International colloquium of planetary geology- Rome, September 22-30, 1975 - Proceedings

INTERNATIONAL COLLOQUIUM OF PLANETARY GEOLOGY

ROME, SEPTEMBER 22-30, 1975

PROCEEDINGS

GEOL. ROMANA, 15 (1976): 271-526, fig., tab., tav., Roma.

INDEX

FOREWARDS

GENERAL GEOLOGY OF ITALY Papers

LocARDI E., LoMBARDI G., FUNICIELLO R., PAROTTO M. - The mam volcanic groups of Latium (ltaly):

relations between structural evolution and petrogenesis

ToRo B. - Gravimetry and deep structure of the Sabatinian and Alban volcanic areas (Latium) . . . . . Expanded abstracts

ALVAREZ W. - The tectonic significance of Mediterranean volcanoes GUEST J.E., RoMANO R. - Volcanic activity on Mount Etna.

ATMOSPHERIC PROCESSES AND EOLIAN FEATURES Papers

Pag.

275

279 301 311 313

GROLIER M.J., ERICKSEN G.E., McCAULEY J,F. - Wind-furrowed sandstone, northeastern margin of the Sechura desert (Peru) . . . . . . . . . 317 Expanded abstracts

CoTERA A.S. - Grain size parameters of eolian deposits . 326

FLUXES AND CRATERING Papers

FASANO A., VITTORI O. - Black spherules in the micron and submicron range . . . . . 329 UNDERWOOD J.R., JR. - Impact structures of the Libyan Sahara: some comparison with Mars 337 Expanded abstracts

FECHTIG H. - Interplanetary dust fiux at 1 A.U. as derived from Lunar microcraters studies and in situ experiments . . . . 341 NEUKUM G. - Cratering in the Earth-Moon system - Some comparison with other terrestrial planets 341 OBERBECK V.R., QuAIDE W.L., ARVIDSON R.E. - Secondary cratering on Mercury, the Moon, and Mars 342

FUTURE PLANS Papers

CoRADINI A., SALOMONE M. - Uses of principal components models in planetary geology 347

MALAVASI M. - The Trapani-Milo stratospheric balloons base . 351

Expanded abstracts

ANDERSON D.M. - Analysis of volatile constituents of the Martian regolith by the Viking GCMS 359 QuAIDE W.L., 0BERBECK V.R., MoRRISON R.H., AGGARWAL H.R., BLANCHARD M.B. - Mars penetrators:

a preliminary analysis of their value for near surface science . . . . 361 ScHABER G.G. - Geologie application of radar data to planetary exploration . . . . 363 SoDERBLOM L.A. - Computerized image processing in terrestrial and planetary geology . . . . 366

MARS Papers

CJCCACCI S., FREDI P., LuPIA PALMIERI E. - Morphometric characteristics of the ejecta blankets of Martian impact craters . . . . . . . . 367 CoRADINI M., ARVIDSON R.E. - Age and formation of Martian fretted terrain. . . . . 377 CoRADINI M., BIANCHI R. - The contact between plains an d cratered terrain on Mars: a geomorphologic

analysis . . . . 383 CoRADINI M., FEDERICO C., FuLCHIGNONI M. - Wind fiow over large escarpments on Mars . . . . 393 MASSON P. - Structure pattern analysis of the Noctis Labyrinthus-Valles Marineris regions of Mars . 399 NuMMEDAL D., GoNSIEWSKI J.J., BooTHROYD J.C. - Geologica! significance of large channels on Mars 407 Expanded abstracts

AGGARWAL H.R., 0BERBECK V.R. - A morphological study of Martian doublet craters CARR M.H. - Change in height of Martian volcanoes with time . . .

GJLBERT M.C., HIGGINS B.B. - Geochemistry of Mars . . . .

LuccHITTA B.K. - Some principles of landform development applied to Mars . MASURSKY H. - The evolution and modification of the crust of Mars . . . .

SAUNDERS R.S. - Martian tectonic history . . . . . . . .

WISE D. U. - Faulting and stress trajectories near Alba Volcano, Northern Tharsis ridge of Mars

419 421 422 424 426 428 430

274 INTERNATIONAL COLLOQUIUM OF PLANETARY GEOLOGY

MERCURY

Papers Pag.

CAPUTO C., CASACCHIA R., CAVARRETTA G., PAROTTO M., CARUSI A., FuLCHIGNONI M. - Mercury cratering I: a catalogue of large craters (belt from

+

ameters, areas, and perimeters of craters having a radius > 10 km . . . . 451 CARUSI A., FuLCHIGNONI M., PoscoLIERI M., CAPUTO C., CASACCHIA R., PAROTTO M. - Mercury cratering

III: a catalogue of large polar craters, some statistica! considerations . . . . 457 CAPUTO C., PAROTTO M., CARUSI A., FULCHIGNONI M. - Mercury cratering IV: a comparison between dif-

ferent physiographic provinces . . . . . . . . . . . . 467

TRASK N.}. - Cratering history of the heavily cratered terrain on Mercury . . . . 471 Expanded abstracts

NESS N.F., BEHANNON K.W., LEPPING R.P., WHANG Y.C. - The magnetic fìeld at Mercury and its impli- cations . . . . 477 ScHULTZ P.H., GAULT D.E. - On the origin of the hilly and Iineated terrain on Mercury . . . . 479

SOLAR SYSTEM Papers

CoRADINI A., MAGNI G., FEDERICO C. - Accretion processes in the early Solar System . . . SHOEMAKER E.M., HELIN E.F., GILLETT S.L. - Populations of the planet-crossing asteroids Expanded abstracts

SoDERBLOM L.A. - Geology of the early Solar System (Mars, Moon, Mercury compared) .

COMPARATIVE VOLCANOLOGY

481 487

490

Papers

PETERSON }.E. - Volcanism in the Noachis-Hellas region of Mars . . . 493

SAUNDERS R.S., MALIN M.C. - Venus: geologie analysis of radar images 507

Expanded abstracts

BIASINI A., FINOCCHI A. - Photogeological map of Boseti Mountains (Main Ethiopian Rift) . . . . 516 GREELEY R. - Lava tubes - Earth, Moon and Mars . . . . 517 TILLING R.I., HoLCOMB R.T., LocKWOOD J.P., PETERSON D.W. - Recent eruptions of Hawaiian volcanoes

and the evolution of basaltic landforms . . . . 519 UGOLINI F. C. - Soils of the high elevations of Mauna Kea, Hawaii, an analogy to Martian soils? . . . . 521

PREMESSA

L'International Colloquim of Planetary Geology, che si è svolto a Roma presso l'Istituto di Geologia e Paleontologia dell'Università degli Studi di Roma dal 22 al 30 settembre 1975, è stata la prima ma- nifestazione europea nel settore della geologia pla- netaria.

