Basin deformation due to laccolith emplacement at Radicofani (Southern Tuscany, Italy)

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1 Boll. Soc. Geol. It., Volume speciale n. 1 (2002), , 6 ff. Basin deformation due to laccolith emplacement at Radicofani (Southern Tuscany, Italy) V. ACOCELLA (*) (**), V. PASCUCCI (*) & G. DOMINICI (***) ABSTRACT The Radicofani basin (Southern Tuscany), developed during regional extension, is mainly filled by a mio-pliocene clastic sequence. During Early Pleistocene, the Radicofani spatter cone formed in the centre of the basin. A broad anticline, whose formation was previously associated with both extensional and compressional tectonics, is present within the basin. In order to better define the anticline, we collected field, seismic and magnetic data. Field work constrained the surface geometry of the anticline, showing that the Radicofani vent is located along the trace of its axial plane. Seismic lines are consistent with field data and show a progressive decrease in the dip of the anticline layers towards the bottom of the Neogene succession. The measured magnetic field shows a systematic relationship with the anticline, suggesting the association with a buried magnetic body. The matching of these data suggests that the anticline is the result of the emplacement of a laccolith within the Radicofani basin, possibly related to the magmatic activity responsible for volcanism at Radicofani. KEY WORDS: anticlines, laccolith emplacement, Radicofani basin. RIASSUNTO (*) Dip. di Scienze della Terra, Università di Siena, Via Laterina 8, 53100, Siena, Italy. (**) Presently: Dip. Scienze Geologiche, Università Roma TRE, Largo S.L. Murialdo, Roma, Italy (acocella@uniroma3.it). (***) Istituto Nazionale Geofisica, Via di Vigna Murata, Roma, Italy. Deformazioni indotte da laccoliti nel bacino di Radicofani (Toscana meridionale). Il bacino estensionale di Radicofani (Toscana meridionale) si è formato dal Miocene superiore (MARTINI & SAGRI, 1993; LIOTTA, 1996); il riempimento del bacino è per la massima parte costituito da una successione argillosa, con intercalazioni sabbioso-conglomeratiche, ascrivibile al Pliocene inferiore (LIOTTA & SALVATORINI, 1994). Nel Pleistocene inferiore, il bacino è interessato da attività vulcanica prevalentemente effusiva, con la formazione del cono di scorie di Radicofani (D ORAZIO et alii, 1994). Un ampia anticlinale è stata riconosciuta nel bacino; la sua formazione è stata associata a tettonica estensionale (formazione di un doppio roll-over; LIOTTA, 1996) o compressiva (formazione di un thrust-top basin; SANI et alii, 1998). Al fine di meglio definire la struttura e la natura dell anticlinale, sono stati acquisiti dati di terreno, sismici e magnetometrici. L analisi di terreno ha permesso di riconoscere l andamento in pianta di tre anticlinali; la traccia del piano assiale dell anticlinale maggiore passa in prossimità del vulcano di Radicofani. Le sezioni simiche mostrano che l anticlinale maggiore coincide con quella osservata sul terreno ed è l unica struttura plicativa con significativa continuazione in profondità; le rimanenti due anticlinali mostrano un estensione del tutto limitata in sezione. La struttura profonda dell anticlinale maggiore è caratterizzata dalla progressiva diminuzione della pendenza degli strati verso il basso della successione sedimentaria. Tale geometria difficilmente si concilia con meccanismi di formazione dell anticlinale per tettonica estensionale o compressiva. L analisi magnetometrica mostra un massimo relativo del campo magnetico nelle tre sezioni effettuate. Il massimo ha una relazione sistematica con la traccia del piano assiale dell anticlinale, suggerendo un associazione con un corpo magnetico sepolto. I dati acquisiti suggeriscono che l anticlinale maggiore riconosciuta, passante per il vulcano di Radicofani, è data dall inarcamento della sequenza sedimentaria in seguito alla messa in posto di un corpo magnetico laccolitico nel bacino; il laccolite sarebbe relazionato all attività magmatica responsabile del vulcanismo di Radicofani. Le stime di pressione magmatica effettuate, attraverso modelli flessurali, per spiegare la deformazione osservata nella copertura sedimentaria del bacino sono in accordo con i valori ipotizzati