Boll. Soc. Geol. It., Volume speciale n. 1 (2002), 151-159, 7 ff. The structure of the Aurunci Mountains (southern Lazio): a balanced cross-section and its restoration DAVID ROSSI (*), SABINA BIGI (**), MARIO DEL CASTELLO (***) & PIO DI MANNA (****) ABSTRACT sfruttano il line-length balancing methods, sono stati calcolati i tassi di estensione e di raccorciamento, che possono essere generalizzati This short note presents a short balanced cross section between all’intera struttura studiata. Questa presenta una notevole comples- Gaeta and Campodimele, based on field data from a geological- sità strutturale, caratterizzata dalla sovrapposizione delle strutture structural map (1:25.000 scale). distensive, legate all’estensione tirrenica su quelle compressive me- The investigated area presents some specific topics, which dif- dio-mioceniche a vergenza nord-orientale. Nell’area infatti sono ri- ferentiate it from the adjacent northern and southern zones. The conoscibili due unità tettoniche principali, sovrapposte mediante un structural setting is characterized by a thin-skinned thrusting which piano di sovrascorrimento a generale andamento appenninico che involved the whole sedimentary succession (Upper Triassic-Upper presenta una traiettoria tipo ramp-flat-ramp. Nel settore sudocciden- Cretaceous). After the structuring of the Apenninic chain, the Plio- tale dell’area il sovrascorrimento determina la sovrapposizione dei Pleistocene extension offsets this sector along NW and subordinated termini triassico-giurassici su quelli cenomaniani; mentre nel setto- NE trending normal faults. re centrale un piano ausiliario posto alla base del cretacico determi- The Aurunci Mountains belong to the southwestern portion of na soltanto un raddoppio della porzione cretacica della successione. the Lazio-Abruzzi (mesozoic) carbonate platform domain. The sedi- Probabilmente associabili alle fasi finali della deformazione sono in- mentary succession shows the characters of inner shelf environ- fine i sistemi di faglie normali a direzione prevalentemente appen- ment, with facies ranging from subtidal to supratidal. The lithotypes ninica; mentre parte delle strutture distensive minori mostrano di- are mainly laminated dolomitic wackestones and calcareous pack- rezioni antiappenniniche e riattivano in senso normale anche faglie stones with benthic assemblages dominated by molluscs, foraminif- preesistenti a direzione N-S. era and green algae. I calcoli di bilanciamento hanno fornito valori di raccorcia- A number of outcrop of reasonable quality has enabled the con- mento pari a 6.6 km (32.8%) mentre i valori di estensione calcolati struction of one balanced cross-section throughout the Aurunci sono pari a 4.9 km (26.9%). Mountains, using the line-length method. This has been made up using as main detachment the Triassic dolostones levels and, as sec- ERMINI CHIAVE ondary ones, the lower portion of the cretaceous succession. T : Sezione geologico bilanciata, Appennino Using specific computers software we obtain a value of 6.6 km centrale, Monti Ausoni-Aurunci. e.g. (32.8%) of shortening, and a 4.9 km e.g. (26.9%) of extension. KEY WORDS: Balanced cross-section, central Apennines, 1. INTRODUCTION Aurunci and Ausoni Mountains. Cross-section balancing has been used widely to pro- duce structural models of the external parts of fold and RIASSUNTO thrust belts and is used in both contractional and exten- sional tectonic regimes. Restoration and balancing is an La struttura dei Monti Aurunci (Lazio meridionale): una se- old method of calculating depth to detachment and short- zione geologico-bilanciata e la sua retrodeformazione. ening (CHAMBERLAIN, 1910; BUCHER, 1933; GOUGEL, I Monti Ausoni-Aurunci costituiscono la parte sudoccidentale 1962) which was modified and adapted by DAHLSTROM del dominio della Piattaforma Laziale-Abruzzese, dove affiora una (1969), HOSSACK (1979); BOYER & ELLIOTT (1982); COO- potente successione carbonatica dal Trias superiore al Cretaceo su- PER (1983); GIBBS (1983); WOODWARD et alii (1985); periore di mare sottile, costituita essenzialmente da calcari fangoso- stenuti, calcari dolomitici e dolomie (ACCORDI et alii, 1988; CARAN- DE PAOR (1987, 1990); DE PAOR et alii (1988) and many NANTE et alii, 1978; CHIOCCHINI et alii, 1994). others. Attraverso una dettagliata analisi di superficie, ottenuta tramite Using the most commonly utilized geometric con- un rilevamento geologico alla scala 1:25.000, ed una analisi struttu- straints for thrust structures, we propose a short balanced rale alla meso ed alla macro-scala, viene proposta una sezione geo- logica bilanciata tra Gaeta e Campodimele. Utilizzando software che geological cross-section, between Gaeta and Campodi- mele in the Aurunci and Ausoni Mountains (southern