r i t e with much pyrite

fluorescing v i t r i t e

fluorescing v i t r i t e

fluorescing ulminite, yellow fluorescing cutinite

fluorescing ulminite with much pyrite, ulminite A: 0,27% Rm, yellow fluorescing resinite

fluorescing v i t r i t e with pyrite

much v i t r i t e , partly fluorescing

drift-wood v i t r i t e

same

fluorescing euti- ni te

ulminite, partly pyritized

fluorescing v i t r i t e with much pyrite

orange fluorescing sporinite

fluorescing v i t r i - nite and l ip t in i te

Table I continued

Sample Number Field Laborat.

Sample Location Formation and Rock Type

Age Rm Rmax Rmin n Oxidat. Remarks % % %

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

34 16961

35 16953

37 16956

38 16758

39 16769

40 16760

41 16761

42 14713

42A 14714

45 14716

47 14719

48 14772

49 14783

50 14720

51 14721

63 14722

57 14898

58 14899

59 14900

61 14902

62 14903

63 14904

64 14905

67 15184

69 15186

71 16089

72 16090

74 16092

Lago di Suviana, western side of dam (BO)

1.5 km WSW of Rovegno, ..i Trebbia-Valley (GE)

1.8 km SSW of Varzi, Fontana (PV)

2 km SSW of S. Sebastia- no Curone (AL)

Roccaforte Lig., 3.5 km SSW of Rmcchetta L. (GE)

2 km SW of Voltaggio (AL)

same

Calafuria, S of Li- vorno, quarry at km 304 of highw. (LI)

same, Torre dei Bocca- le Castel of Populonia, 6 km NNW of Piombino ( U )

1.6 km WSW Ponte agli Sto l l i , road Greve- Figline (FI)

0.1 km WSW Ponte agli Stoll i (FI)

Ponte a Poppi, road to Camaldoli, Avena (AR)

near Vertel l i , road Poppi-Montemignaio (AR)

Road Dicomano (F I ) - S. Gadenzo, km 122.6

0.5 km WNW Passe del la Raticosa (FI/80)

near Codo]o, Gordana valley, 5.5 km W Pontre- moll (MS)

same, 20 m above NA 67

Giarede, Gordana Valley 6 km WSW Pontremoli (MS)

same, 9 km WSW Pontre- moll, 0.6 km E Nace (MS)

Road to Cirone 0.7 km E Pracchiola (MS)

1.5 km NW Berceto (PR}

i km S Calice (SP), Vara Valley

near Armorane (PR), Baganza-Valley

Crostolo-Valley, near Vezzane (RE)

Suviana fm, L. Miocene 1.43 graywacke

Ostia fm (Torri- Cenomanian/ 2.31 glia unit), gray- Turonian wacke

Ranzano fm, Oligocene/ 0.49 mudstone L. Miocene

same L. Oligocene 0.29

same M. Oligaoene 0.41

same U. Oligocene 0.52

same thin coal same (0.38) measure

Macigno fm, gray- U. Oligmcene/ 2.56 wacke L. Miocene

same same 2.04

Macigno, gray- U. Olig./ 4.89 wacke L. Miocene

Macigno, bottom of U. Oligocene 1.16 fm, graywacke

Macigno, top of L. Miocene 1.19 fm, graywacke

Petrignacola (Se- Oligocene 0.67 nario) fm, graywacke

Cervarola fm, L. Miocene 0.99 graywacke

Cervarola fm, L. Miocene 0.56 bottom of fm, graywacke

Monghidoro fm, U. Cretac./ 0.60 carbonac, grayw. Paleocene Macigno, 150 m U-Oligocene (1.65) above bottom, graywacke

same same (1.83)

same, bottom same 1.71

same, top of fm U-Olig./ 2.22 L. Miocene

Pracchiola, gray- L. Miocene 2.43 wacke

Ostia fm, gray- Cenomanian/ 1.19 wacke Turonian

Macigno, gray- U. Oligocene/ 3.22 wacke L. Miocene

M. Sporno fm, car- U. Paleocene/ 0.37 bonatic graywacke L. Eocene

Gessifera fm, U. Miocene 0.22 shales between (Messinian) gypsum

near Clchero, I km Gottero fm, U. Cretac./ (2.72) 5W M. Ramaceto ( G E ) graywacke Paleecene

Tare-Val ley, Piane Macigne fm, gray- U, O l ig . / 2.01 di Carn ig l ia , 2 km wacke U. Miocene SW Tornolm IPR)

Cresta del G h f f i , Gottero fm, gray- U. Cretaceous(2.98) 2.6 km N Passe del wacke Paleocene Bocce (GE)

50 +

35 ++

20

25

30

40

20

2,79 2.24 17 +

2.46 1.90

6.73 3.06

2.53 1.94

2.63 2.16

+ drift-wood v i t r i t e

probably u lmini te A, cerpoco l i in i te 0.32 Rm (n=lO)

ulmini te B

seam coal, much pyr i te

sapropel ic, very much pyr i te

16 (+)

14 (+)

25 ++

18 +

35 +

16

15 +

14 + seam coal

12 +iF value uncertain, reworked graphite

13 +++ value uncertain

01 (+) d r i f t wood

28 (+) rewarked graphite

26 (+)

39 (+)

3.50 2.48 21

25

25

++ co I1 . ,u lm . , i i gh t - orange f l . l i p t i n .

corpohuminite values, u lmini te A gives 0.14 % Rm {n=15), negative a l te ra t i on of spor in i te

7 +iF value uncertain

13 + 2 ref lectance groups,also 1.72% Rm (n=12), reworked graphite

12 +t* value uncertain

Table 1 continued

Sample Number Sample Location Formation and Field Laborat. Rock Type

Age Rm Rmax Rmin n Oxidat. Remarks % % %

75 16093 2,5 Km E Passo del same Bocco (PR)

76 16094 1.8 km E Passe del same Bocco (GE)

77 16095 0.7 km ENE Passo del same Bocco (GE)

80 16096 Road Fivizzano (SP)- Orocco-Caio fm, Passe del Cerreto, carbonatic gray- km 26 wacke

81 16097 Road Favale di Malvaro Val Lavagna fm, (GE)-Parazzuolo, near slate pass

82 16098 1.5 km NE To r r i g l i a (GE) road to Porto

83 16099 1.4 km NNE To r r i g l i a (GE) road to Propata

85 16101 Elba-lsland, road La- cona-Marina di Campo

85a 16102 Elba-lsland, 3 km W Marina die Campo

86 16103 Piombino (L I ) , 0.6 km S of docks

87 16104 1.5 km N Lore Ciuffenna (AR), road to Trappola

88 16105 Pratomagno (AR), Croce di Pratomagno

89 16106 Highway Bibbiena {AR)- Passo dei Mandr io l i , km 189

90 16107 same, km 193 (Badia same Prataglia I (AR)

91 16108 2.5 km E Badia Pra- taglia (AR)

92 16109 1.8 km E Badia Pra- taglia (AR)

93 16110 Passo Mandrioli (AR)

94 16111 Highway P. Mandrioli- Cesena, km 202 (FO)

95 16112 Highway bifurcation 4.5 km S S. Piero in Bagno {FO)

96 16113 Road S. Piero B. (FO) M. Fumaiolo, 1.2 km SSW Alfero

97 16114 Sarsina (FO), km 234 of highway, Mandrio- li-Cesena

100 16117 2.5 km NW M. Gottero, forest road, 0.6 km S M. Bertola (SP)

101 16119 coast SW Riva same Trigoso (GE)

102 16120 NW entrance of same Deiva Marina (GE)

103 16121 Road S. Margherita- Antola fm,carbo- Portofino near Por- natic graywacke to Pedale (GE)

104 16122 same, E of Paraggi Ranzano fm, near (GE) base, conglomerate

105 16123 Highway Busalla-Scoffera Antola fm~ 1.9 km E Montoggio (GE) carbonatic gray-

wacke

106 16124 Road Vobbia-lsola del Ranzano fm, Cantoned2.75 km SE Vob- conglomerate bietta (GE)

107 16125 Vigoponzo, S S. Sebas- same, sand- tiano C. (AL) stone

same (2.89) 8 +++

same (2.10) 5 +++

same (3.13) 11 +++

U. Cretaceous 1.37 8 +

L. Cretaceous 4.47 5.31 3.94 17 (+)

2.80 2.59 22 (+)

75 (+)

Ostia Sandstone fm, U. Cretaceous 2.67 graywacke (Turonian?)

