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The role of extensional tectonics at different crustal levels on granite ascent and emplacement: an example from Tuscany (Italy)
Tectonophysics 354 (2002) 71 – 83 www.elsevier.com/locate/tecto
The role of extensional tectonics at different crustal levels on granite ascent and emplacement: an example from Tuscany (Italy) V. Acocella*, F. Rossetti Dipartimento di Scienze Geologiche, Universita` Roma TRE, Largo S.L. Murialdo, 1, 00146 Rome, Italy Received 27 November 2001; accepted 24 May 2002
Abstract The role of regional extension on the rise and emplacement of granites in the crust is still debated. Pluton ascent and emplacement widely occurred in Tuscany (Italy) since late Miocene during the post-orogenic collapse of the inner Apennines, and are presently occurring in the geothermal areas of Amiata and Larderello. Tuscany offers a preferred test site to study the role of regional extension on pluton ascent and emplacement at different crustal levels. Ductile extension enhanced the segregation and ascent of granitic melts in the lower crust, controlling pluton emplacement in correspondence with the brittle – ductile transition. In the brittle crust, magma ascent occurred through subvertical faults and fractures compatible with the regional extension direction; pluton emplacement mainly occurred by means of roof lifting. The case of Tuscany suggests that the extensional structures enhance melt segregation and ascent in the ductile crust, but are not efficient alone to provide a pathway for the ascent of granitic magmas in the brittle-extending crust. The estimated magmatic strain rates due to pluton emplacement in the geothermal areas are much larger than the regional tectonic strain rates. This suggests that regional tectonics did not control magma emplacement in the brittle crust and explains why nontectonic processes (roof lifting) accommodated the space required for pluton emplacement. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Magma transport; Pluton emplacement; Regional extension; Tuscany
1. Introduction The role of regional extension in the ascent and emplacement of granites has been a matter of study and debate. Extension has been commonly solicited as an efficient environment for the ascent of granitic melts at mid-crustal levels. Known examples include the Basin and Range (Gans et al., 1989; Wernicke et al., 1987; Davis et al., 1993), Sierra Nevada (Tobisch *
Corresponding author. Tel.: +39-06-54888027; fax: +39-0654888201. E-mail address: [email protected] (V. Acocella).
et al., 1993), Papua New Guinea (Hill et al., 1995), Greenland (Strachan et al., 2001), Yemen (Geoffrey et al., 1998), the Cyclades (e.g., Gautier et al., 1993) and the Tyrrhenian Sea (Jolivet et al., 1998). In these cases, the activity of low-angle ductile extensional shear zones enhanced decompression and rise of the isotherms, inducing the ascent of granites by removing the load on the roof (e.g., Lister and Davis, 1989). In the upper crust, one (to our knowledge) case highlighted the control of extensional structures on granite emplacement (Greenland; Hutton et al., 1990); such control is, however, debated (Hutton et al., 2000), as it has been suggested that the granite
0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 0 2 ) 0 0 2 9 0 - 1
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emplaced through roof uplift and floor depression, rather than the activity of extensional structures (Grocott et al., 1999). Regional extensional structures (including high- and low-angle normal faults) in the upper crust do not seem to play an important role in solving the ‘‘space problem’’, or the creation of the space required to accommodate granite emplacement (e.g., Hutton, 1988a; Tikoff et al., 1999). The inability for regional extensional structures to accommodate pluton emplacement at shallow levels applies, in particular, to large plutons and batholiths (Paterson and Fowler, 1993; Hanson and Glazner, 1995). Rather, granite emplacement at shallow crustal levels has been often related to the activity of strike-slip faults, mainly due to strain partitioning processes in oblique convergence settings (Busby-Spera and Saleeby, 1990; Glazner, 1991; Tikoff and Teyssier, 1992; Tobisch and Cruden, 1995; Tikoff and de Saint Blanquat, 1997; de Saint Blanquat et al., 1998; Wilson and Grocott, 1999). In such a tectonic framework, the emplacement of granites along localised zones (such as relay zones, pull-apart structures or tension gashes) is the most common mechanism (e.g., Guinebertau et al., 1987; Hutton, 1988b; Schmidt et al., 1990; D’Lemos et al., 1992; McCaffrey, 1992; Morand, 1992; Moreau et al., 1994; Vigneresse, 1995a; Castro and Fernandez, 1998). All these studies suggest that regional extensional structures may control the ascent of the granitic magma, but do not control the emplacement of the plutons at shallower crustal levels. In this work, we aim to test this hypothesis, investigating the role played by extensional structures on the ascent and emplacement of the Mio – Pliocene granites of the Tuscan Magmatic Province of Italy. Tuscany provides the opportunity to study the ascent and the intrusion of granites at different structural levels within an extending crust as a consequence of the collapse of the inner Apennine chain. The mechanisms of pluton emplacement in Tuscany have been discussed in Rossetti et al. (2000) and, subordinately, in Acocella (2000). Based on these data, in this work, we first review the modalities of pluton ascent and emplacement, and then we propose a new interpretation for the role of regional extensional structures on the ascent and emplacement of the granites at different crustal levels.