L'idea di organizzare questo congresso è nata durante la Hawaian Planetology Conference, tenuta a Hilo (Hawaii) nell'ottobre 1974, il cui motivo do- minante è stato l'alternarsi di lavori sulla geologia dei pianeti con escursioni di campagna e di dibat- titi sulla natura e formazione dei crateri locali.

A Roma - per la sua posizione favorevole al centro di interessanti aree vulcaniche - uno dei temi principali è stato la vulcanologia comparata.

Sono state organizzate escursioni guidate per con- sentire ai partecipanti di approfondire le conoscen- ze sui centri vulcanici dei Vulsini, dei Sabatini e dei Colli Albani. È stata organizzata anche una escursione aerea sui centri vulcanici attivi dell'Italia meridionale. Di particolare interesse è stata la visita all'Etna che si trovava in quel periodo in una fase di attività.

Il congresso è stato suddiviso in diverse sessioni in cui si è discusso della geologia generale dei pia- neti, del flusso di materia interplanetaria e della craterizzazione, delle atmosfere dei pianeti, dei pro- cessi eolici, degli aspetti teorici della formazione del Sistema Solare e dei progetti futuri.

L'organizzazione è stata curata dal Gruppo Ita- liano per gli Studi Planetari, costituito da ricerca- tori del Laboratorio di Astrofisica Spaziale del CNR

- Frascati, dell'Istituto di Geologia e Paleontologia dell'Università degli Studi di Roma, del Centro di Studi per la Geologia dell'Italia Centrale del CNR -Roma.

Un particolare ringraziamento per la collabora- zione fornita durante l'organizzazione va rivolto al Dr. Steve Dwornik, al Dr. Donald Wise, alla Si- gnora Sheena McLahanan, al Dr. John Guest, ai Dr. Ronald Greeley, al Prof. Alfred Rittman, al Dr. Romolo Romano, al Dr. Enzo Locardi e al Dr. Gianni Lombardi.

Il congresso è stato finanziato dal Ministero Af- fari Esteri (Direzione Generale della Cooperazione Culturale, Scientifica e Tecnica) e dal Consiglio Nazionale delle Ricerche (Servizio Attività Spa- ziali).

La stesura di questi Proceedings è stata curata da un Comitato di Redazione composto dal Dr.

Claudio Caputo, dal Dr. Andrea Carusi, dalla Dr.

Angioletta Coradini, dal Dr. Costanzo Federico, dalla Dr. Paola Fredi, dal Dr. Marcello Fulchi- gnoni, Dr. Gianni Lombardi e dal Dr. Maurizio Parotto.

Il Comitato di Redazione è particolarmente rico- noscente al Prof. Bruno Accordi, Direttore di Geo- logica Romana che ha voluto ospitare questi lavori nella rivista contribuendo anche concretamente alla realizzazione dei Proceedings.

Altri contributi per la stampa sono stati erogati dai Comitati per le Scienze Fisiche e per le Scien- ze Geologiche e Minerarie del Consiglio N azionale delle Ricerche e dalla Regione Lazio.

276 /NTERNATIONAL COLLOQUIUM OF PLANETARY GEOLOGY

FOREWARD

The International Colloquium of Planetary Geology - which took piace in Rome in September 1975 at the Istituto di Geologia e Paleontologia of the Uni- 'Uersity - has been the first European meeting on planetary geology.

The idea of organizing this colloquium was born during the Hawaian Planetology Conference held in Hilo (Hawaii) in October 1974 whose leit-motiv was the alternating of papers on the geology of planets and fie/d excursions as well as on the nature of foca!

crater formation sensu lato.

In Rome - because of its favourable location at the center of an interesting volcanic area - one of the most important topics was the comparative vol- canology. Guided fie/d trips have been organized to allow the attendants to become familiar with Vulsini, Sabatini and Alban Hills volcanic centers. Also an overfli'ght has been dane to show the active volcanoes of Southern Italy. Particularly interesting has been the visit to Mt. Etna which at that moment was in a new eruptive stage.

The colloquium was divided into sessions dealing with genera! geology of planets, interplanetary flux rates an cratering, planetary atmospheres and aeolian processes, theoretical aspects of Salar System forma-

tion and future planning.

Organization was carried out by the Italian Con- sortium for Planetary Studies, which is composed of researchers belonging to the Laboratorio di Astrofisica

Spaziale del CNR-Frascati, to the Istituto di Geo- logia e Paleontologia dell'Università degli Studi di Roma and to the Centro di Studio per la Geologia dell'Italia Centrale del CNR-Roma.

A personal thank to al! the american and european people who helped the organizers as Dr. Steve Dwornik, Dr. Dona/d Wise, Mrs. Sheena McLahanan, Dr.

John Guest, Dr. Ronald Greeley,Prof. Alfred Ritt- man, Dr. Romolo Romano, Dr. Enzo Locardi and Dr. Gianni Lombardi.

The meeting was sponsored by the Ministero Affari Esteri (Direzione Generale della Cooperazione Cul- turale, Scientifica e Tecnica) and by the Consiglio Nazionale delle Ricerche (Servizio Attività Spaziali).

The Editoria! Board having provided for the draft of these Proceedings was composed of: Dr. Claudio Caputo, Dr. Andrea Carusi, Dr. Angioletta Cora- dini, Dr. Costanzo Federico, Dr. Paola Fredi, Dr.

Marcello Fulchignoni, Dr. Gianni Lombardi and Dr.

Maurizio Parotto.

The Editoria! Board is particularly thankful to Prof. Bruno Accordi, Director of Geologica Romana, who accepted these papers in the review also contri- buting concretely to the realization of the Proceedings.

Further contributions for the press were given by the Comitato per le Scienze Fisiche and the Comitato per le Scienze Geologiche e Minerarie of the Con- siglio Nazionale delle Ricerche as well as by the Regione Lazio.

The proceedings that are included in this document are a result of many years of field and laboratory studies by researchers in Italy, the United States, France, Germany and Great Britan.