per la messa in posto di altri laccoliti conosciuti (CORRY, 1988). TERMINI CHIAVE: anticlinali, messa in posto di laccoliti, bacino di Radicofani. INTRODUCTION The post-thickening evolution of Southern Tuscany has been characterized by extensional processes and related magmatic activity (fig. 1; CARMIGNANI & KLIGFIELD, 1990; SERRI et alii, 1993; CARMIGNANI et alii, 1994; JOLI- VET et alii, 1998). From Late Miocene to Late Pliocene, extension occurred through high-angle N-S to NW-SE normal faults and subordinate NE-SW systems (FAZZINI & GELMINI, 1982; LIOTTA, 1991; CARMIGNANI et alii, 1994), responsible for the development of N-S to NW-SE basins (MARTINI & SAGRI, 1993). Magmatic activity accompanied extensional processes since Late Miocene (MARINELLI et alii, 1993; SERRI et alii, 1993), mainly through anatectic intrusives ( Ma) and subordinate volcanic products ( Ma), displaying a contamination with sub-crustal magmas (PECCERILLO et alii, 1987; INNOCENTI et alii, 1992; SERRI et alii, 1993; PECCERILLO, 1999). The NNW-SSE trending Radicofani basin is located on the westernmost part of the Tuscan Magmatic Province (fig. 1). The basin started to form during Miocene; however, most of its infill (up to approximately 2000 m) is made up of a Lower Pliocene argillaceous marine sequence, with sand and conglomerates intercalations (CALAMAI et alii, 1970; GANDIN & SANDRELLI, 1992; IAC- CARINO et alii, 1994; LIOTTA & SALVATORINI, 1994). During Early Pleistocene (at approximately 1.3 Ma; D ORAZIO et alii, 1991) a spatter cone formed in the centre of the Radicofani basin, feeding a shoshonitic-latitic ultrapotassic lava flow (D ORAZIO et alii, 1994, and references therein). The extensional structure of the basin is mostly inferred from seismic lines (LIOTTA & SALVATORINI, 1994; LIOTTA, 1996). The western part of the basin is bordered

2 750 V. ACOCELLA ET ALII 0 N Larderello 600 Tyrrhenian Sea Magmatic products (Late Miocene-Quatern.) Sedimentary deposits (Early Mioc.-Quat.) Undifferentiated substratum Southern Tuscany 20 km by a set of normal faults, regularly spaced every few hundreds of meters. The dip of these faults progressively decreases and becomes subhorizontal reaching the triassic evaporites, found in deep wells (CALAMAI et alii, 1970; LIOTTA, 1994; LIOTTA, 1996). The eastern part of the basin is bordered by normal faults, approximately spaced every kilometre (LIOTTA, 1994; LIOTTA, 1996; SANI et alii, 1998). The basin is thus bordered by opposite-verging normal faults, showing a graben-like structure (LIOTTA, 1996, and references therein). The inner part of the central Radicofani basin shows a N-S trending broad anticline (CALAMAI et alii, 1970; LIOTTA, 1996; SANI et alii, 1998). This feature has been mainly interpreted as the result of a double roll-over, due to the progressive tilt of the layers approaching the opposite-verging normal faults at the borders of the basin (LIOTTA, 1994; LIOTTA, 1996). However, recent works interpreted the structural setting of the Radicofani basin as mainly due to neogenicquaternary compression, responsible for the development of a thrust-top basin (BOCCALETTI & SANI, 1998; SANI et alii, 1998). According to these authors, the anticline found within the Radicofani basin is the result of compressional pulses (SANI et alii, 1998) Amiata 800 Radicofani basin (Fig. 2) normal faults; faults uplift curves of Neogene deposits (height in meters a.s.l.) bedding of Pliocene deposits in the Amiata area Fig. 1 - Schematic geological map of Southern Tuscany (Italy). The uplift curves of the Neogene marine deposits and the bedding of the Pliocene layers in the Amiata area are schematically reported. These features permit to define the broad dome centered on Amiata, due to the emplacement of an intrusion (ACOCELLA, 2000). Carta geologica schematica della Toscana meridionale. Sono riportate le curve di sollevamento dei depositi marini neogenici e la giacitura dei depositi pliocenici nell area del M. Amiata. Tali evidenze permettono di riconoscere un duomo centrato sul M. Amiata e dovuto alla messa in posto di un intrusione (ACOCELLA, 2000). These contrasting tectonic interpretations show how debated is the mechanism of formation of the basin and the anticline within. In order to better define the anticline, we