2. Regional extensional tectonics in southern Tuscany Crustal thickening occurred in Tuscany at the Oligocene – Miocene boundary (Brunet et al., 2000) during the development of the northern Apennine orogenic wedge (Jolivet et al., 1998 and references therein). The post-thickening evolution was characterized by widespread extension due to the collapse of the Apennines (Carmignani et al., 1994; Jolivet et al., 1998). Extension occurred at the back of the eastward-migrating Apennine orogen, as a consequence of the progressive eastward shifting of the Apennine subduction (Jolivet et al., 1998 and references therein). In the northern Tyrrhenian area (Fig. 1), extensional tectonics developed since early Miocene in Alpine Corsica (Jolivet et al., 1990), progressively shifting eastward with an overall E –W direction of extension (Carmignani et al., 1995). Extension resulted in the development of shallowly eastward-dipping ductile extensional shear zones. These, active at the brittle– ductile transition, initially accompanied the nearly isothermal exhumation of the deep-seated metamorphic rocks and, successively (late Miocene – Pliocene), magmatic activity (Jolivet et al., 1998 and references therein) (Fig. 1). From late Miocene to late Pliocene, N – S to NW – SE normal faults were active in southern Tuscany, bordering several extensional basins (e.g., Carmignani et al., 1994; Pascucci et al., 1999) in a nonrotational setting (Mattei et al., 1996) (Fig. 1). NE – SW strike-slip faults interrupt the continuity of adjacent basins throughout southern Tuscany (Fig. 1). These faults, being coeval and kinematically compatible with the NW – SE normal faults, have been interpreted as transfer systems (Liotta, 1991). Moreover, N – S-trending dextral fault segments along the coastline of southern Tuscany have been recognized as active features during pluton emplacement (Acocella et al., 2000; Rossetti et al., 2001) (Fig. 1). These faults, being coeval and kinematically consistent with the E – W to WNW – ESE regional extension direction, have been interpreted as accommodating strain compatibility in the brittle crust during crustal-scale extensional simple shearing at depth (Rossetti et al., 2000).
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Fig. 1. Schematic map of the main structural and magmatic features of southern Tuscany (Italy). Insets show: (a) an ideal E – W section through the central part of the Tuscan Magmatic Province; (b) the eastward migration of extension and related magmatism along an ideal section. The age of the intrusion at Amiata is not radiometric and is inferred from the age of the beginning of the uplift (3.5 Ma) and the presence of an active geothermal field (0 Ma) (Acocella, 2000).