Planetary studies are based upon a succession of facts, proceeding from telescopic observations of a celestial body to analysis of returned lunar samples. Between these two extremes lies the te- dious research performed by dedicated scientists. The great dust storms on Mars, for example, were observed in the 1800's by Schiaparelli using a telescope. In 1971, the Mariner 9 mission to Mars observed a global dust storm. After a short period of time, and very fortuitously, the dust storm subsided and the cameras an board the spacecraft were able to photograph surface features that were totally unobservable in the past, including the largest volcano in the solar system and a valley that would extend from Sicily to Milano. PI's in the NASA Planetary Geology Program (including scientists from the University of Rome, University of Paris, University of London, and the Max Planck Institut) were delighted with these discoveries and with the hitherto unknown evi- dence of eolian and fluvial activity.

The coordination of efforts to unravel the secrets of Mars is the responsibility of the Planetary Geology Program office at NASA Headquarters. Individuai scientists, doing work in their home universities or laboratories worked on specific topics. In was recognized that in order to optimize science returns not only from Mariner 9, but other space missions, it was necessary to have a well coordinated data exchange program. This program not only includes the exchange of published reports, but also a personal coordination on an individuai basis. It was the desire of the program office that this exchange take piace not only on a personal

camaradie

basis, but also in forma!

meetings where latest findings can be presented, diswssed, and critiqued by their peer groups. The strength of the Planetary Geology Program, or any science program, lies in its ability to have a free exchange of results along with a criticai discussion of same.

The International Colloquium of Planetary Geology is a beautiful example of the mutuai cooperative needs of the Planetary Geology Program. Scientists from the participating five coun- tries met in the beauttful city of Rome to not only discuss results of their own work, but also to learn about the volcanological phenomena in the Alban Hills and Mt. Etna in Sicily. The gra- cious hospitality of the host scientists, along with their scientific capability, were the important factors in the success of the colloquium and the resulting Proceedings Document.

STEPHEN

E.

Chief of the Planetary Geology Program Offìce

NASA Hdq - Washington

279

ENZO LOCARDI GIANNI LOMBARDI

CNEN, CSN della Casaccia, S. Maria di Galeria - Roma Istituto di Mineralogia e Petrografia dell'Università - Roma RENATO FUNICIELLO - MAURIZIO PAROTTO

Istituto di Geologia e Paleontologia dell'Università - Roma

THE MAIN VOLCANIC GROUPS OF LATIUM (ITALY): RELATIONS BETWEEN STRUCTURAL EVOLUTION AND PETROGENESIS

The petrogenesis ami volcanic successions of the recent volcanic groups of the Tuscan and Latian regions do not fit well in the various geodynamic schemes proposed for the Meùi- terranean basin.

The Authors analyze the structural evolution from the Tyrrhenian coastline to the Apen- nine from the first phase of the plicative tectonics to the distensive tectonics, in relation both to the development of the Tyrrhenian basin ancl the volcanic and magmatic succession.

Such analysis allows to rccognize an evolutive correspondence between the formation of the Tyrrhenian basin and the sequencc of volcano-tectonic phases.

The information on the genesis of the related magmas provided by recent data of isotope geochemistry fit well with the proposcd approach to the problem.

\Vork carried out with the contribution of C.N.R.

Revised from the Guidebook for the Field Trips of the International Colloquium of Planetary Geology. R. Funiciello and M. Parotto contributed to the parts dealing with the main regional tectonic features and the Alban Hills volcano; G. Lombardi contributed to thc part concerning the acidic volcanic groups and the petrogenetic considerations.

PREFACE

In the Quaternary, most of the area overlooking the Tyrrhenian basin featured an active volcanism and the volcanic products emitted, coeva! and con- tiguous, cover the majority of the known petro- chemical range.

The criteria of the global tectonics cnabled to attribute the different types of volcanism to given geodynamic processes and it is striking that ali main geodynamic processes were simultaneously active in such a limited area.

The correct petrological defìnition of volcanites and the attribution of the different volcanic systems to one or the other crustal or subcrustal environ- ment, was the outcome of the work of many re- searchers in Italy in the past few years. The area o n which more confused an d often contrasting indi- cations exist, however, is represented by Latium, with its series of alkaline-potassic volcanic groups.

The typical co-magmatic province ( << Roman Province >), WASHINGTON) was recognized thanks to

thc petrographic works at the beginning of this century and, since then, the area involved is sup- poscd to havc represented a maximum frequency of pctrographic works relevant to volcanic rocks.

Nonetheless, the results of these studies were not satisfactory on thc whole in defìning magmatism genctically and the few generai theories produced have not withstood criticism for a long time. The more rcccnt approach to the problem was more cffectivc since it applied isotopc geology and seis- mic data to thc more recent volcanic areas. The overall picture seems to be reliable on a small scale, but the impact with the various !oca! geologie reali- ties is again unsatisfactory. It seems, however, that a rcsearch instrument was neglected in trying to solve this geologie problem, that is the same ge- ology.

We are attcmpting here to suggest a geologie approach, hoping that we chose the suitable re- prcsentation scale, before coping with the possi- bility of adapting onc or the othcr geodynamrc modcl.

R. FUNICIELLO and !VI. PAROTTO are members of the lTALIAN CO:\ISORTJUM FOR PLANETARY STCDIES.

Introduction

The volcanic activity m Latium took piace in an elongated belt between the Tyrrhenian coast and the Apenninc range from the Late Pliocene to less than 50,000 years ago.

Considering the deep structures, the volcanism appears to be at the western border of an arca characterized by a marked crustal increase; the minimum crustal thicknesses corresrond to thc Tyrrhenian basin (about 10 km); at its eastern margin the crust rapidly increases and reachcs its

maximum thickness below the axts of the Apen- mne range.

Geodinamically the volcanism of Latium is relat- ed to the post-miocenic distensive tectonics which is responsible for the format10n of the Tyrrhenian basi n.

The volcanoes of Latium belong to two mag- matic scries, clearly distinct from each other, even if they scttled in the samc region and, at least in part, at the samc time.

The first series includes the acidic, rhyolitic and

rhj~odacitic volcar>.o~s of the Tolfa-Cerite area and of Mt. Cimino. The second includes the Vulsini, Vico,

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Sabatini and Alban Hills ( also calle d the <<Lati an vol- cano >>) groups, which show a marked potassic alkaline charactcr. The first series has strong petrographic ana- logies with the Tuscan Magmatic Province (from the island of Elba to Mt.