Antola fm, bottom, U. Cretaceous 1.49 carbonatic gray- (Santonian) wacke Elba f lysch fm, U. Cretaceous(2.68) (2.79){2,64) 2 9raywacke

same same 5.50 6.51 3.83 14

Canetolo fm, Paleocene/ 2.58 2.80 2.43 8 ++ graywacke L. Eocene

Cervarola fm, l ig- L. Miocene 1.56 100 (+) n i t i c layer between graywackes

Cervarola fm, L. Miocene 1.36 64 + graywacke

same same 1.13 25 ++

same 0.80 50 (+)

same, bottom of same 0.80 fm

Marnoso-arenacea fm L.-M. Miocene 0.69 (Mandrioli uni t) , 9raywacke

same same

same same

Marnoso-arenacea fm, M. Miocene graywacke

Petrignacola fm Oligocene (M. Comero), grayw.

Marnoso-arenacea fm, U. Miocene uppermost part, graywacke

Gottero fm, graywacke

66

60

0.87 18

0.76 10

0.66 30

0.64 16 ++

0.50 43 (+)

U. Creta- (1.96) 3 +++ ceous/Paleo- cene

same 2.56 2.88 2.13 34 +

same 2.75 3.19 2.54 47 (+)

U. Cretaceous 1.37 13 + (Sant./Maestm~

Oligocene 0.73 21 (+)

U. Cretaceous 2.12 2.33 1.90 6 ++

Oli9ocene 0.59 8 (+)

same 0.31 30 (+)

(+)

(+) 1 (+)

(+) j (+)

value uncertain

reworked graphite

value uncertain

reworked graphite

strongly deformed slate with v i t r i - te layers

reworked graphite

much v i t r i t e , no fluoresc, l i p t i n i t e , reworked graphite

value uncertain reworked graphite

reworked graphite

reworked graphite

no fluorescent l i p t i n i t e

no fluorescing l i p t i n i t e

v i t r i t e

seam coal

reworked graphite

0.74% Rm

value uncertain

2 reflectance groups: also 1.97% Rm (n=9)

Table I continued

Sample Number Sample Location Field Laborat.

Formation and Age Rock Type

Rm Rmax Rmin n % % %

Oxidat. Remarks

NA 108 16126 Uppermost Parma-Valley, 3.3 km W Valditacca, near Lagoni (PR)

NA 109 16078 1 km SE Fabbrica Curone (AL)

NA 112 16081 Tidone-Valley {PC), 0.7 km NE Calghera

NA 113 16607 near M arradi, Lamone- Valley (Pl)

NA 116 16610 1.3 km SW W. Faggiola, Senio-Valley (FI)

NA 117 16611 Poggio di Stignano, Santerno-Valley {FT)

NA 118 16612 0.4 km N S. Pellegrino, Santerno-Valley

NA 119 16613 1.5 km E Firenzuola (FI)

NA 120 16614 Peglio, Diaterna- Valley (FI)

NA 121 16615 O.g km W. Bordignano Diaterna-Valley (FI)

NA 122 16616 0.7 km NW M. Canda, P. Raticosa(Fl)

NA 123 16617 1.2 km NE M. Canda, P. Raticosa (Fi)

NA 127 16669 O.g km SW Pieve S. Loren- zo, Lunigiana (LU)

NA 129 16661 0.5 km NE Fivizzano (MS)

NA 130 16662 1.3 km NE 8azzano (RE), Dolo-Valley

NA 131 16668 Rocca Camera, 3.5 km N Vidic iat ico (go)

NA 132 16664 Lago del Brasimone, 3.2 km WSW Castigliane Pepoli (BO)

NA 133 16665 Highway to Prato, 2.5 km SSE Castigliane Pepoli (BO)

NA 134 16762 2.7 km ENE Castig]ione Pepoli, near Bruscoli

NA 135 16681 AGIP Borehole Ponte del l 'Ol io 1,19 km SSW Piacenza, d. 5319-5325m

NA 135a 16685b same, depth 4992-5000 m

NA 136 16680 same, depth 4830-4838 m

NA 137 16679 same, depth 3968-3971 m

NA 138 16678 same, depth 3422-3425 m same

NA 139 16677 same, depth 3279-8281 m same

Macigno fm, top, L. Miocene 1.82 graywacke (?)

Ranzano fm, Oligocene (0.40) graywacke

same same (0.38)

Marnoso-arenacea M. Miocene 0.55 fm, graywacke

same same 0.53

same same 0.61

same, top of fm same 0.74

same~top of fm same 0.87

same, intercalation same 0.71 within overlying ol isthostr ,

same, top of fm same 0.66

Sporno fm, carbo- Paleocene/ 0.55 natic, graywacke L. Eocene

same same 0.59

Macigno fm~gray- U. Oligocene 2.05 wacke

same U. Olig./ 1.75 L. Miocene

Cervarola fmx L. Miocene 1.73 graywacke

Cervarola fm, L. Miocene 1.52 graywacke

Cervarola (Casti- same (0.97) glione) fm, top, graywacke

same same 1.02

same same 1.14

Molasse deposits L Miocene (0.75) of Po Plain, sandstone

same same 0.59

same same 0.64

same M, M1oceoe 0.SR

same 0.57

same 0.52

NA 140 16676b same, depth 2850-2852 m same same 0.49

NA 141 16675 same, depth 2708-2709 m same same 0.51

NA 142 16674 same, depth 1972-1975 m same same 0.41

NA 144 16672 same, depth 877-880 m Luretta fm, detr i - L. or M. (0.38) t ical limestone Eocene

NA 145 16671 same, depth 494-498 m same same (0.32)

NA 146 16685a borebole Pontremoli i , autochthonous cry- M./L. Carbo- 8.4 1.8 km WSW Pontremoli (MS)3 s ta l l i ne basement, niferous or depth 3117-3118 m schis t older

NA 147a 16682 same, depth 2585-2589 m tec ton ica l l y U. Carbonif. 4.81 brecciated sand- (?) stone and coal

(NA 147 a: read depth 2885--2889 m)

2.07 1.66 60 ++

12 (+)

11 +

18 (+)

io +

30 (+)

25 (+)

14 (+)

20 (+)

25 (+)

I0 +

15 (+)

21 +++

14 (+)

32 (+)

15 (+)

(6) ++

13 +

31 ++

(51 -

29

44

12

18

14

26

10

6

6

6

11.0 0.8 7

6.10 4.30 6

reworked graph@re

ulminite A or B

much pyrite

d r i f t wood v i t r i t e

no fluorescing ] i p t i n i t e

very l i t t l e organi( matter

orange fluorescing l i p t i n i t e

same

yellow and orange fluorescing l ip- t i n i t e

same

well measurable v i - t r i n i t e , yel low f luoresoing l ip t i - nite

same

same

same

very small part i - cles, yellow fluorescing l iptinite, microfauna

same

0.6% Rmin(20%)~ graphite

3.09% Rmin(20%)~ meta-bituminite much pyrite

Table i continued

Sample Number Sample Location Formation and Age Field Laborat. Rock lype

Rm Rmax Rmin n Oxldat. Remarks % % %

,A 147b 16683 same

IA 147c 16684 same

IA 147 16682-84 same mean of 147a§

~A 148 16952 AGIP offshore borehole sandstone with Martina I , 12 km SSE coaly laminae Pianosa Isl.,d.2943.7 m

~A 149 16951 same, depth 2202.5 m same

~A 150 16950 same, depth 1412.3 m same

~A 151 16949 same, depth 1411. 6 al

IA 152 16996 i km WSW Petrignacola, Parma-Valley (PR)