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3. Magma ascent and pluton emplacement in southern Tuscany Magmatic activity accompanied extension in southern Tuscany since late Miocene, with an overall eastward migration, following the extensional deformation front (e.g., Marinelli et al., 1993; Serri et al., 1993; Jolivet et al., 1998). The Tuscan Magmatic Province (Fig. 1) is made up of granites (7.0 Ma – present) and subordinate volcanic products (5.0 –0.3 Ma), displaying a contamination with subcrustal magmas (Innocenti et al., 1992; Serri et al., 1993). Magmatic activity is responsible for a positive thermal anomaly in southern Tuscany from late Miocene to present, culminating in the active geothermal areas of Larderello and Amiata (e.g., Mongelli et al., 1998) (Fig. 1). The exposed and buried intrusive stocks in southern Tuscany have been interpreted as apophyses of a larger deep-seated batholith (Pasquare´ et al., 1983; Marinelli et al., 1993; Baldi et al., 1994; Barberi et al., 1994; Franceschini, 1994; Bossio et al., 1995; Gianelli et al., 1997). We now review the data relative to the ascent and emplacement of the plutons outcropping in southern Tuscany, partly presented in Rossetti et al. (2000) and Acocella (2000). A main distinction is made here, grouping the plutons accordingly to their different crystallization depths: (i) greater than 2 kbar: Montecristo, Giglio, Monte Capanne and
Porto Azzurro of Elba island (off-shore granites); (ii) less than 2 kbar: Campiglia, Gavorrano, Amiata and Larderello (on-shore granites). Table 1 summarizes the published data concerning the conditions for pluton emplacement and the associated tectonic features. (i) The Montecristo monzogranite (Fig. 1) has a Rb/Sr cooling age of 7.0– 7.3 Ma (Ferrara and Tonarini, 1985). Using the ternary plot of normative composition (quartz, albite and orthoclase, recalculated to 100%), a pressure of crystallization starting at about 5 kbar was calculated (Innocenti et al., 1997). The Giglio monzogranite (Fig. 1) has a Rb/Sr cooling age of 5.0 Ma; according to the mineral assemblages (ternary plot of normative quartz, albite and orthoclase), a minimum crystallization pressure of approximately 4 kbar was calculated (Westerman et al., 1993). The Elba monzogranites (Fig. 1) have a cooling age between 6.8 and 5.5 Ma (K/Ar and Rb/Sr methods: Ferrara and Tonarini, 1985); thermobarometric data in the aureole of Porto Azzurro pluton indicate an emplacement depth slightly larger than 2 kbar (Duranti et al., 1992). All these plutons preserve magmatic concentric foliation, with solid-state recrystallisation processes increasing towards their boundaries; such a fabric developed as a consequence of a flattening-type deformation (Boccaletti and Papini, 1989; Burrelli and Papini, 1992; Westerman et al., 1993; Rossetti et al., 1999). Top-to-the-east exten-
Table 1 Main features associated with the emplacement of the monzogranite intrusions of southern Tuscany Pluton
Location
Minimum age (Ma)
Emplacement pressure (kbar)
Main structural features associated
Montecristo
off-shore
7.0 – 7.3 (1)
5 (2)
Giglio
off-shore
5.1 (3)
>4 (3)
Elba
off-shore
6.8 – 5.5 (1)
2 – 3 (5)
Campiglia Gavorrano Amiata Larderello
on-shore on-shore on-shore on-shore
5.7 (13, 14) 4.9 – 4.4 (14) 3.5 – 0 (17) 3.8 – 0 (18)
V 1 (13) ? 1 – 1.7 (17) f 1 (18)
shallow-dipping extensional shear zones (E – W extension) (3) shallow-dipping extensional shear zones (E – W extension) (6, 8, 9, 12) shallow-dipping extensional shear zones (E – W extension) (4, 7, 10, 11) N – S right-lateral faults; domed roof (15) N – S right-lateral faults (16) NE – SW structures (?); domed roof (19, 20) NE – SW structures (?); domed roof (?) (20, 21, 22, 23)
Related references: 1 = Ferrara and Tonarini, 1985; 2 = Innocenti et al., 1997; 3 = Westerman et al., 1993; 4 = Boccaletti and Papini, 1989; 5 = Duranti et al., 1992; 6 = Burrelli and Papini, 1992; 7 = Daniel and Jolivet, 1995; 8 = Rossetti et al., 1999; 9 = Rossetti et al., 1998; 10 = Keller and Pialli, 1990; 11 = Bouillin et al., 1994; 12 = Lazzarotto et al., 1964; 13 = Barberi et al., 1967; 14 = Borsi et al., 1967; 15 = Acocella et al., 2000; 16 = Rossetti et al., 2001; 17 = Gianelli et al., 1988; 18 = Del Moro et al., 1982; 19 = Acocella, 2000; 20 = Gianelli et al., 1997; 21 = Batini et al., 1983; 22 = Mongelli et al., 1998; 23 = Dallmeyer and Liotta, 1998; ? = unknown data (see text for further details).