collected field, seismic and magnetic data within the basin. The matching of our data permits to propose a new interpretation for the development of the Radicofani anticline, suggesting that it formed as a result of the emplacement of a laccolith within the sedimentary sequence of the basin. FIELD DATA Field work was performed in the central part of the Radicofani basin, aiming at evaluating the surface geometry of the anticline by measuring the attitudes of the outcropping Lower Pliocene deposits. At a broader scale, the deposits within the Radicofani basin make up a monocline moderately (approximately 10 ) plunging towards the east. Such a monocline marks the eastern part of a dome-like structure formed by Lower Pliocene deposits and centered on M. Amiata, west of Radicofani (fig. 1; ACOCELLA, 2000). The Amiata dome is the result of the uplift induced by pluton emplacement (ACOCELLA, 2000). Fig. 2 shows the collected field data in the central part of the Radicofani basin: the measured bedding of the Lower Pliocene marine deposits allowed us to recognize three broad anticlines with a mean NNW-SSE elongation. The trace of the axial plane of the largest anticline, approximately 12 km long and 4-5 km wide, passes immediately east of the Radicofani spatter cone (fig. 2). The northern part of this anticline was recognized through seismics by SANI et alii (1998). A shorter anticline is present on the SW part, along the Paglia River; this feature is approximately 7 km long and 3 km wide (fig; 2) and coincides with the anticline recognized by LIOTTA & SAL- VATORINI (1994) and LIOTTA (1996). The third, shortest anticline, recognized NW of Radicofani, is approximately 2 km long and is arranged with an en-echelon configuration with regard to the previous one (fig. 2). The three observed anticlines can be thus interpreted as positive culminations within an overall east-dipping monocline in the central part of the Radicofani basin. SEISMIC DATA The deep structure of the anticlines recognized on the field is shown in three line drawings of ENE-WSW trending seismic profiles crossing the Radicofani basin (fig. 3). The sections (fig. 4) show some common features, which are here highlighted. Four main unconformities are present in the Mio- Pliocene sedimentary succession of the basin (fig. 4). Their age is constrained from deep wells data. The upper unconformity, C, marks the transition from the G. sphaeroidinellopsis seminulina Zone and the G. margaritae Zone (Early Pliocene); unconformity C is located at the bottom of the Pliocene deposits; unconformity B is located at the bottom of the Messinian deposits and unconformity A is located at the bottom of the Langhian deposits (PASCUCCI et alii, 1999). The overall structural setting of the basin is made up of opposite-verging normal faults, more closely spaced on the eastern side (fig. 4).

3 BASIN DEFORMATION DUE TO LACCOLITH EMPLACEMENT AT RADICOFANI 751 Fig. 2 - Map of the central Radicofani basin, showing the bedding of the Lower Pliocene deposits, as recognized by field analysis. Solid lines indicate the axes of the three recognized anticlines. The trace of the axial plane of the largest anticline passes through the Radicofani vent. Carta della porzione centrale del Bacino di Radicofani, con le giaciture dei depositi del Pliocene inferiore misurate sul terreno. Linee a tratto continuo si riferiscono all asse delle anticlinali riconosciute. La traccia del piano assiale dell anticlinale maggiore passa per il centro eruttivo di Radicofani. The broad anticline is the main deformative feature within the sedimentary infill in the central part of the basin in the three seismic sections (fig. 4). The trace of its axial plane coincides with the trace of the axial plane of the major anticline (passing through the Radicofani vent) observed on the field. Similarly, the dips of the Pliocene layers are consistent both in field and seismic data (fig. 4). The anticline in the seismic profiles shows the progressive decrease in the dip of the layers towards the bottom of the Neogene succession (fig. 4). The curvature of the anticline in the sections is in fact higher in the deposits overlying the C unconformity and rapidly decreases, becoming negligible, in the deposits overlying the B unconformity.