NA 154 15997 Groppo 5ovrano, Bratica Valley, 5,5 km S of Cornig]io (RR)

NA 155 16998 Grammatica, Bratica Bratica fm, Valley, 4.5 km S of graywacke Corniglio {PR)

NA 156 17000 Quarry 0.9 km N Ponte del l 'Ol io (PC), Nure Valley

NA 157 17001 same

NA 158 17002 same

NA 161 17D03 1,2 km NNE Castel di Casio (BO), Trepbi~ - Valley

NA 162 17004 Road Castel d'Aiano Zocca, 2,2 km NE Castel d'AJano (BO)

NA 164 17005 Highway Vergato-Bologna, Calvenzano, cemetery (Be)

NA 165 17006 I km SE Castello Carpi- neti (RE)

NA 166 17007 Secchia-Valley, Ponte Secchia near Lusigna- na (RE)

NA 167 16999 l km NE of Ranzano (RE)

NA 168 3363 Coal mine Ribolla (GR)(abandoned 1959)

NA 169 3357 Coal mine Baccinello (GR)(abandoned 1959)

NA 170 15060 Passe del Vestlto, Apuan Alps (MS)

NA 171 15082 Isola Santa, Apuan Alps (LU)

NA 172 ]8417 Road Pontremoli- Borgotare, 1.4 km SE Bratte (MS)

NA 173 18418 Road Chiavari-Borgonovo, quarry at bifurcation to Cichero

NA 175 18430 0.5 km SE Passe Due same, base of Senti; 10 km W Pentre~ fm moI~ (MS)

NA 176 18433 Passe del Bocce, same 1.3 km SW S. Maria del Taro (PR)

NA 177 18434 same, 1.6 km 8W same S. Maria del Taro (PR)

NA 178 18438 same, 0. i km E same, top of Giariette (GE) fm

NA 179 18436 Ligurian coast,1 km same SW Levanto (SP)

same same

same same

same

Eocene (?)

same

Miocene/ Olig. ?

same same

Petrignacolo fm, U. Eocene/ graywacke L. Oligocene

Groppo-Sevrano fm, U.Eocene mi caceous graj:~.

Luretta fm, calcare- L. or M. n i t i c sandstone Eocene

same, st rat igr , same 0.44 50 m under NA 156

same, s t rat igr , same 0.43 100 m under NA 156

Monghidoro fm, U. Cretaceous 1.06 9ra~Avacke Pale~cene

Carpineti fm, L.-M. Miocene 0.45 sandstone

Loiano fm, Oligocene 0.42 sandstone

Carpineti fm, L-Miocene 0.4/ sandstone

Ranzano fm, Oligecene 0.60 graywacke

same, slumping same 0,57 level, micaceous sandstone

intramontane post- L. Pliocene 0.75 tectonic melasse, sandstone

same same 0.59

graphite-serici te- U, Carboni- 14.8 schist ferous (?)

Pseudomacigno fln U. Oligocene graphite-sericite (L.-Miocene ?) schist

Gotte~ fm, U. Cretac./ 1,08 9raywacke L. Palemeene

same same (3.33)

same

same

same 3.19

L. Paleo- 3.62 cene (?)

U. Cretac./ 3.88 L. Paleocene

5.84 6.58 4.87 13 - 3.54% Rmin(20%),

4.92 6.85 3.09 19 2.13% Rmin(20%) metabituminite, l i t t l e pyr i te

5.22 6,64 3.89 38 2.20% Rein(20%)) mean of 147 a+b+c

0.66 12 well measurable

0.54 17 same

0.38 10 same

0.38 14 same

1.19 14 (+)

1.75 5 +

1.42 11 (+) driftwGod

0.49 11 (+)

17 (+)

17 (+)

10 +

12 (+)

10 ++

9 +

18 (+)

23 (+) ulminite B

35 seam coal

100 - seam coal

17.1 0.6 5 - graphite

16.5 0.2 11 - graphite

16 much v i t r i n i t e

14 +++ value uncertain

1.75 51 + much v i t r i t e~ rewerked graphite

3.23 70 + much v i t r i t e , reworked graphite

3D +

36 § much v i t r i f y , reworked graphite

51 + much v i t r i t e


less oxidized. The degree of oxidation is best indicated by the degree of transfor- mation of pyrite into iron hydroxides. Relatively often the coal itself showed signs of oxidation such as local increase of reflectance, porous structure, desiccation cracks, and oxidation rims. In these cases only microscopic fields without visible indications of oxidation were measured. In Table 1 the degree of oxidation is indicated for every sample. Thirty-eight of the total number of 186 investigated samples were either too strongly oxidized to give reliable reflectance values or did not contain any measurable organic material.

All measurements were done on polished blocks of particulated rock or coal samples (grain size 8 mm). In many cases polished blocks of whole rock pieces cut perpendicular to the bedding were also used. Measurements were made using a LEITZ-Orthoplane photometer microscope with off immersion lenses (magnification 500 times, monochromatic light of 546 nm, field of measurement 8 micron). The mean random reflectance (Rm) was determined by measuring as many single points as possible. Moreover, for high rank coaly inclusions Rmax and Rmin were also determined under polarized light. A more detailed description of this technique is given by M. TEICHM~LLER (1982).

Sample location

In order to obtain information about the diagenetic and anchimetamorphic state and its variations in the elastic sediments of the Northern Apennines, sampling of surface exposures was extended from the mountains near Genoa to those in Tus- cany, and from the Tyrrhenian Sea to the Po Plain. The geology of that area is shown in the sketch map of Fig. 2, which also gives the obtained Rm-values at their locations. The sample numbers together with the respective tim-values can be found in Fig. 8, and Table 1 gives more detailed information about the sample locations and measurements. Samples which showed too strong oxidation were omitted in figures and tables.

Thanks to the collaboration of the AGIP Mineraria Oil Company, core samples could be studied (insertion in Fig. 8); these come from three boreholes situated in the internal zone (Martina 1), in the central zone (Pontremoli 1), and in the external zone of the Apennines at the border of the Po Plain (Ponte dell'Olio 1).

The eoalifieation pattern

The mean values of average vitrinite reflectance (Rm) measured on the samples from the entire Northern Apennine area are distributed over a large range independently of the stratigraphic position of the samples. However, the regional distribution of the values shows some regular trends. The general pattern shows much higher values in the internal zones of the chain near the Ligurian and Tyr- rhenian Seas than in the intermediate and external zones bordering the Po Plain. Moreover, vitrinite reflectance is commonly higher in the deeper tectonic units (nappes) than in the upper ones.

This generally applicable pattern is disturbed only locally by tectonic comp- lications. These general features have already been treated by the present authors in preliminary papers (tlEUTTER et al., 1978, 1982).

870

The Coalification Pattern in the Northern Apennines

Coalification gradient within normal sequences

Because steep and sufficiently high slopes with exposures of relatively un- disturbed sediments are searce in the Northern Apennines vertical gradients can only be inferred from samples taken at considerable horizontal distances. There- fore, erros due to tectonic phenomena as well as to horizontal variations of palaeo- temperature may occur. The increase of coal rank with depth (i. e. the vertical Rm gradient) ean be best observed in deep horeholes drilled through thick and un- disturbed sequenees. Such sediments are found especially in foredeeps and back- deeps of the chain.

In the AGIP borehole Ponte dell'Olio 1, situated about 20 krn south of Piacenza, vitrinite reflectance increases from 0.40 ~ Rm at 2000 m to about 0.60 :~ Rm at a depth of 5000 m. This corresponds to a coalification gradient of 0.07 ~ Rm/km in a Miocene sequence belonging to the folded sub-Apenninic sediments of the Po Plain. This autochthonous Miocene is overlain by Paleocene-Eoeene flysch sediments of the Liguride Luretta-Sporno Unit which forms the external border of the uppermost nappe of the Apennines. The allochthono~s cover yielded a (possibly unreliable) value of 0.32 ~ Rm at a depth of 500 m, which seems to fit the gradient of the normal sequence. From surface samples of the same formation, however, values af about 0.45 ~~ Rm were obtained; this suggests that diagenesis was slightly more advanced in the nappe than in the underlying Miocene sediments (Fig. 4; Fig. 8: Insertion).