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sional shearing is observed at the boundaries of the plutons, overprinting the previous magmatic foliation (Daniel and Jolivet, 1995; Jolivet et al., 1998; Rossetti et al., 1999) (Fig. 2). The shear zones belong to a set of low-angle, eastward-dipping extensional systems, mostly active at the depth of the regional brittle– ductile transition (Jolivet et al., 1998 and references therein). Shearing affected the intrusions during their final stage of cooling (Rossetti et al., 1999) and continued after pluton solidification, forming lowgrade mylonites and cataclasites along the boundary of the intrusions (Daniel and Jolivet, 1995; Rossetti et al., 1999) (Fig. 2). Fission tracks data on the Capanne intrusion at Elba (Bouillin et al., 1994) indicate that the final cooling and exhumation of the pluton occurred in the last 2 Ma. At Giglio island, brittle faulting controls the contact between the monzogranite and the country rocks, where no evidence of thermal metamorphism is observed (Lazzarotto et al., 1964; Rossetti et al., 1999). The Elba and Giglio examples thus indicate that the final exhumation of the plutons occurred after their cooling.
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(ii) The quartzmonzonite pluton outcropping at Campiglia (Fig. 1) has a K/Ar cooling age of 5.7 Ma and, according to the mineral assemblages, was emplaced at a depth f 1 kbar (Barberi et al., 1967). The Gavorrano monzogranite (Fig. 1) has K/Ar and Rb/Sr cooling ages of 4.9 and 4.4 Ma, respectively (Borsi et al., 1967; Ferrara and Tonarini, 1985). No data exist on its depth of crystallization, due also to the fact that faulting disrupted the original contacts with the host rocks. Both at Campiglia and Gavorrano, it has been recognized that pluton ascent occurred in correspondence with releasing bends formed along N – S dextral faults (Fig. 3; Acocella et al., 2000; Rossetti et al., 2001). At Campiglia, a subhorizontal schistosity in the thermal aureole, acquired during a flattening-type deformation, is folded forming a dome bordered by a rim synform, with an axis parallel to the main fault trace (Fig. 3). This feature is interpreted as due to the ascent of the cooling intrusion, suggesting roof lifting as the main mechanism of emplacement (Acocella et al., 2000).
Fig. 2. Schematic cartoon showing the tectonic framework (from ductile to brittle) affecting the intrusions in offshore Tuscany. Even though field examples are from the Giglio intrusion, solid-state, eastward-directed extensional shearing is documented throughout all the intrusions (Rossetti et al., 2000). (a) Milonitic fabric from the roof of the intrusion with fractured feldspar in a quartzitic matrix; shear sense sinistral. (b) Spaced S – C foliation at the border of the intrusion; shear sense dextral (c) Eastward-directed, low-angle extensional faults cutting across the cooled intrusion.
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Fig. 3. Block diagram showing the ascent of the Campiglia pluton within the releasing bend induced by dextral faults at the Miocene – Pliocene boundary and the roof uplift induced by pluton emplacement. Inset A shows N – S-trending fracture cleavage associated with dextral fault strands in the thermal aureole; the granite lies few hundreds of meters at depth. Inset B shows the domed roof of the intrusion within the hostrock. The subhorizontal schistosity and boudinage induced by the flattening due to the emplacement of the granite is also shown.