Amiata). The second series, instead, is linked to the al- kaline volcanoes of Campania (from Roccamonfina to the Phlegraean Fields and Vesu- YÌus). Latium represcnts the area indeed where these system3 converge and an analysis of the principal regional structures may help to clarify which geodynami- cal processes conditwned the development of the voi-

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FH;. 1 - Contour ma p of the ^<< l\Ioho Discontinui tv>>. Contour interval: 5 km.

1: Cenozoic to Rccent volcanics. 2: Postorogenic succèssions. 3: Orogenic complex.

4: Hercvman Europe (foreland). (Redrawn from: GmsE & MoRELLI, 1975).

-- Carta strutturale della ^<< discontinuità di l\lohorovicic •>. Equidistanza: 5 km.

l : vulcani ti terziarie e quaternaric. 2: successioni postorogene. 3: complessi orogeni.

4: Europa ercinica (avampaese). (Ridisegnato da: C lESE & MoRELLI, 1975).

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Mesozoic-Cenozoic struc- tural units

The volcanism in Latium ts found m sedimentary complexes of the Apennine Orogene which were formed m different periods and phases and show genetic and evolutive differences.

Some complexes are related to a Mesozoic sedimentary cycle which developed in pe- lagic areas quite dose to the forcland (Vmbria-Marche- Molise basin, Tuscan basin,

GENERAL GEOLOGY OF ITALY 281

Panormide ridge, etc.); others, instead, are pro- ducts of sedimentation in interna! basins and show typical ophiolitic sequences (furrows of thc Li- guride complex). The emplaccment of these com- plexes was accompanied by piling up of thick series of flyschoid units which are younger in age pro- ceeding from the Tyrrhenian to the Adriatic Sea (from Oligocene to Quaternary). Overthrusting within the single units and of the different com- plexes is the main tectonic feature. These over- thrusts lead to notable movements towards E and NE, in some cases over large distances.

In particular, in Centrai I taly, severa! tectonic phases which occurred since the Upper Miocene are responsible for the present fìeld relations bet- ween the pelagic Umbria-Marche-Sabina facies and the carbonate shelf units of Latium and Abruzzi.

In the Early Neogenc a plicative tectonics led to a complex series of overthrusts; in the Pliocene- Quaternary, instead, a distensive tectonics deter- mined the formation of a rifting which led to Horst and Graben systems and also to horizontal dis- placements.

Neogenic structural units

Locally three main Graben may be distinguished:

Tiber Valley, Siena-Radicofani and Pontina Plain.

They probably are just different elements of a 'lingle large structure which after its formation was dissected and in part covered by volcanic products.

The formation of thc principal Graben is mainly related to the Lower Pliocene distensive tectonic cycle. In this period, parallel to the Tyrrhcnian coast some crustal blocks sank even more than one thou;and meters, along step faults systems. The most important of these sinking belts may be traced from Tuscany, near Siena, through the area oc- cupied by the alkalinc volcanoes, down south to the Pontina Plain.

The transgressing Pliocene sea therefore reached the left bank of the Tiber valley where it was blocked by the Apenninc Range and in the area thick dq::osits of day and sandy clay formations w ere sedimented. W estwards, the Graben near the Tyrrhenian coast was also bordered by a discon- tinuous alignment of a series of Horst composed of sedimentary rocks of the Sicilide-Liguride com- plexes or of the TusGm units. The discountinui- ties were mainly caused by a secondary, transversal Graben system.

The structure of the main Graben is complicateci by the presence of other secondary tectonic ele-

ments, e.g. the Mt. Cetona Horst which, with an elongation parallel to the main Graben divides it in two scctions, thc Siena-Radicofani and the Tiber vallcys respectively.

Whilc during the Middle-Upper Pliocene, the Tyrrhenian basin began its rapid sinking, a belt between thc Apennine and the prescnt Tyrrhenian coast started to rise. The Lowcr Pliocene sedi- ments which fìlled thc principal Graben, therefore, were uplifted as far as l 000 m above the present sea level along the axis of the uplifted structure.

At the same time towards the Tiber valley and the Tyrrhcnian coast, the marine sedimentation, even though interrupted by various regressions, con- tinued on the flanks of this structure.

I t is in connection with this regional swelling that the acidic magmatism of southern Tuscany and of northern Latium occurred. The Rocca- strada group to thc north and the Tolfa-Cerite to the south settled about 2.5 m.y. ago. Similar acidic Yolcanic complexes such as Mt. Amiata, Radicofani and Mt. Cimino, instead, are younger, between 1.4 and (Mt. Amiata) 0.4 m.y. Such age differences (about l m.y.) between petrographically similar vol- canics may be explained by considering the local structural evolution. The swelling, indeed, occur- red along belts parallel to the Tyrrhenian coast- linc, with the earlier movements developing west- wards and thc more recent ones in the areas fur- ther inland.

The climax of this uprising coincided with the volcanism and therefore the age of the aforemen- tioncd Yolcanic systcms may also mark the age of thc maximum uprising in the specifìc areas.

Quaternary structural units

Thc rcgional swelling reached its peak at the bc:ginning of the Pleistocene. Aftcrwards a major collapse of the raised belt occurrcd beginning from the area now occupied by the Vulsini volcanoes, slightly south of the present maximum culmina- tion. The present altitude and position of the top of the Plio-Pleistocenic sediments indicates that the collapse continues to the south and coincides with the area occupied by the other Vico, Sabatini and Alban Hills alkaline volcanic groups.

lt can be concluded that the alkaline volcanism settled in a Graben, just after its formation, in a structural situation which has strong analogies with that typical of the Rift Valley systems. The bor- ders of the Graben may be identifìcd with those

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Frc. 2 - Geologica! sketch of the volcanic area in Centra! Italv. Volcanic sequences. 1: Alkaline potassic volcanics (Pleistocene to Recent); (a: Vulsini volcanoes;

b: Vico volcano; c: Sabatini volcanoes; d: Alban Hills volèanoes). 2: Acidic volcanics (Plio-Pleistocene). Sedimentary formations. 3: Travertine. 4: Recent (continental and coastal sediments). 5: Upper Miocene- Pliocene- Lower Pleistocene (day and sand). 6: Carboniferous- Lower Miocene (mainly sedimentary

sequences). 7: Faults. 8: Buried faults. (Redrawn from: LoCARDI & MoLIN, 1974; CAPUTO et al., 1974).