The AGIP borehole Martina 1, situated offshore 27 km south of the southern coast of Elba in the Tyrrhenian Sea, shows a coalification gradient of about 0.19 '~ Rm/km (Fig. 8: Insertion) in an Oligocene-Eocene sequence. The differences between the coalification gradients of the two boreholes may be explained by the increased heat flow in the internal parts of the Apennines. Indeed, the present temperature gradient in Ponte dell'Olio 1 is 19 ~ C/km, whereas that of Martina 1 is 82 ~ C/km (AGIP, 1977). In both cases relations between Rm values and corresponding present temperatures indicate that coalification may be attributed to the present temperature field. With regard to Martina 1 there is a special problem because much higher temperatures than those encountered in the drilling were expected on the basis of the generally high heat flow of the Tyrrhenian area (Fig. 6).

As pointed out earlier, the reconstruction of vertical palaeo-gradients of Rm from surface samples is almost impossible. Nevertheless, in some places increases of Rm ean be observed from stratigraphieally higher to lower positions. The samples Na 26, 26 A and 27, showing 0.51~/0, 0.54~ and 0.61~ Rm, embrace a stratigraphic sequence about 400 m thick from the Ranzano Formation (Oligocene) to the Val-Luretta Formation (Paleocene-Eocene). This latter formation is also present in the upper part of Ponte dell'Olio 1, which is situated only about 8 km north of that sampling area. The resulting vertical gradient of 0.25 ~ Rm/km seems to be influenced by some lateral effect because it is relatively high. I t suggests that this Liguride nappe (Fig. 1: Sporno Unit) was heated in an originally more internal position where the temperature gradient was higher than that of its present external position.

The increase of coalification with depth is also displayed by two samples from the Cervarola Formation of the Pratomagno (to the northwest of Arezzo). NA 88

56 Geologische Rundschau, Bd. 72 87"1

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o77 Eocene(Luretta- Sporno Unit)

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Reftectonce

Fig. 4. Coal ranks in the AGIP borehole Ponte dell'Olio 1. Diagenesis in the Liguride Luretta-Sporno Nappe is slightly more advanced than in the underlying autochthonous sediments of the border of the Po Plain. Note the low Rm values at a depth of 5000 m.

They reflect the reduced heat flow of the sub-Apenninic part of the Po Plain.

(1.86 ~ Rm) and NA 87 (1.56 ~ Rm) include a height difference of about 1000 m in this gently eastwards dipping graywacke sequence. The resulting coalification gradient of 0.20. ~ Rm/km is a minimum value, however, since the location of Na 87 appears to be downthrown relatively to NA 88 along a normal fault (Fig. 5: Section F).

Yert iea l var ia t ion of rank w i t h i n t h e n a p p e p i l e

In addition to the normal increase of coalification within the stratigraphic sequences, a general increase can be observed from the uppermost to the lowermost tectonic units of the Apenninic pile of nappes. This feature can be estimated only along longitudinal sections because transverse sections are too. strongly influenced by lateral variations of coal rank within the individual units themselves.

The Ligurian coast between Rapallo and La Spezia corresponds to an approxi- mately longitudinal profile which cuts across the different nappes of the Northern Apennines because of the northwest axial clip of the chain in that region. The samples taken along this coast reveal an increase from 0.78 =~ Rm (NA 104, Ran- zano Fm. - - a R B - - near Portofino) and 1.87 ~ Rm (NA 108, Antola Flysch

fA--) within the uppermost Liguride unit, to 2.56 ~ 2.75 =~ and 8.88 ~ Rm (NA 101, 102, 179, Gottero Sandstone - - aG- - ) within the Liguride Gottero nappe, and to 5.07 ~ Rm (NA 1, Macigno Fm. - - m O - - near La Spezia) in the Tuscan

876


nappe. In sample NA 1 an Rmax value of 6.22 0/0 was measured, suggesting a level at the boundary between anehimetamorphism and epimetamorphism (TEIClt- Mi)LLER et al., 1979; BBEITSCHMID, 1989,). In the same way, there is no inversion (but gaps, see p. 885) between 4.47 ~ Rm in sample NA 81 from the Lavagna Slates (--cB--), 2.67 o/o Rm in the Ostia Sandstone near Torriglia (NA 82, - -cB-- ) and the Rm values of the Antola Flysch ( ~ A - - ; NA 105: 2.12~ NA 88: 1.49 ~

There are no reflectance measurements from samples taken at the boundary between the Ligufides and the underlying Macigno Formation, but since the lowermost Liguride rocks are olistostromes they cannot have been subjected to important heating before being affected by submarine muddy landslides. The existing Pan data do not reveal any inversions between the Macigno Formation and their Liguride cover, nor could any inversions be detected from one Liguride nappe to the other (p. 886).

The lowermost tectonic unit of the internal parts of the Northern Apennines is the Lower Tuseanide unit, frequently called "Tuscan Autoehthon', which is best exposed in the Apuan Alps (--M'--) . Here, the entire Mesozoic-Cenozoic Tuscanide sequence and locally outcropping Palaeozoic rocks are epimetamorphic. Correspondingly, organic material in rocks of Carboniferous age (NA 170:17.1 ~ Rmax, 0.6O/o Rlnin) and of Oligoeene-Miocene age ("Pseudomacigno', NA 171: 16.5 ~ l~max, 0.2'~ Rmin) have been converted to graphite, as evidenced by their high Rmax and low Rmin values. Generally the contact between the Lower Tuscanides and the Tuscan Nappe is marked by a distinct metamorphic hiatus. This break is verified by the coal rank of the Macigno Formation of the Tuscan Nappe at the border of the tectonic window of the Apuan Alps where Rm values of 8.22 0/o (NA 64), 2.05 ~ (NA 127)7 and 1.75 ~ (NA 129) were obtained.

This trend of increasing metamorphic degree from top to bottom through the pile of nappes is also present in the external zones of the Apennines, but it is not so evident because the level of coal rank is generally lower than in the internal zones. For example, north of Arezzo (Bibbiena) the Liguride Senario Sandstone (Fig. 2: - - a C - - ; Fig. 5: - -aS- - ) is much less coalified (NA 49:0.67 ~ Rm) than the Cervarola Formation beneath (--aMC--; NA 50:0.99 ~ Rm, NA 89:1.18 ~ Rm; Fig. 5: Section F). Section E (Fig. 5), which runs parallel to the strike in the external zones of the Apennines south of Bologna, shows slightly higher Rm values in the southeast within the autochthonous Marnoso-Arenacea Formation (--fma-- ; NA 118, 120 about 0.70/0 tim, and more externally NA 116, 117, 121 about 0.6 ~ Rm) than in the northwest within the tlanzano-Bismantova Molasse sequence (--at lB--, NA 88, 164: 0.48'~ Rm). These molasse sediments which belong to the uppermost Liguride unit were deposited unconformably on top of the Late Cretaceous - - Paleocene Monghidoro-Flysch before and during the nappe movement. Folding and perhaps heating of that flysch formation may have occurred already during the pre-Apenninic "Ligurian Phase" (see p. 885).

Horizontal variations of coalification

In the maps shorting the distribution of Rm values in the Northern Apennines (Figs. 2 and 8), the horizontal decrease of coal rank from the internal (southwestern) to the external (northeastern) zones is obvious and may be traced within

877


0

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S SW Riomoggiore Beverino H. Coppigliolo PONTREHOLI

0

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Fig. 5. Six geological sections (A--F, for Section F and legend see pp. 880--881) through the Northern Apennines with ir~dieation of mean values of vitrinite reflectance and the

respective sample numbers. The traces of these sections are shown on Figure g.