The Amiata and Larderello areas show gravity minima, positive heat flow, uplift of hundreds of meters of early Pliocene marine deposits and the culmination of the ‘‘K-horizon’’ seismic reflector, interpreted as the present brittle – ductile transition (Pasquare´ et al., 1983; Cameli et al., 1993; Marinelli et al., 1993; Baldi et al., 1994; Carmignani et al., 1994; Mongelli et al., 1998; Gianelli et al., 1997; Liotta and Ranalli, 1999). These features have been interpreted as due to the emplacement, up to 5 km deep, of buried granitoids since early Pliocene (Del Moro et al., 1982; Barberi et al., 1994; Gianelli et al.,
1997; Manzella et al., 1998; Mongelli et al., 1998). At Amiata, petrologic data from the host rock around the pluton and from the xenoliths in the erupted lavas suggest emplacement at a depth of 1 – 1.7 kbar (Gianelli et al., 1988). At Larderello, the mineral assemblages suggest a depth of crystallization of 1 kbar (Del Moro et al., 1982). The space required for the emplacement of the Amiata pluton (Fig. 1) was mainly accommodated by roof lifting, as shown by the domed attitude of the pre-uplift Pliocene marine deposits (Acocella, 2000) (Fig. 4). The NE – SW elongation of the pluton and its roots, shown by
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Fig. 4. Crustal section showing the shape of the Amiata pluton (from the interpretation of gravity data) and the roof uplift induced by its emplacement (from field data). Inset shows a portion of the dome, formed as a consequence of pluton emplacement, as revealed by the attitude of the Pliocene marine deposits.
seismic lines, gravity data, heat flow data and the elongation of the uplifted overburden (Calamai et al., 1970; Pasquare´ et al., 1983; Gianelli et al., 1988; Orlando et al., 1994; Gianelli et al., 1997) is parallel to the NE –SW structures (fractures and faults) found in the area (Fig. 1). Despite the large amount of both surface and subsurface data and modelling, no mechanisms of emplacement have been proposed so far for the Larderello intrusion. The intrusion induced a broad uplift (>600 m) of Pliocene deposits, as shown by surface (Baldi et al., 1994; Bossio et al., 1995) and seismic data (Pascucci et al., 1999). Even though a link between extensional shear and the rise of the intrusion has been suggested (Cameli et al., 1993; Dallmeyer and Liotta, 1998), seismic lines, gravity and heat flow data point out N – S and NE – SW elongations for significant portions of the Larderello pluton (Batini et al., 1983; Gianelli et al., 1997; Mongelli et al., 1998). These trends are parallel to the N – S and NE – SW structures (fractures and faults) found in the area (Fig. 1).
4. Magmatic rates vs. tectonic rates in the brittle crust We try to estimate the magmatic and tectonic strain rates occurred during early Pliocene in Tuscany. Based on the previously calculated velocity ( f 2 cm/year) of migration of the extensional axis in the inner part of the northern Apennines (Faccenna et al., 1997 and references therein), a regional tectonic strain rate e˙t f 10 15 s 1 is estimated. Calculating a magmatic strain rate is more challenging and several assumptions have to be made. Amiata is probably the best pluton for these calculations, as its overall geometry is relatively known: the pluton has a laccolith shape and is fed by a NE –SWtrending conduit f 10 km long (Gianelli et al., 1988, 1997; Orlando et al., 1994; Acocella, 2000) (Fig. 4). We assume a density contrast with the host rock Dq = 400 kg m 3 (Orlando et al., 1994), a mean granite effective viscosity l = 108 Pa s (Talbot, 1999), g (acceleration due to gravity) = 9.8 m s 2, a length of the feeder conduit L = 104 m and a width w = 101 m.