Lazio). Data come from field mapping and conventional structural analysis. We use the line-length balancing (*) Dottorato di Ricerca in Scienze della Terra, Università method (DENNISON & WOODWARD, 1963) to calculate the degli Studi di Roma «La Sapienza» (P.le A. Moro, 5 – Roma), E-mail: shortening and extension of the section. [email protected] (**) Dipartimento di Scienze della Terra Università degli Stu- di di Roma «La Sapienza» (P.le A. Moro, 5 – Roma), E-mail: [email protected] 2. REGIONAL SETTING (***) Dottorato di Ricerca in Scienze della Terra Università degli Studi di Bologna (Via Zamboni, 67 – 40129 Bologna), E-mail: The Aurunci and Ausoni Mountains, in Southern [email protected] (****) Collaboratore esterno al Dipartimento di Scienze della Lazio area, represents the most internal part of the Lazio- Terra Università degli Studi di Roma «La Sapienza» (Via Farneti Abruzzi carbonate paleo-platform sensu CHIOCCHINI & S. Giorgio, 17 – 03020 Pico FR). MANCINELLI (1977). This area is comprised among the 152 D. ROSSI ET ALII Fig. 1 - Structural scheme of Souther Latium: 1) Quaternary deposits; 2) Plio-Pleistocene vul- canic sediments; 3) Miocene silicoclastic sequences; 4) Argille Varicolori formation; 5) Lazio- Abruzzi carbonate platform deposits; 6) Upper Triassic out- cropping formations; 7) Thrust faults; 8) Normal faults; 9) minor faults. – Schema strutturale del Lazio meridionale: 1) Depositi quater- nari; 2) Sedimenti vulcanici Plio-Pleistocenici; 3) Sequenze silicoclastiche mioceniche; 4) For- mazione delle Argille varicolori; 5) Depositi di piattaforma Lazia- le-Abruzzese; 6) Affioramenti del Triassico superiore; 7) Faglie in- verse; 8) Faglie normali; 9) Faglie di minore importanza. Amaseno River in the West, the Latina Valley in the 3. STRATIGRAPHICAL DATA North, the Garigliano River in the East and the Tyrrhen- ian Sea in the South (fig. 1). The sedimentary succession The bottom of the sedimentary succession cropping cropping out in the whole area belongs to the carbonate out in the area consists of massive dolomites covered by platform domain. It consists of neritic carbonate deposits limestone beds from Upper Triassic to Lower LIASSIC of Upper Triassic to Upper Cretaceous age (ACCORDI et (fig. 2) (CHIOCCHINI & MANCINELLI, 1977; CARANNANTE alii, 1988; CARANNANTE et alii, 1978; CARBONE, 1992; CA- et alii, 1978). Above this sabkha deposits, marine lagunal TENACCI, 1978; CHIOCCHINI & MANCINELLI, 1977; CHIOC- subtidal limestones (Middle Liassic-Upper Jurassic), CHINI et alii, 1984; DE CASTRO, 1962; GROSSI, 1933; PRA- rarely and poorly dolomitized occur. The facies analyses TURLON, 1964). Such sequences are typical of subsiding suggests the development of a carbonate inner shelf, carbonate platform areas where the accumulation rate characterised by moderate but variable water energy. counterbalances the relative sea level changes and causes The presence of layers containing rudstones and oolitic fluctuations from supratidal to shallow subtidal environ- deposits suggests the occurrence of depositional environ- ment (ACCORDI et alii, 1988; CARANNANTE et alii, 1978; ment characterised by high energy. The Lower Creta- CHIOCCHINI et alii, 1994). ceous deposits suggests the onset of a carbonate plat- From South to North the structural setting consists of form depositional system ranging from tidal flat to a deformed fold belt, trending NW-SE and bounded to lagoon. The analysis of textural parameters and vertical N-E by a main thrust fault (ACCORDI, 1963, 1964, 1966; organisation of the lithofacies assemblages allow to rec- CASSETTI, 1896; CASSETTI, 1900; CIPOLLARI & COSENTINO, ognise a cyclic organisation of the facies, arranged in 1991, 1995; FRANCHI, 1924). It placed the meso-cenozoic shoaling upward sequences. In the lower part, the sedimentary succession over the upper Miocene terrige- sequence is composed of interbedded wackestones and nous deposits cropping out in the Latina Valley, as men- packstones, sometimes dolomitized. The middle part tioned by many authors (ACOCELLA et alii, 1996; BENEO, (Lower Aptian-Cenomanian) is mainly composed of 1942; CARRARA et alii, 1995; DI FILIPPO, 1978; DI FILIPPO wackestones with interbedded micrites and laminated et alii, 1979; DI FILIPPO & TORO, 1980; DI FILIPPO et alii, dolomicrites. Interformational conglomerates, often dol- 1986; BARTOLE, 1984; MOSTARDINI & MERLINI, 1989). omitized, and discontinuous red layers (paleosoils) are The main tectonic transport is northeastward; in contrast interbedded and suggest the occurrence of depositional the amount of slip determined is not well defined environment characterised by restricted water circula- (PAROTTO, 1980; PATACCA et alii, 1990; TOZZI & CAPO- tion or short emersions. The bioclastic limestones (crys- TORTI,