878


T. Aveto T. Perino

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Ponte delI'Otio 1

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~- L, km

879


W SW Ponte agti StoUi E NE[ SW G r e v e I F i g l i n e V a l d a r n o Fnella Puliciano Croce di P r a t o r n a g n o

~. ~ - - ~ 136-88 0.99-50 / / 1.19-/8 \~ 156-87 I ~1

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4 k i n

fluaternary~ Le ge n d: Pliocene and Miocene deposits - - ' Messini~l'n gypsum of the Po Plain

Ranzano-Bismantovu molasse- se uence Aoto,o F ,y .o . (U. Eocene - U~,,~iocene} (U.Cretctceous -PaleocJ

Fig. 5 (continued from pages 878 and 879).

each distinct nappe. In the external zones, where the vertical differences in coalification are small, the decrease of Rm values towards the Po Plain is almost m- dependent of the sample location in the pile of nappes. The overall horizontal trend has been disturbed only locally by folding and faulting after the coalify- ing thermal event.

Starting near the northwestern border of the Apennines towards the Tertiary Piemont Basin, a series of samples was gathered in the Ilanzano-Bismantova Mo- lasse sequence which was deposited upon the uppermost Liguride nappe during its transport (REUTTEtl & GllOSCUIITI-I, 1978). As this area did not suffer profound subsidence, the Rm values of the samples are rather low. They show a regular northwards directed decrease from 0.59 ~ to 0.29 ~ (NA 106, 89, 107, 88). Near Varzi, where some huge faults transect the Apennines, the trend appears to be interrupted by the somewhat high Rm value of 0.49 ~ (NA 87).

As shown in Fig. 1 the uppermost Liguride unit is composed of different sedimentary complexes of Cretaceous and Paleocene-Eoeene age which mostly unconformably underlie the Ranzano-Bismantova Molasse sequence. The decrease of coal rank from the internal to the external side of the mountain range becomes evident when the tlm values of the Antola-Flysch (--fA--; NA 108, 88, and 105: 1.4--2.1 ~/0 Rm) are compared with those of the Monghidoro and Cassio Flysehes ( - - fMC~; NA 81, 82, 58, and 21:0.75--0.58 ~ Rm).

The Gottero Sandstone (--aG--) belongs to a deeper Liguride unit (Gottero Unit, Fig. 1). Therefore its Rm level is much higher than that of the uppermost Liguride nappe. Nevertheless it shows the same horizontal trend of Rm decrease towards northeast. The samples taken at the Ligurian coast (NA 101, 102, and 179: 2.56, 2.75, and 8.88 ~ Rm) or those taken northeast of Chiavari (Passo del Bocco, NA 176--178:8.19--8.62 ~/0 Rm) reveal coal ranks at the boundary bet-

8 8 0


P. deLPonte Mandriofi BagnodiRomagno Volbiono Sorsino T.Sovio NE Popp EArno Partino 0.09-92 10.07-93

= I t I 0 . 0 0 - 9 ~ I ~ - - 0.ss-95 I . . . . 90 0.0&-96 I 0.00-97 I [ 0.67-49 113-89 0.80-90 0.76-9,; u.oo . l ! 11

r ~ 0 . . . . . / - - " M x ~ x - L&km

Ophiolites~ associated with CB, cbo cbo (Jurassic 1

Undifferentiated sedimentor~ complexes under[y fCM~FDSE, fC (U.Jurassic-UCretaceous);ohstostromes at the border of the Po Plain, intercalated with Neogen formations.

Cr Camp ex �9 [U.Cretaceous-Paleocene)

C a i o Ftysch (U.Cretaceous-P~teocene)

~ ca.eto~o seq ..... ~ Se.o,~o So..=to.e (pateocene-Oligocene } [Otigocenel

Nodino Se, ndstone (ind Cervarota Sandstone (U.OLigocene- MMi ocene)

Olistho.stromes and variegated shales associo, ted with Modino and Ce va o a Sandstones | Oligocene-Miocene)

Nacigno Formation ~ Narnoso-Arenacea (U.Ohgocene.L.Miocene)~ ...:~_~ ~ Forrna ion L,~ U.Niocene)

~ - - ~ 1 < . . . . . ic-O,i ~, . . . . . . fT . . . . . Nappe, ~ Gyp . . . . . d anhydrites Urnbro-Marc-es~ and Po PLain (Triassic)

~ I~letcmorphic c o m p t e x ~ easement complex of Tuscany (EarLy Po, ieoeozoic or oLderg) (Pal~eozoic-Otigocene)

ween diagenesis and anchimetamorphism ( ~ 3.5 ~ B_m; M. TEICHMULLEIt et al., 1979; BltEITSCHMID, 1982). Heating in that part of the Cottero Formation was much stronger than in its outcrops west of Pontremoli (NA 175:1.75 ~ IIm) and north of Pontremoli (NA 172:1.08 ~/0 tim).

It must be mentioned that it was very difficult to obtain samples from the Gottero Formation that were not extremely oxidized. A revision of earlier measured samples as well as the results of selected new samples (NA 172--179) proved that the tlm level is considerably higher than the values presented in the preceding papers (REUTTER, M. & R. TEICHM~rLLEtt • ZANZUCCHI, 1978, 1982). The new data are much better compatible with measurements of the illite cristal- ]inity in the Passo del Bocco and Monte Ilamaceto area (Fig. 5: Section B) by VENWUREL~I & F~EV (1977).

Within the Macigno Formation of the Tuscan Nappe (Fig. 1: Tuscanides II) the samples NA 1, 64, 13 and 14 (Fig. 5: Section C) once again reveal the transverse decrease of coalification from 5.07 0/0 tlm near the Ligurian coast to 1.gg tlm ~ 1Rm north of Pontremoli. In this Section C a break due to a complicated structure orth of NA 14 (p. 884) is indicated by NA 62 (2.48 ~ Rm) from the Cervarola Sandstone, but farther north, independently of the provenance of the samples, the decrease again becomes regular until t im values of 0.88 0/0--0.42 ~ (NA 17, NA 16) are met in the Miocene sediments of the anticline of Salsomag- giore at the edge of the Po Plain.

Also in Section A (Fig. 5), running through the external zone of the Apennines from the Aveto Valley to the AGIP borehole Ponte dell 'alia 1, the decrease of coalification towards the Po Plain is evident (NA 7:2.590/0 Rm, NA 156--158: 0.49--0.48 ~ Rm) although the samples come from very different tectonic units. The deepest unit of this section, apart from the Neogene sediments of the Po

881


Plain which dip under the nappes of the Apennines, is represented by the Cer- varola Sandstone of the "Bobbio Window" ( - -aMC-- ; NA 4 and 6). All other surface samples of tMs section are from various Liguride units.

In Sections D and F, a decrease of the diagenetie state of the sediments can be traced from the Modino-Cervarola Unit ( - - a M C ~ ) towards the Po Plain. In Sec- tion D the decrease of rank from the southwest to the northeast is obvious in Liguride allochthonous units, the maximum thickness of which probably exceeds 4 kin; whereas in Section F the same trend is evidenced by the Rm values in the Modino-Cervarola Unit and the Marnoso-Arenaeea Formation ( - - fma-- ) of the Umbro-Marches. The former unit was overthrust upon the latter which may be considered as autochthonous. Although the tectonic situations represented in the two sections are fundamentally different, the degree of coalifieation in both cases seems to depend only on the distance between the sample location and the border of the Po Plain. The longitudinal Section E illustrates the transition from Section F, in tile southeast, to section D, in the northwest.

Although only very few data exist from southern and western Tuscany, it may be hypothesized that in these internal parts of the Apennines, the mean reflectances also gradually diminish towards east or northeast. This assumption is based on the comparison of the Rm values of the Maeigno Formation at the Tyrrhenian coast south of Livorno and north of Piombino (NA 42: 2.560/o Rm; NA 45: 4.89 ~ Rm) with those from the same formation between Greve and S. Giovanni Valdarno (south of Florence; NA 47, 48:1,16 and 1.19 ~ Rm). In this latter area the Macigno Formation is involved in the huge frontal anticline of the Tuscan Naope ( - - m O - - ; left side of Section F).