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The latter value is purely indicative and is based on the fact that the mean aspect ratio (L/w) of large feeder dikes on earth is 103 – 104 (Ernst et al., 1995). These values permit to calculate the rate of pluton filling Q (Cruden, 1998) as: Q ¼ gLw3 Dq=12l
ð1Þ
From Eq. (1), Q is f 33 m3 s 1 at Amiata. Given the possible volume (v f 2000 km3; Acocella, 2000) of the intrusion, the pluton infilling time is t f 2000 years. The strain rates required to emplace the 2.5km-thick tabular intrusion at Amiata in 2000 years are ˙ f 10 11 s 1. This value is the same as the one em ˙ = 10 11 s 1) we obtain applying numerical mod(em els (Fig. 7b in Cruden, 1998), based on the infilling rate and assuming a radius for the tabular intrusion r f 15 km (Acocella, 2000 and references therein). These calculations are consistent with the magmatic strain rates deduced from cores of deep wells at Larderello and Amiata. In these studies, the fact that hydrothermal quartz in the aureole of the plutons is associated to fluid inclusions showing a temperature of 550 jC is interpreted as due to high (10 10 – 10 12 s 1) strain rates related to pluton emplacement (Gianelli, 1994). We do not have any information on the magmatic strain rates associated with the emplacement of the other plutons in Tuscany. Assuming the magmatic strain rate values for Amiata and Larderello as representative for pluton emplacement above the regional brittle – ductile transition (we refer to the one active during the Miocene– Pliocene boundary), it appears that the magmatic strain rates in the brittle crust of Tuscany are larger than the tectonic ones.
5. Discussion The depicted scenario allows us to outline the role of the extensional structures on the mechanisms of magma ascent and emplacement at different crustal levels. The deeper crystallized (offshore) intrusions give insights on the role of extensional shear zones on (i) melt segregation and magma ascent within the ductile crust and (ii) pluton emplacement at the Miocene – Pliocene brittle– ductile transition. The absence
of the K-horizon (present brittle –ductile transition) in the offshore domain indicates that extension re-equilibrated the previous crustal portion, driving exhumation of the deeper crustal levels. The shallower crystallized (onshore) intrusions give insights on the ascent and emplacement of the plutons in the brittle crust. Here, the K-horizon is steeply rising in correspondence with the geothermal areas of Amiata and Larderello, confirming that shallower crustal levels are exposed. The deeper crystallized off-shore granites (Montecristo, Giglio and Elba) rose within an extensional context, characterized by extensional shear zones active at the regional brittle– ductile transition. This suggests that decompression induced by the shear zones enhanced melting at deeper crustal levels and the segregation and ascent of the Montecristo, Giglio and Elba granitic melts in the ductile crust. We have no clear indication for the mechanism (diapirism or fracturing) of ascent through the ductile crust, but we can put constraints on the emplacement of the plutons. The magmatic concentric foliation pattern in the offshore plutons can be interpreted as due to the stopping of the ascending plutons. In addition, the increasing solid-state character of the foliation towards the pluton boundaries suggests that the cooling plutons behaved as expanding, solid-state diapiric bodies within more resistant host rock (England, 1990; Weinberg and Podladchikov, 1994). Eastwarddirected extensional shearing then affected the intrusions as solid-state bodies. The fact that the eastward-directed shear occurred at low-grade conditions suggests that shearing was active (at the brittle– ductile transition) at the final cooling of the plutons. Two major factors contributed to the emplacement of the plutons at the brittle– ductile transition: (i) the change in rheology between the ductile and the brittle crust and (ii) the localised noncoaxial shear deformation. (i) At the brittle –ductile transition, in fact, the differential stress required for flow at a given strain rate increases dramatically (e.g., Ranalli, 1995). As a result, the further rise of magma by density contrast alone is highly inhibited, unless the brittle crust is fractured (Vigneresse, 1995b). (ii) Eastward-directed extensional shearing localised the emplacement of the intrusions, as the noncoaxial fabric of the shear deformation operated in catalysing deformation (e.g., Passchier and Trouw, 1998). The Miocene –Pliocene brittle– ductile transition thus acted as a major boun-