Schema geologico dell'area vulcanica nell'Italia centrale tirrenica. Successioni ,vulcaniche - 1 : Vulcani ti alcalino-potassiche (Quaternario); (a: Vulcani Vulsini;

b: Vulcano di Vico; c: Vulcani Sabatini; d: Colli Albani o Vulcano Laziale). 2: Vulcaniti acide (Plio-Pleistocene). Successioni sedimentarie. 3: Travertino.

4: Quaternario (sedimenti continentali e costieri). 5: Miocene superiore - Pliocene - Pleistocene inferiore (sedimenti sabbiosi e argillosi). 6: Carbonifero - Mio- cene inferiore (successioni prevalentemente sedimentarie). 7: Faglie. 8: Faglie sepolte. (Ridisegnato da: LoCARDI & MouN, 1974; CAPUTO et al., 1974).

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GENERAL GEOLOGY OF ITALY 283 of the Lower Pliocene sea at the beginning of the

distensive tectonic phase.

These series of positive and negatives movements which took piace before the acid and the alkaline potassic volcanism, respectively, offer an oppor- tunity for speculation on the genesis of the mag- mas, which, however, are out of the scope of the present work.

Main tectonic lines at the margins of the vol- canic outcrops

The main tectonic alignments of the regwn are plotted in fig. 3. The southern ramifications of the << Tuscan nappe>> (TN) sink below the Vul- sinian volcanics, as well as the Neogenic units (Tevere Graben, Siena-Radicofani Graben). Gravi- metrie data show that the Tuscan nappe rapidly sinks more than 2000 m below the volcanics. In the Sabatini area, however, there are culminatiom of the Tuscan formations, which may be inferred also from the nature and distribution of the sedi- mentary volcanic ejecta. Similar structures outcrop south of the volcanic area, on the Zannone island.

Also the front of the <<Monte Modino-Monte Cervarola nappe>> (CV) disappears below the vol- canic outcrops and we have no more evidence of its continuation to the south.

The << Umbria-Marche-Sabina complex >> (UM) has some culminations near the contact with the volcanics and then disappears below the alluvium of the Tiber and below the volcanics.

From these field relations the existence may be inferred of large systems of normal faults which cut the Graben occupied by the volcanics to the north and to the east.

Along the western margin of the Latina V alley the Lepini-Ausoni Range dips below the volcanics.

Drilling showed that a NW -SE oriented step fault- ing characterizes the carbonate structure towards

sw.

Other important sedimentary structures are found inside the volcanic area. The biggest is Mt. So- ratte, a mainly calcareous relief constituted by large overthrusts of Tuscan or Sabina units. Other simi- lar nuclei are aligned with it and may be followed even across the Tiber valley towards the Corni- colani mountains.

In fact geophysical data show that all these nuclei are p art of a single large NW -SE oriented struc- ture. This is truncated to the WNW by a normal fault which also separates, on the eastern margins,

the Vico from the Sa ba tini volcano; to the south it stops against the Tiburtini mountains.

Along the Tyrrhenian coastline, there are culmi- nations of the sedimentary structures of the base- ment which then rapidly sink towards the Tyrrhe- nian basin along a system of step faults parallel to the coastline.

The distensive tectonics which controlled the evolution of the Graben and of the volcanism has been active up to the recent.

Only the fractures connected to and cau~ing the last volcano-tectonic phases may be easily recog- nized. This is the case of the NW -SE oriented fracture system which caused the sinking of the whole western sector of the Latian volcano and caused also large movements in the Sabatini, pro- voking the imposing phreatomagmatic activity which closed the volcanic cycle of Latium.

Structure of the substratum below the vol- canics

The basement of the Tolfa- Cerite- Manziate acidic volcanic group is the same as the similar Tuscan groups of Mt. Amiata and Roccastrada and is mainly constituted by flyschoid allochthonous formations of the Sicilide and Liguride complexes.

In the late Pliocene this Flysch was uplifted as a Horst system which bordered the main Neogenic Graben to the west. The Flysch series, Cretace- ous-Paleogenic in age, is made up by calcareous and arenaceous rocks with variable clay compo- nents, and reaches a maximum thickness of 2000 m.

The acidic volcano of Mt. Cimino and the Radi- cofani group, instead, are on a substratum made up of Pliocenic day and sandy clays of the princi- pal Graben, which may reach a thickness of over 1,500 m. The same formation is the one which, nearly everywhere, underlies the outcrops of the alkaline volcanoes. South of the Vulsini group, the Flysch relief of Mt. Razzano seems to be related to the Mesozoic-Paleogenic ridge of Mt. Cetona.

But drilling dose to the two ends of the ridge showed, on their internai slopes, Pliocene sediments steeply plunging towards the center of the Vulsini area, which is thus at the intersection of two (Apennine and anti-Apennine) Pliocenic Graben systems.

From drill cores we know that along the Mt.

Cetona-Mt. Razzano alignment marine sediments of the Lower Pliocene prevail, whilst on the flanks a series from marine to transitional to continental sediments of the Middle-Upper Pliocene and of

FrG. 3 - Comparison between Skylab and ERTS pictures (a) and regional tectonic lines (b) in Centrai ltaly. The sketch of fig. 3b (where area of fig. 3a is framed) shows that distensive tectonic lines cut across the structures due to previous compressive tectonics. These lines shape a series of Horst and Graben which follow continually from Tuscany to the Pontina Plain also under the Latian Volcano covcr. LEGEND. 1: Latian alkaline potassic volcanics (Pleistocene). 2: Acidic volcanics (Upper Pliocene- Pleistocene). 3: Norma! faults (dashes indicate the downthrown si de). 4: Graben (G) an d Horst (H) (Pliocene - Pleistoccne). 5: idem, beneath the volcanic cover. 6: Northeast- wards-dipping structures of the Panormide Complex. 7: Axes of folds generally overturned cast- or northeastwards.

8: Overthrust of the Umbria-Marche-Sabina Complex; it continues southwards beneath the volcanics, as far as Capo Circeo (Uppermost Miocene- Lower Pliocene). 9: Fronts of structures belonging to the Panormide Complex (Up- permost Miocene). 10: Front of the Cervarola nappe (Middle Miocene). 11: Front of the Tuscan nappe; it is likely

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GENERAL GEOLOGY OF ITALY 285

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to continue southwards (to Zannone Island ?); (Middle Miocene). 12: Caldera or crater rim. PC = Panormide Complex (carbonate shelf sequences). UM = Umbria-Marche-Sabina Basin Complex (pelagic sequences). TN =

Tuscan nappe. CV = Mt. Cervarola nappe.