The horizontal variation of Rm values within the nappes and formations of the Northern Apennines e/early illustrates the well-known phenomenon that heating in the internal zones of an orogen is much stronger than that in the external zones. FREY et al. (1980) and Br~EITSCHMID (1982), who determined not only the vitrinite reflectance but also the illite eristallinity and fluid inclusion temperatures, found the same trend in the Helvetlc nappes to the north of the Aar-Massiv. It is equally well developed in the Molasse and some of the nappes of the Pr~alpes northwest of the Mont-Blane-Massif (KOBLE~ et al., 1979).

Although from the northwest to the southeast the Northern Apennines become considerably wider as they diverge structurally, coal ranks in the internal and external zones remain at the same level along strike. Therefore, the horizontal gradient in the northwestern cross sections (Sections A and C) is much greater than the horizontal gradient in the southeast (Section F). There are two possible causes of this phenomenon: either the isolines of the heat flow which was re- sponsible for the coalification were denser in the northwest than in the southeast, or orogenic compression and shortening were much more effective in the narrow northwestern part of the mountain range.

Heating due to magmatic activity

The samples mentioned so far owe their coal ranks to wide spread regional diagenesis and metamorphism. Moreover~ in Tuscany and the adjacent Tyrrhenian Sea intense heating was locally produced by magmatic activity, Which began in the Late Miocene (e. g. M. Capanne granodiorite of Elba, 7 m. y., p. 890) and then

882


Fig. 6. Present heat flow densities in the Italian peninsula and adjaeent areas, from CE~MAK, 1979.

spread towards the east and northeast during the Pliocene and Pleistocene. The actual high heat flow of Tuseany with its local extreme maxima is due to this magmatism (Figs. 6 and 8; HAENEL, 1980). Thus some samples owe their coal rank to this younger thermal event. NA 85 taken from the Late Cretaceous Flyseh of western Elba ( - - fCM--) yielded 5.50 ~ Rm, 6.51 ~ Rmax, and 8.88 ~~ Rmin. The sample location is about 200 m away from a normal fault which separates it from the M. Capanne granodiorite. The Elba Flyseh is frequently penetrated by granitic dikes. Another example of very young heating is presented by NA 169 (Baecinello) and NA 168 (Ribolla), both located in Plioeene sediments near Gros- seto. As erosion cannot have removed much overlying sediment, their Rm values of 0.59 ~ and 0.75 ~ indicate very intense heating not far from the former surface.

Breaks in the trends of coalifieation

The most evident anomaly within the general pattern is found at the external side of the Tuscan Nappe along its overturned frontal anticline, which can be fol-

883

K.-J. I~EUTTER et al.

lowed for 250 krn ~from M. Orsaro (east o.f Pontremoli) to M. Cetona (80 km to the northwest of Orvieto). Samples were taken from the Macigno Formation ( - - toO- - ) of the normal limb of this anticline near Pontremoli (NA 14), near Fivizzano (NA 12), and between Greve and S. Giovanni Val d'Arno (NA 47, 48). The resulting Rm values are fairly uniform (1.88 ~ 1.08 ~ 1.16.0/# and 1.19 0/o). It is therefore surprising that the reflectanees in the Cervarola Sandstone lying immediately at the northeast of that anticline are significantly higher (NA 62: 2.48 ~ NA 11:2.49 o/~, NA 20:1.98 ~ and NA 87:1.56 ~ Rm). NA i08 coming from the stratigraphically uppermost Macigno strata of the overturned limb of the frontal anticline has an intermediate value of 1.82 ~ Era. As there is no reason to suppose an elongated heat source paralleling the frontal anticline of the Tuscan Nappe, the burial depth (and hence the mutual geometric relations) of the two units must have been different during heating.

The situation in the M. Orsaro area must be seen in close connexion with that in the Gordana Valley to the west of Pontremoli. The samples NA 61, 10 and 59

SW vqi Gordona Id.Orsaro NE 2.22-61 E t , - 1 4 6 I

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Fig. 7. A cross section through the central zone of the Northern Apennines near Pont- remoli (Section a) shows that the coalification pattern in the Macigno Formation (--toO--), which belongs to the Tuscan Nappe, and the Cervarola Sandstone (--aMC--) is strongly disturbed by post-metamorphic tensional tectonics. By means of the Rm values a reconstitution of the Pontremoli structure can be achieved (Section b) if the palaeogeothermie conditions are supposed to have been more or less uniform during the regional heating, Samples in brackets are proiected into the section, (Section a: read

2.43--62 instead of 0.43--62)

884


are located from west to east in a stratigraphically descending order from the top to the base of the Macigno Formation whose thickness exceeds 2000 m. So it is surprising that in this direction the Rm values of these samples decrease from 2.2 ~ (NA 61) at the top of the formation to 1.71 ~ (NA 59) at its base within the anomalously short horizontal distance of only 8 km (Fig. 7; Fig. 5: Section B). It might be concluded that within the Tuscan sequence of the Pontremoli area a huge anticline, having a steep flank in the southwest and an overturned flank in the northeast, had been arched up for several thousand metres over the level of the Cervarola Sandstone when coalification reached its present degree. Thereafter (and possibly after further thrusting to the northeast, p. 887), it must have collapsed owing to strong tensional downfaulting. Thus, besides other effects causing a disturbed pattern of coal ranks, the frontal Macigno Formation ( - -mO-- ) must have subsided with respect to the Cervarola Sandstone ( - -aMC-- ) at the external side of the Tuscan Nappe.

At the front of the Modino-Cervarola Unit there are some irregularities which are similar to those at the front of the Tuscan nappe since locally higher Rm values (NA 181:1.52 ~ NA 84:1.48 ~ appear in more external positions than lower values (NA 15:r ~ NA 2:l.]0:0/0). East of Borgo San Lorenzo, at the stratigraphic base of the Cervarola Sandstone only 0.56 ~ Rm (NA 51) were measured, which is in contrast with the Rm of about 0.8'~ found near the internal border of the Marnoso-Arenacea Formation (NA 119, 91). Here again, overthrusting prior to coalifieation and subsequent faulting may explain these minor anomalies.

Repeated heating is revealed by the samples NA 108 and 104, which were col- lected west of Chiavari at the Ligurian coast (Portofino). The Late Cretaceous Antola Flysch ( fA ) yielded 1.37 ~ Rm while, about hundred metres away, in the unconformably overlying Oligocene Ranzano Sandstone, Rm was found to be only 0.73 ~ The Antola Flysch was folded and dislocated for the first time during the main orogenic phase of ,the Western Alps in the Late Eocene ("Ligurian Phase"), and for the second time in the course of the Apenninic orogeny during the Miocene ("Tuscan Phase" = Late Miocene). Its present coal rank, at least in the internal zone of the Apennines, is evidently due to a subsidence connected with the first tectonic movements. The Oligocene sediments, which are posttec- tonic with respect to the first movements and pretectonic or syntectonic with respect to the Apenninic orogeny, owe their coalification to a younger, but less effective thermal event. The same unconformity is also present south of Bologna, but the differences in the reflectances between the Monghidoro-Flyseh of Late Cretaceous to Paleocene age and the transgressive Loiano (Ranzano) Molasse of Oligocene age are not so much pronounced (NA 81:0.62~ Rm and NA 58: 0.60 Rm versus NA 88:0.48 ~ Rm and NA 164:0.42 ~ Rm; Section E of Fig. 5) as in the foregoing ease.