4 752 V. ACOCELLA ET ALII Fig. 3 - Location of the seismic and magnetic profiles in figs. 4 and 5 respectively. Ubicazione dei profili sismici e magnetici nelle figg. 4 e 5. A magnetic survey was performed in order to further characterize the largest anticline passing through Radicofani. The magnetic analysis consisted of measuring (with a spacing of 50 meters) the intensity of the magnetic field along three transverse sections, approximately located in correspondence with the trace of the seismic profiles (fig. 3). Fig. 5 shows the results of the 3 magnetic profiles. The grey line displays the measured data, corrected considering the daily variations of the magnetic field. The black line displays the polinomial best fit curve (7th order), calculated in order to better show the higher wavelength anomalies. Fig. 5 also reports the location (arrow) of the trace of the axial plane of the major anticline, as shown by field and seismic data. The three magnetic profiles show a correlation between the maximum intensity of the magnetic field, characterized by an increase of nt, and the trace of the axial plane of the major anticline. In the three cases, the magnetic maximum is located between m east of the trace of the axial plane. These data suggest a systematic relationship between the presence of the anticline and the variations in the magnetic field. Since the variations in the intensity of the magnetic field do not depend upon the lithological variations at surface, they have to be related to buried masses with a different magnetic susceptibility. Being the magnetic field a dipole, the interpretation of the profiles in fig. 5 is not straightforward. A quantitative interpretation of the magnetic data in order to constrain the depth and the size of the buried masses is beyond the purpose of the paper and will be considered elsewhere. For a qualitative interpretation, considering the orientation of the magnetic profiles, their latitude and a cylindrical shape for the buried magnetic body, it is possible to infer that the body is located between the point of maximum and minimum intensity of the magnetic field (BREI- NER, 1973; FEDI & RAPOLLA, 1993, and references therein). The possible location of the magnetic body in the sections in fig. 5 thus broadly coincides with the location of the trace of the axial plane of the anticline. The magnetic data thus suggest that the anticline passing through Radicofani is associated with a buried magnetic body. The smaller anticline recognized on the field along the Paglia River can be also recognized in the seismic profiles Line 12 and Line 13, immediately west of the main anticline (fig. 4). However, its very limited lateral and deep extent suggests that this fold has a negligible importance if compared with the major anticline. Similarly, the smallest anticline found on the field is not shown in the seismic profiles. The profiles thus show that the anticline passing through the Radicofani vent is by far the largest fold recognized within the basin: if compared in size, the remaining anticlines can be considered negligible. Finally, the resolution of the seismic profiles Line 12, passing through the Radicofani vent, does not show the magmatic intrusion feeding the spatter cone, suggesting that the profiles do not permit to discriminate magmatic intrusions. MAGNETIC DATA DISCUSSION Field survey allowed us to recognize three anticlines forming positive culminations within the east-dipping monocline related to the uplift of the Amiata area in the Radicofani basin (fig. 2). The anticline passing through the Radicofani vent is the largest feature. Field data are consistent with seismics, which show that the largest anticline passes through Radicofani and that the remaining anticlines have a negligible extent at depth. Moreover, the seismic profiles show that the largest anticline is characterized by a decrease in the dip of the layers towards the bottom of the sedimentary sequence (fig. 4). This feature cannot be consistent with the formation of the anticline as due to extensional tectonics, as previously suggested (LIOTTA, 1996), since a progressive increase in the dip of the layers is expected within rollovers (WHITE et alii, 1986). Similarly, the geometry of the anticline in section is not consistent with the geometry of folds formed in compressional domains. The development of the anticline cannot thus be explained by compressional tectonics, as proposed by SANI et alii (1998).

5 BASIN DEFORMATION DUE TO LACCOLITH EMPLACEMENT AT RADICOFANI 753 Fig. 4 - Line drawings of the seismic profiles (see fig. 3 for location) across the Radicofani basin, showing a major anticline passing through the Radicofani vent. Profili sismici attraverso il bacino di Radicofani; è evidenziata l anticlinale maggiore passante per il centro eruttivo di Radicofani.

6 754 V. ACOCELLA ET ALII Fig. 5 - Magnetic profiles (see fig. 3 for location) across the Radicofani basin. The measured data (grey lines), the filtered data (black lines) and the location of the trace of the axial plane of the largest anticline are shown. Profili magnetici attraverso il bacino di Radicofani. Sono riportati i dati misurati (in grigio), i dati interpolati (in nero) e l ubicazione della traccia del piano assiale dell anticlinale maggiore.