- Confronto tra immagini Skylab ed ERTS (a) c lineamenti tettonici regionali (b) nell'Italia Centrale. Dallo schema di fig. 3b (in cui è indicata l'area della fig. 3a) si rileva che i motivi della tettonica distensiva tagliano le strutture do- vute alla precedente tettonica compressiva. Essi configurano una serie di Horst e Graben, che si seguono ininterrotta- mente dalla Toscana alla Pianura Pontina, anche sotto la copertura vulcanica laziale. LEGENDA. 1: Vulcaniti laziali alcalino potassiche (Pleistocene). 2: Vulcani ti acide (Plioccne superiore - Pleistocene). 3: Faglie distensive (i trattini in- dicano il lato ribassato). 4: Graben (G) e Horst (H) (Plio-Pleistocene). 5: idem, sotto la copertura vulcanica. 6: Strut- ture monoclinalichc del Complesso Panormide, con immersione generale verso nord-est. 7: Assi di pieghe, general- mente rovesciate verso est o nord-est. 8: Fronte di accavallamento del Complesso umbro-marchigiano-sabino; proba- bilmente esso prosegue verso sud sotto le vulcaniti, almeno fino a Capo Circeo (Miocene superiore- Pliocene inferiore).

9: Fronti di accavallamenti all'interno del Complesso Panormide (Mioccne superiore). 10: Fronte del Complesso M.

Modino- M. Cervarola (Miocene medio). 11: Fronte della Falda Toscana; probabilmente esso continua verso sud, sotto le vulcani ti (Miocene medio). 12: Margini di caldere o di crateri. PC = Complesso Panormide (facies di piat- taforma carbonatica). UM = Complesso di bacino umbro-marchigiano-sabino (facies pelagica). TN = Falda Toscana.

CV = Complesso di M. Modino- M. Cervarola.

the Pleistocene may be found. The substratum of the centrai part of the Vulsini area, therefore, seems to be made up by the formations which filled a Graben in the Lower Pliocene, were then uplifted in a ridge in the Upper Pliocene and, in the Pleis- tocene, sank before and during the alkaline potas- sic volcanic activity. The sunken belt corresponds to the continuation of the Pliocenic Graben of Radicofani and of Tevere.

In the Vulsini volcanoes the area which collapsed in the Pleistocene is defined by a seri es of NW- SE and NE-SW faults and corresponds nearly ex- actly to the eastern margins of the volcanics. The day sediments of the Lower Pleistoccnc show stcp faulting towards Lake Bolsena, near Mt. Cetona and Mt. Razzano ridges, and the sediments of the Upper Pliocene and of the Pleistocene at the east- ern margin are similarly faulted. It is against the corresponding fault planes that we find thick (hun- dreds of meters) volcano-sedimentary series of the first phase of the Vulsini activity.

The centrai eone of the Vico volcano set in piace in correspondence to the axis of the ridge which formed in the Middle-Upper Pliocene. The top of the sedimentary substratum of this volcano is hundreds of meters below the main level of the substratum at the margins. The sunken blocks seem to h ave developed along NW -SE an d NE- SW alignments. In the western sector Flysch units, belonging to the continuation of the Mt. Cetona- Mt. Razzano ridge, constitute the substratum. In the center there is a NW -SE belt of day an d sands of the Middle-Lower Pliocene and to the west similar sediments of the Upper Pliocene and Pleis- tocene. To the north, between the sediments of the basement and the alkaline products of the Vico volcano, acidic products of Mt. Cimino set in piace.

Also the Sabatini volcanic group, like the Vul- sini one, occurred in the vicinity of a previous transversal structure with respect to the main tec- tonic axis. Indeed, a sequence of terrains is ob- served along NE-SE trending belts. A Flysch ridge at the NW boundary of the group, delimits the Lower Pliocene sediments with tectonic contact.

These sediments are followed, along parallel belts, by the Middle- U p per Pliocene an d Marine Pleis- tocene sediments. Immediately north of the Sa- batini group, such sequence system resumes the Apennine trend.

The substratum of the Sabatini volcano appears to have notably sunken and the top of the sedi- ments was reached, in welJs, below sea level. The main depression has a WNW-ESE elongation and

stretches from Lake Bracciano to the area south of Cesano, Formello and Sacrofano. From Bac- cano to Sutri another minor depression with a NW -SE direction may be recognized an d i t ap- pears to be a continuation of the one going from Tuscania towards Lake Vico.

Recently, drilling carried out for geothermal exploration showed that the principal depression (from Bracciano to ESE) is interrupted near the eastern margins of the Lake by a steep structural high of the preneogenic basement. This structure is composed of Flysch units and rises up to 200 m below sea level. The tectonic significance of this Flysch nucleus inside the Pliocenic basin will be discussed in the chapter dealing with the evolution of the Sabatini group.

The Alban Hills volcano occurred in an area which tectonically represents the convergence point of the Graben of the Pontina Plain, of the princi- pal tectonic lines of the Sabina mountains and of the overlapping front of the calcareous mesozoic range of the Lepini-Ausoni Mts. The most recent units which outcrop at the margins of the volcanics are Pleistocenic and they surround the southern and the western part of the apparatus, where they are found in deep cuts. Only data from some shallow wells in the volcanics are available. They show that the contact of the volcanics with the Plio-Pleistocenic clays and sandy clays of the sedi- mentary basement occurs at various levels. Depres- sions reach a depth of -70 m b.s.l. whilst culmi- nations are on average 100 m higher.

The Plio-Pleistocenic units may reach a thick- ness of some hundreds of meters. Below them is the deep substratum that here is made up of a sequence of calcareous and marly formations of the Sabina series. The nature of the deep sub- stratum, as deduced from analyses of the sedi- mentary ejecta in the products of the final phreato- magmatic activity appears to be fairly homogene- ous. Only at the southern margins is there a tran- sition to another carbonate facies typical of a sedi- mentation in a shelf-edge environment, probably related to the nearby structure of the Lepini Moun- tains.

The acidic volcanic groups

A first group comprises the Tolfa (about 40 sq km) and the Cerite (25 sq km) massif and some minor and isolated nuclei of less importance such as Mt. Calvario, la Tolfaccia, Mt. S. Vito etc.

They are the erosional relics of a larger volcanic complex. Sands and pebbles derived from weath-

GENERAL GEOLOGY OF ITALY 287 ering of the volcanics surround the volcano-tectonic

horst on which the volcanoes settled.