Another type of discontinuity in the coalification profile was found at the base of the huge slab of Antola Flyseh ( - - fA--) near Torriglia (Section B of Fig. 5). In its basal strata 1.49 ~ Rm was measured (NA 88), while in the underlying thinly bedded graywacke flysch (Ostia Sandstone, - - c b o - - ) Rm is 2.67'/0 (NA 82), and farther to the northeast in the same formation, not more than 100 m beneath the Antola Flyseh, it is 2.81 ~ (NA 85). This gap in the degree of coalifieation can only be explained by nappe movement of the Antola Flyseh over

885


the Ostia Sandstone which led to the tectonic replacement of the original, more strongly eoalified, material by less strongly eoalified rocks. This result is quite in- teresting, as it was previously not known whether there was any tectonic movement after the Eocene Ligurian Phase at this contact. A similar coalification gap of the same origin is supposed to exist between the Ostia Sandstone and the underlying Lavagna Slates of the Gottero unit (NA 81:4.47 ~ Rm), but the scarcity of samples of that area does not permit any precisation.

The anchimetamorphism of the Lavagna slates is generally considered to be older (Late Eocene?) than the Apenninic nappe tectonics, which are of Miocene age. This would imply that possibly all Liguride units of the area between La Spezia, Genoa, Ottone and Varese Ligure from the Gottero Unit up to tile Antola Flysch had suffered previous tectonic deformation and heating. If this is true, the Rm values of the Liguride formations show that pre-Miocene heating rapidly decreased from the southwest to the northeast, that means away from the West Alpine mountain arc. In the course of the Apenninie orogeny the Ligurides were transported as nappes towards the northeast in such a manner that their more eoalified parts (southwest of the Antola unit and the Gottero nappe) were era- placed now in the internal zone of the Apennines. However the subsequent Apen- ninic heating led in that internal zone to eoal ranks of about the same level. Probably for this reason no inversions of coal rank could be detected between the Liguride nappes and the Tuscanides. Perhaps some of the irregularities in the coalification pattern of the Gottero Sandstone in that Ligurian area (the Rm values of NA 10i and 102 at the coast are smaller than those of NA 176--178 from the Passo del Boeco northeast of Chiavari) are due to the two thermal events whleh certainly showed quite different heat flow anomalies.

Indications of late thrust movements

Except for the Late Eocene thermal event, coalification of the rocks of the Northern Apennines obviously occurred after the nappes had been piled up. And yet there is evidence of thrust movements that were subsequent to the regional Apennine coalifieation (disregarding the magmatic activity in Tuscany).

In this respect the Pontremoli 1 drilling is very instructive (Fig. 8: Insertion; Fig. 5: Sections B and C; Fig. 7). The Macigno Formation at the surface near the borehole presents Rm values of about 1.7 ~ (NA 57--59). At a depth of 2885 m, "graphitic" schists of probably Carboniferous age show values of 5.22 ~ Rm, 6.64 ~ Rmax, and 2.20 ~ Rmin (NA 147), which indicate a rank close to low- grade metamorphism (epimetamorphism). At a depth of 8117 m, graphite (NA 146:11.0 ~ Rmax, 0.8 ~~ Rmin) was found within a basal complex of quartzitic micasehist. If the coal ranks measured in the borehole and at the surface were produced by the same thermal event after the nappe emplacement, the resulting tlm gradient would be relatively high. It would require a young (Late Miocene) high heat flow which, however, is in contrast with the present temperature gradient of only 9,7 ~ C/km (108 ~ C at a depth of 8520 m). For this reason and because of the unusually high metamorphic degree (garnet) and the great thickness of the mieasehists in the lowermost part of the borehole (500 m), it is more likely that these rocks represent a Hercynian or even older basement belonging

886


to a former cool foreland area, whose sedimentary cover was dislocated towards the northeast and replaced by allochthonous material from more internal zones. If this is true (no radiometrie data are available), it would prove the transport of rocks affected by Apenninic metamorphism (NA 147; if this sample is of late Carboniferous or younger age no Hercynian metamorphism can be expected) over rocks lacking Apenninic metamorphism and, hence, coalification prior to nappe movement.

Besides the probable inversion with respect to Apenninic metamorphism in Pontremoli 1, it may be inferred from the lithologic description of the borehole that between the Palaeozoic rocks of NA 147 and the overlying Modino-Cervarola Unit ( - - b M - - and --aMC---- in Sections B and C of Fig. 5) or Tuscan nappe a metamorphic gap exists which also owes its existence to post-coalification thrusting. It resembles the metamorphic gap existing in the Apuan Alps between the epimetamorphic lower Tuscanides and the anchimetamorphic Tuscan nappe (p. 887). As the metamorphism of the Lower Tuscanides is very young (KLmFIELD et al., 1977: last metamorphic event 11 m.y.) an explanation of the spectacular metamorphic hiatus of the Apuans requires not only a tectonic replacement of the rocks originally overlying the metamorphic sequence but also a sudden inter- ruption of heating. Its probable cause is lateral thrusting of the metamorphic Tuscanides upon a relatively cool substrate. The findings in the Pontremoli 1 drilling support this view of an allochthonous position of the Lower Tuscanides.

Estimates of vertical uplift

From the coal rank, especially that of the external zones, some conclusions can be drawn regarding the vertical uprise of the mountain chain. Near the southwestern border of Marnoso-Arenacea Formation Rm values up to 0.80 ~ (NA 91) and 0.87 ~ (NA 119) have been measured. As this formation is generally considered to be autochthonous except for some sliding along Upper Triassic evaporites, the heat flow which caused the coalification probably was not higher than the present heat flow. About 10 km to the southwest from the internal border of that formation, near Borgo San Lorenzo, the present heat flow was determined to be 60 mW/m ~ (BoccALETTI et al., 1977). If a heat conductivity of 2.0 W/m ~ C in the alternating shales and graywackes is assumed, a geothermal gradient of 80 ~ C/km during coalification results. According to BOSTmK et al. (1979), who published a diagram showing the relationship between effective heating time, maximum rock temperature, and vitrinite reflectance, 0.8 ~ Rm correspond to 180~ after 10 m. y. of heating. Also in the young sediments of the Upper Rhine-Valley, where heating is very short, a similar temperature/Rm relation exists (M. TEICHMOLLER, 1979: Fig. 9). Subtracting from 180 ~ a hypothetic surface temperature of 15 ~ a temperature difference of 115 ~ C is obtained which equals a thickness of about 3.8 km. This quantity of autochthonous and allochthonous (Modino-Cercarola Unit) sediments must have been eroded since the Early Pliocene.

In the northwestern segment of the Northern Apennnines, the Ranzano Bisman- tova Molasse sequence of the uppermost Liguride nappe shows Rm values between 0.5 ~ and 0.6 ~ Rm even in relatively external positions (NA 26, 166, 167). The heat flow and the thermal gradient during coalification are unknown, but

57 Geologische Rundschau, Bd. 72 887


they cannot have been high because of the rather frontal position of these Li- gurides during the nappe transport. Therefore, assuming effective minimum temperatures of about 80~ (BosTICK et al., 1975; M. TEmHMOLLES, 1979) and gradients of 80 40 ~ C/kin the thickness of the overlying Molasse sediments must have been about 2000 m, which is supported locally by field evidence. So, during their gravity movement these Ligurides were probably hidden beneath a thick uniform cover of synorogenic sediments, which were eroded only in the course of the following uplift of the chain.