7 BASIN DEFORMATION DUE TO LACCOLITH EMPLACEMENT AT RADICOFANI 755 Fig. 6 - Proposed model of christmas-tree laccolith emplaced within the sedimentary sequence at Radicofani. Modello proposto di laccolite tipo «christmas-tree», per spiegare le deformazioni osservate nella coltre sedimentaria a Radicofani. These evidences indicate that the mechanism of formation of the anticline at Radicofani cannot be simply explained in terms of regional (extensional or compressional) tectonics and that other possibilities have to be considered. Magnetic data suggest a relationship between the anticline and a buried magnetic body (fig. 5). The evidence that the anticline passes through the Radicofani vent implies a relationship with magmatic activity. These considerations suggest that a magnetized magmatic body, possibly related to the volcanic activity of Radicofani, intruded within the sedimentary sequence in the basin. The intrusion would be responsible for the uplift of the overburden and the development of the observed anticline. A laccolith-like intrusion may best explain the collected field, seismic and magnetic data. The mechanical features of the sedimentary infill within the basin offer preferred conditions for the development of laccoliths (CORRY, 1988; SCAILLET et alii, 1995). Christmas-tree laccoliths, characterized by multiple piled tabular intrusions decreasing in width towards the top (CORRY, 1998, and references therein), may properly explain the increase in the curvature of the anticlines towards the top of the seismic profiles (fig. 6). Applying a simple flexural model, it is possible to verify whether the observed anticline at Radicofani might be induced by a laccolith with a realistic magmatic pressure (fig. 6). The possible depth range of the laccolith is, from the curvature of the sediments in the seismic profiles, between approximately 0 m and 1500 m b.s.l.. This corresponds to an elastic thickness h of the overburden between 500 and 2000 m. The uplift H due to the intrusion is approximately m and the mean width W of the anticline is approximately 4000 m. The use of flexural models to evaluate the magmatic pressure requires that h<<w and H<<W; for this purpose, we consider the lowermost values of h and H. The flexural rigidity D of the sedimentary sequence overlying the laccolith is (TURCOTTE & SCHUBERT, 1982): D = E h 3 /12 (1-ν 2 ) (1) Being the Young s modulus E 20 GPa (TURCOTTE & SCHUBERT, 1982), the Poisson s ratio ν 0.25 (TURCOTTE & SCHUBERT, 1982) and h 500 m, the resulting D 2, Nm. This permits to calculate the magmatic pressure p required to form the anticline using the equation (POL- LARD & JOHNSON, 1973; TURCOTTE & SCHUBERT, 1982; KERR & POLLARD, 1998): H = (p-q) W 4 /24 D (2) where the lithostatic load q 10 MPa, H 200 m and W 2000 m. The resulting magmatic pressure is p 57 MPa. Such a value is consistent with the calculated magmatic pressures required for the emplacement of laccoliths with similar dimensions (JOHNSON & POLLARD, 1973; CORRY, 1988). CONCLUSIONS Field, seismic and magnetic data were collected to investigate the mechanism of formation of the anticline in the Radicofani basin. Field work recognized a major anticline, whose axis passes through the Radicofani vent. The seismic profiles show that the anticline is characterized by the progressive decrease in the dip of the layers towards the bottom of the sedimentary sequence. The measured magnetic field shows a systematic relationship with the anticline, suggesting the association with a buried magnetic body. The matching of the collected data suggests that the anticline is the result of the emplacement of a laccolith within the Radicofani basin. Its emplacement would be related to the magmatic activity responsible for volcanism at Radicofani. The computed magmatic pressure required to form the anticline is consistent with the values commonly inferred for laccolith emplacement. ACKNOWLEDGEMENTS The authors wish to thank A. Lazzarotto for his encouragement and helpful discussions, D. Liotta and F. Sandrelli for stimulating discussions, A. Di Ponzio, R. Funiciello, M. Marchetti and T. Meloni of Istituto Nazionale di Geofisica, for the magnetic surveys, L. Burbi, A. Bernasconi and S. Merlini of AGIP for the use of the seismic profiles and the reviervers G. Pasquarè and M. Sagri for helpful suggestions. This work has been supported by C.N.R. funds (responsible A. Lazzarotto).

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