The radiometric age ranges measured on severa!

samples representative of different facies and areas range between 4.2 and 2.1 m.y.; the maximum frequency is around 2.5 m.y.

Volcanologically two principal phases may be dis- tinguished: an initial ignimbritic activity - with basai levels of chaotic tuffs - and a subsequent effusi o n of viscous lava flows that led to the con- struction of a series of morphologically prominent lava domes which raised and perforated the exist- ing ignimbritic plateau. The two phases must have been very dose in time, as testified by strong petrographic analogies of the products (both ignim- brites and lava flows range from rhyolites to quartz- latites) and confirmed by the absolute dating results which show overlapping ranges of values for the two episodes.

Also minor quartzmonzonitic hypoabissal bodies, which cross only the sedimentary basement, and widespread products of intense late magmatic ac- tivity, are connected to this magmatism.

Strong analogies with the situation observed in the Tolfa-Cerite are shown by the Mt. Cimino group, positioned much farther east of the Tyrrhe- nian coastline. Also here the volcanism occurred in an area of tectonic instability where there is convergence of severa! fault systems. Mt. Cimino corresponds to a volcano-tectonic horst connected with a sizeable magmatic intrusion.

The products of Mt. Cimino activity are found over an area of more than 300 sq km, but most of the outcrops are covered by the more recent alkaline products of the Vico volcano. The ac- tivity began about 1.4 m.y. ago and unti! about l m.y. a succession of ignimbritic emissions and lava domes extrusions occurred.

Locally ignimbrites reach thicknesses of more than 150 m and domes constitutes reliefs of up to 1000 m a.s.l. Both ignimbrites and lavas have a quartzlatitic composition, whilst petrochemical dif- ferentiations are found in the products of the last phases of activity of the Cimino volcano. These are represented by latitic lava flows and later by olivin-latitic lava issued from a series of centrai and eccentric apparata.

After the end of the activity of the Mt. Cimino volcano, collapses occurred in the area. They were related in part to the eruption itself, but most were caused by the new distensive tectonic phase which led to the formation of the main Graben at present occupied by the alkaline volcanoe&.

The alkaline potassic volcanics groups

The distribution of the different alkaline volcanic groups and their evolution is in generai quite com- plex. Morphologies tend to be very flat and ero- sion scarce. Flattening is principally caused by the succession of collapses which accompanied the vol- canic evolution and by the calderas and volcano- tectonic sinking which concluded the volcanic epi- sodes, but there is also a marked influence of the characteristics of the most widespread volcanic pro- ducts which are large pyroclastic flows.

In the following part we shall give a brief des- cription of the main volcanic groups from north to south.

The V ulsini volcanoes

The southernmost products of the Vulsini vol- canoes directly overlie the l m.y. old Mt. Cimino ignimbrites and are covered by a 0.5. m.y. old pyroclastic flow of the Vico volcano. The activity therefore began in this interval of time and proba- bly at its lower Iimit. The mo~t recent age found up to now is 0.06 m.y. for a lava flow of the Latera apparatus but there are severa! later volcanic epi-

~odes in the same V ulsini area. Traces of the initial activity are found in wells and in some relics which outcrop on the eastern margins of Lake Bolsena. In the first phase, large thick volcano- sedimentary basins developed. They show that a strong subsidence was active together with the vol- canic activity of this first phase. The directions of the currents in the volcano-sedimentary basins in- dicate that a N -S oriented volcanic relief occupied the central-eastern sector of the Vulsini area.

The Montefiascone volcanic group belongs to a later stage, but is stili older than 0.5 m.y. It in- volves a series of centrai volcanoes and of fissural eruptions and its products occupy the southeastern part of the Vulsini area.

A volcano-tectonic phase followed where the ex- i,ting vokanic structures were dis<;ected by series of NW-SE, N-S and E-W faults. This phase was characterized by the sinking of the Bolsena basin depression and uprising, along a system of approxi- mately N -S oriented step faults, of a structure which divides the new depression from the eastern volcano-sedimentary basins. These tectonic move- ments were accompanied by a fissural volcanism which led to the effusion of huge quantities of pyroclastic flows and lavas, starting from about 0.4 m.y. ago.

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Principal fractures supplying the volcanic activity. 9: Rim of caldera. 1 O: Minor volcanic apparatus. 11 : Trace of the section (see fig. 5).

- Carta schematica dei vulcani Vulsini. 1: Prodotti finali del vulcano di Latera, successivi e/o contemporanei alla formazione della caldera (più recenti di 0,06 m.a.). 2: Margini esterni delle colate piroclastiche del vulcano di Latera (0,2 - 0,1 m.a.). 3: Margini dei prodotti prevalentemente !avici relativi alle fasi iniziali del vulcanismo di Latera (0,4 m.a.). 4: Colate piroclastiche e lave del gruppo di Bagnoregio. 5: Colate piroclastiche del vulcano di Vico (0,5 m.a.).

6: Gruppo vulcanico di Montefiascone. 7: Paleovulcano di Bolsena e relative facies vulcano-sedimentarie. 8: Princi- pali fratture alimentatrici dell'attività vulcanica. 9: Orlo di caldera. 1 O: Apparati vulcanici minori. 11: Traccia della

sezwne (vedi fig. 5).

These products towards E and NE covered the marginai basins and they also expanded further on over the sedimentary units borderi11g the previous basin.

Starting from 0.3 m.y. the principal volcanic ac- tivity shifts westwards to the great strato-volcano of Latera, that emitted products now covering ali the western area of the Vulsini zone. The new centrai strato-volcano is characterized by a first phase with prevailing lavas and tuffs, a second phase of massi ve pyroclastic ftows (0.2 - 0.1 m. y.) and, finally, at 0.06 m.y., the formation of the caldera of Latera.

The evolution of this caldera is very complex.

On its northern slopes, at the top of the series, pyroclastic materials occur which have antidune structures, interrupted by radiai channels, typical of a base surge genesis.

The originai craters of phreatomagmatic explo- sion are now masked by a series of more recent ash and scoriae cones which are at the margins or cross the caldera.

Part of the Latera products expanded towards the Bolsena depression, covering the tectonic struc- tures which delimited the basin to the west.

GENERAL GEOLOGY OF ITALY 289

But there have been other volcanic activities also after the sinking of the Bolsena depression.