Conclusions: Orogenic and palaeogeothevmal development

I:(EUTTER ~ GROSCURTH (1978) and REUTTER (1980) developed a modified plate tectonic model for the Northern Apennines: Subduction directed to the southwest (o~ veest, before the rotation of Italy during the Late Tertiary) led to the con- sumption of the residual oceanic area, which had not been consumed during the Late Eocene orogeny of the Western Alps and Corsica. Thus, a forearc system (DICKINSON .& SEELu 1979) was generated within the oceanic Liguride sediments from the eastern rim of the Corso-Sardinian microcontinent to the inner side of the Western Alps. In this configuration the Ranzano-Bismantova Molasse was deposited in a forearc basin, whereas in the trench-like depression at the front of the forearc (foredeep) the turbidites of the Macigno Formation or southwestern equi- valents accumulated. The Liguride forearc was obducted upon the edge of the Adriatic plate when its continental crust became affected by subduction. However, this crust was evidently only pulled down to a limited extent (this burial depth is indicated by blue-green amphibolites in some very internal Liguride rocks at M. Argentario and on Giglia Island, immediately south of the map area of Fig. 2, and on Gorgona Islands WSW of Livorno), and after suffering some compressional deformation it rose up again. According to REUTTER et al. (1980) this uplift was made possible by a detachment of the Adriatic crust from the lower lithosphere (lower crust or mantle). It is postulated that during the following ensialic phase of orogeny a process of crustal deformation and detachment from the sinking lower lithosphere advanced towards the interior of the Adriatic Plate (Fig. 8). At the same time, the gap which was opening between the separating parts of the lithosphere was filled with asthenospheric material. Evidence of the lithospheric detachment results from crustal studies which show that during continental collision (Oligocene-Miocene boundary) the eastward prolongation of the continental crust of Corsica was apparently underthrust under that of the Adriatic Plate (LETz et al., 1978). The uprise of the Adriatic crust from beneath the obducted forearc may have determined the transition from forearc to gravity nappe tectonics. The Lignride material and underlying Tuscanide units could slide along the slope between the advancing frontal downwarping and the following crustal upbuck- ling.

At the beginning of the Apenninic orogeny, i.e. during the Late Oligocene "forearc period", the heat flow in the foredeep, which may have been situated at the southwestern edge of the Adriatic Plate, probably was very low as a conse- quence of crustal subsidence. However, at the southwestern border of the Ban- zano-Bismantova Molasse basin, close to the Alpine units of the Western Alps and Corsica, the heat flow may have been relatively high. These internal zones of the

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Alps were probably heated by uprising melts from the Penninie lithosphere which is generally considered to have undergone deep subduction towards the east (or southeast) during the Alpine (i. e. pre-Apenninic) orogeny. Since within the uppermost Liguride nappe beneath the t/anzano-Bismantova sediments (aNB-), the coal rank decreases from southwest to northeast (Fig. 5: compare the values of the Antola Flysch fA in Section B, left side~ with those o~ the Sporno- Luretta Unit - - F D S - - in Section A, and those of the Monghidoro Flyseh - - f C M - - in Section E) the palaeo-heat flow density during the Oligocene must also have decreased in that direction, similar to the present heat flow pattern (Fig. 6).

When orogeny advanced to the northeast within the Adriatic Plate its continental crust was subjected to different successive movements and deformations which determined further geothermal development. During the foredeep stage, when crustal downwarping led to the sedimentation of the graywackes and the arrival of submarine slides (olistostromes) and Liguride gravity nappes, the heat flow must have been extremely low, perhaps less than 40 mW/m 2 (corresponding to values now existing in the Po Plain; Figs. 6, 8). Thus, heating and eoalifieation of the Oligocene-Mioeene graywackes and the Liguride pile of nappes cannot have been very effective during this stage, although in the internal zones the thickness of the Ligurides probably exceeded 10 km.

The heat flow is supposed to have increased already during the following stage of eompressional tectonics (e. g. by upfolding). It must have increased considerably more during the stage of crustal uprise which was caused by tectonic and thermal processes in the uppermost mantle (according to BEUTTEt/ et al., 1980: detachment of a relatively thin crust from its lower lithosphere, underflow of asthenospheric material, and partial melting). These processes are indicated by anomalous conditions at the crust-mantle boundary and in the upper mantle (Fig. 8; GIESE eL al., 1981: low velocity layer; HA*K et al., 1981: local high conductivity; PANZA et al., 1980: soft upper mantle). Also the northeastern edge of the Corsican crust that, aeeording to LETZ et a]., 1978, was underthrust down to 60 km beneath the detached southwestern border of the Adriatic ernst probably underwent partial melting, thus contributing to the heat transport. Only after nappe tectonics had come to an end (Late Miocene), heat flow reached local maximum values (more than 100 roW/me; Figs. 6, 8) in the internal zones Of the Apennines by magma ascent. The ages of these "postgeosynclinal" plutonie and volcanic rocks decrease from the Tyrrhenian Sea into the interior of Tuscany (BonTOLOTTZ & PASSERINI, 1970: M. Capanne (Western Elba) granodiorite 7 m. y., M. Amiata 0.4 m. y.) and provide further proof that heating was not a stationary process.

In a system of a continuously advancing orogeny like the Northern Apennines, both the development of heat flow and the diagenetic and metamorphic processes advanced together with the tectonic development. Therefore, it may be supposed that a heat flow pattern which was probably similar to the present one (Fig. 6) moved from southwest to northeast during the Apenninie orogeny. In the Early Miocene its minimum values, which are now found in the Po Plain near the northeastern border of the Apennines, must have been situated at the southwestern border of the eontinental crust of the Adriatic Plate, i. e. near the present Tyrrhenian shelf edge.

890


If the metamorphic processes are to be classified as preogenic, synorogenic, or postorogenic not only the stage of the evolving orogeny but also the zone under consideration (internal, central, external) have to be distinguished. Nevertheless, some general statements are possible. Preorogenic heating (with respect to the Apenninic orogeny) is recognized in some of the Liguride nappes (p. 885). The Apenninic regional coalification is younger than the emplacement of the nappes in their stacking order (post-nappe stacking) because heating was somewhat re- tarded with respect to gravity sliding. Further thrusting of the nappes or the nappe pile as a whole took place after regional heating, which means that this thermal event was pre-final thrusting (late-synorogenie) or, in other words, these last eompressional tectonics were postmetamorphic. A later thermal event, con- temporaneous with tensional tectonics (post-compressional) and related to mag- marie activity, occurred and is still active in Tuscany. Regional metamorphism following nappe emplacement, and postmetamorphic thrusting in a continuously advancing orogeny have recently been described in the Swiss Alps too (BI~EIT- SCHMID, 1982).

Summarizing the palaeogeothermal history of the Northern Apennines, three thermal events can be distinguished: the first was preorogenie, the second and most imoortant regional event was late-synorogenic, and the last one was postoro- genie. The tectonic history as well as the palaeogeothermal inferences show that there was only a short period in which regional heating could take place. In the internal zones loading with alloehtbonous material started in the Early Miocene and already during the Messinian (Latest Miocene) the crust had returned more or less to the present level. Hence, if the younger contact metamorphism is disregarded, regional heating cannot have lasted more than 16 m. y., and as heat flow continuously increased starting from very low values, only the last part of this time span was decisive for the present deglee of coalification.

Acknowledgements

Special thanks are due to eand. geol. I. Elsing who carried out the reflectance measurements.

Re[erences

AOlP: Temperature sotterranee. - - 1890 p., Milano, 1977. BOCCALErrI, M., FAZZUOLI, M., LODDO, M., & MONC~LLI, F.: Heat Flow measurements

on the Northern Apennine Arc. - - Teetonophysics, 41, 101--112, 1977. BORTOLOTTI, V., & PASSEalNI, P.: Magmatic activity. - - In; S~STINI, G. (ed.), Develop-

ment of the Northern Apennines geosyncline. Sediment. Geol., 4, 599~624, 1970. BOSTICK, N. H., CASHMAN, S. M., McCuLLOH, T. H., & WADDELL, C. T.: Gradients of

vitrinite reflectance and present temperature in the Los Angeles and Venturff Ba- sins, California - - In: OLTZ, D. F. (ed.): Low temperature metamorphism of kerogen and clay minerals, 65--69, Los Angeles, 1979.

B•EITSCHMID, A.: Diagenese nnd schwaehe Metamorphose in den sedimentiiren Abfol- yen der Zentralschweizer Alpen (Vierwaldst~ittersee, Urirotstock). - - Eel. geol. Helv., 75, 881--880, 1982.

~ERMAK, V.: Heat Flow map of Europe. - - In: ~EaMAK, V., & BrB~CH, L. (eds.), Ter- restrial Heat Flow in Europe, 3--40, Springer, Berlin, 1979.

C. N. B. (Conslglio Nazionale delle Ricerche): Sezioni geologicostrutturali in scala

$91


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The coalification pattern in the Northern Apennines and its palaeogeothermic and tectonic significance