For example two islands, relics of cinder cones erupted in Lake Bolsena; a crater w est of Monte- fiascone, that is probably a phreatomagmatic center;

the scoriae cones and, above al!, fissures filled with scoriae which were aligned along NE-SW, N-S and NW -SE directions. These volcanic activities occurred during a recent distensive tectonic phase which had effects in ali the Vulsini area but es-

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In the sunken areas these formations are covered by epi-volcanics and lacustrine sediments which do not outcrop even at the periphery of the volcano.

lt may be said, therefore, that following an in- tensive activity of acidic pumice and ignimbritic eruption, a collapse of the centrai part of the vol- cano occurred and this consequently led to the formation of a sedimentation basin. After this epi- sode a mainly lavic centrai volcano settled at about 0.8-0.6 m.y. In the subsequent 200,000 years from

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cene day and sand. 7: Flysch (pre-Neogene). 8: Faults.

- Sezione attraverso il gruppo vulcanico vulsino. 1: Vulcanismo finale di Latera (0, 1 - 0,03 ? m.a.). 2: Colate piro- clastiche di Latera (0,2- 0,1 m.a.). 3: Vulcano-strato di Latera (0,3- 0,2 m.a.). 4: Prima fase del vulcanismo di Mon- tefiascone (0,7 - 0,5 m. a.). 5: Paleovulcano di Bolsena e facies vulcano-sedimentarie relative (0, 9 - 0,7 m. a.). 6: Argille

e sabbie plio-pleistoceniche. 7: Flysch preneogenico. 8: Faglie.

pecially in a belt which extends from the Latera volcano towards SE.

The Vico volcano

The identification of the different phases of ac- tivity of the Vico volcano is simplified by the fact that the volcanism mainly occurred from a princi- pal center, thus building up a eone. A terminai caldera allows us to recognize parts of the series which do not outcrop elsewhere.

The products of the Vico volcano in part directly overlie the most recent (1.0 m.y.) acidic volcanics of Mt. Cimino. The first products of the Vico volcano underlie the oldest dated lavas (0.8 m.y.) and are represented by thick ignimbritic ftows and pumice tuffs with a trachytic-rhyolitic composition.

It may be deduced from drilling, that these early formations which outcrop at the western and north- ern margins of the volcano, sank hundreds of meters towards the center of the volcano itself.

fissures crossing the centrai eone, huge pyroclastic ftows erupted, followed by a collapse and the for- mation of the caldera. From pericalderic fissures the effusive activity continued together with in- trusion of subvolcanites. The last activities are related to a series of fissures which cross the west- ern sector of the stratovolcano (Vetralla lava 0.09 m.y.), and are mainly represented by apparata and products related to recent phreatomagmatic activity.

The Sabatini volcanoes

The dispersion of the eruptive centers and the complications due to the tectonics which accom- panied the volcanic activity are responsible for the marked complexity of the Sabatini volcanic group.

Stratigraphic relations with the nearby volcanoes and radiometric age dating show that the volcanism in the Sabatini, Vico, Vulsini and Alban Hills areas is almost contcmporary. The first products are constituted by acidic trachytic pyroclastic ftows and

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FIG. 6 - The main formations of the Vico volcano. 1: Strato-volcanoes (a), craters (b) and scoriae cones (c) follow- ing the sinking of the caldera. 2: Pyroclastic flows and various products connected with the phase of caldera sinking.

3: Pyroclastic flows (ignimbrites <<A •>, << B >>, << C >>), more ancient of 0.54 m.y. 4: Limits of the initial mainly lava volcano. 5: Caldera rims.

- Principali unità del Vulcano di Vico. 1: Strato-vulcani (a), crateri (b) e coni di scorie (c) successivi alla forma- zione della caldera. 2: Colate piroclastiche e prodotti vari connessi con la fase di sprofondamento della caldera. 3: Co- late piroclastiche (ignimbriti <<A •>, << B >> e << C >>), più antiche di 0,54 m. a. 4: Limiti del vulcano iniziale, prevalente-

mente lavico. 5: Margini della caldera.

GENERAL GEOLOGY OF ITALY 291 then by markedly alkaline lavas and pyroclastics

alternateci with epivolcamc lagoon, lacustrine and fluvial deposits. The emission of a great cover of

pyroclastic flows followed, the most recent of which was dated 0.4 m.y. The next formation (0.3 - 0.2 m.y.) is a pyroclastic and effusive system charac-

FIG. 7 - Schema tic evolution of the Vico volcano. a: The substratum is made up by quartzlatitic domes and ignimbrites on a sedimentary basement. b: Alkaline potassi c la vas, building up the first eone of Vico, get as far as 1 O km from the cra- ter. c: The region is peneplained by the destruction of the terminai part of the eone and by an ignimbritic eruption. d: A slightly eccentric caldera has formed

and the eone of Monte Venere is built up. (From LocARDI, 1965).

- Evoluzione schema ti ca del Vulcano di Vico. a: Il substrato è costituito da domi ed ignimbriti quarzolatitiche, che giacciono sul basamento sedimentario. b: Lave al- calinopotassiche costituenti gran parte del primo cono di Vico, estese fino ad una decina di km dal cratere. c: La regione viene peneplanata con la distruzione della parte terminale del cono e con un'eruzione ignimbritica. d: Si forma una caldera leggermente eccentrica e si accresce il cono di Monte Venere, a chiusura dell'attività.

(Da LOCARDI, 1965).

terized by its extension and by the abundance of inter- calations of reworked pro- ducts and lacustrine deposits.

These products may be re- cognized ali along the peri- meter of the volcanic area and they have been grouped in a single main cycle beca- use they seem to constitute a centralized system and be- cause the tectonics which accompanied this volcanism kept the same style of block sinking.

In the subsequent phase (0.18 - 0.09 m.y.) a great mass of pyroclastics extre- mely rich in clayey and car- bonatic ejecta from the ba- sement were set in piace mainly by flows. Finally, explosion products exist ( cin- ders, microbreccias, and py- roclastic breccias), which covered the originai mam edifice.

These products mark the beginning of the second cy- de, characterized by a di- stensive tectonic phase which led to the latera! spreading of two principal blocks and to a volcanic activity along new fracture system.

The principal fracture that separates the two blocks co- incides with the southern and western sides of Lake Bracciano and can be follo- wed north and south thro- ugh a succession of segments with Apennine and antia- pennine direction.

At present the two blocks are displaced at about 14 km m a NNW-SSE direction, which 1s thes result of a combination of latera! spreading toward