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dary for the plutons, as also confirmed by the fact that their final exhumation (mainly controlled by normal faulting) occurred later, after their complete cooling. The onshore granites (Campiglia, Gavorrano, Amiata and Larderello) were emplaced at shallower crustal levels, in a brittle-extending upper crust. The releasing bends formed by the subvertical N – S dextral faults constitute the pathways for the ascent of granitic magma in the brittle crust (Campiglia and Gavorrano). Field data show that, where the original contacts with the host rock are preserved (Campiglia), roof uplift accommodates the space required for pluton emplacement. At Amiata, pluton emplacement occurred through roof lifting (Acocella, 2000); a similar mechanism is likely to have occurred also at Larderello, as suggested by the significant (>600 m) uplift of the Pliocene marine deposits in the area. The ascent of granitic magma at Amiata and Larderello possibly occurred through NE – SW-trending structures, as suggested by the shape of the pluton and of its roots (Calamai et al., 1970; Batini et al., 1983; Pasquare´ et al., 1983; Gianelli et al., 1988, 1997; Orlando et al., 1994; Mongelli et al., 1998). These may have constituted long-lasting pathways for magma rise, as at Larderello, where anomalous geothermal activity has been occurring in a nonrotational setting in the last 4 Ma (Del Moro et al., 1982; Mattei et al., 1996). The interpretation of the NE – SW structures possibly associated with the Amiata and Larderello plutons is not straightforward in such an extensional framework. We interpret these as subvertical transfer systems of the regional normal faults, accommodating portions with different extension modalities (i.e. Liotta, 1991). Independently from the interpretation, it appears that the N – S and NE – SW steeply dipping fracture systems constitute a preferred pathway for the ascent of granitic magmas in the extending brittle crust in southern Tuscany. As a consequence, pluton ascent and emplacement at shallow levels in Tuscany were not directly controlled by the activity of the normal faults responsible for regional extension. The review of the ascent and emplacement mechanisms at deeper and shallower crustal levels in southern Tuscany thus suggests the following points. (i) The activity of the extensional shear zones in correspondence with the regional brittle – ductile transition constitutes an efficient mechanism for the
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segregation and rise of granitic magmas in the ductile crust. The granites (Montecristo, Giglio and Elba) cooled in correspondence with the regional brittle – ductile transition, which acts as a rheological boundary during their ascent. (ii) Above the regional brittle –ductile transition, pluton ascent occurs along subvertical faults, consistent with regional extension, but not along the normal faults responsible for regional extension. In fact, as suggested by the lack of intrusions within the normal faults, these structures did not provide significant pathways for magma ascent. This suggests the inability of these extensional structures to play a major role for the transfer of magma in the brittle crust. (iii) Roof uplift is the main mechanism to accommodate the space required for pluton emplacement in the brittle crust, as shown at Amiata and Campiglia and supposed at Larderello. The reviewed data thus permit to propose a model for the ascent and emplacement of granites in an extending crust, consisting of concurrent shear processes active at different crustal levels (Fig. 5). Decompression triggered by ductile extension is an efficient mechanism for the segregation and rise of granites in the ductile crust (Fig. 5a). Most of the granitic melts stop their ascent at the regional brittle– ductile transition, which constitutes a major rheological boundary (Fig. 5b). In the brittle crust, the further ascent of granitic melts occurs through subvertical fractures and not along normal faults. Localised opening (i.e. releasing bends) along the deep-reaching subvertical fractures provides a preferred pathway for magma ascent in the brittle crust (Fig. 5c). Granites then mainly emplace by lifting the overburden, solving the space problem at the shallowest levels (Fig. 5c). As a result of the prolonged activity of the extensional regime, the deep-seated plutons are later exhumed as cold bodies. Fig. 5d shows a rheological profile of an ideal extending crust with a high ( f 40j/km) geothermal gradient and summarizes, to the right, the fields of compatibility for the possible processes controlling pluton ascent and emplacement at different crustal levels. The fact that the calculated magmatic strain rates in the upper crust (Larderello and Amiata) are larger than the tectonic ones implies that, analogously to what was pointed out in other settings (Petford et al., 2000; de Saint Blanquat et al., 2001), the ascent and the emplacement of the plutons cannot be adequately
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Fig. 5. Schematic model (not to scale) proposed for granite rise and emplacement in southern Tuscany (Miocene – Pliocene boundary). (a) The decompression induced by the activity of low-angle extensional shear zones enhances the rise of granites in the ductile crust. (b) The granites resided in correspondence with the regional brittle – ductile transition and are locally sheared. (c) The ascent of further magma above the regional brittle – ductile transition occurs along subvertical fractures compatible with the regional extension direction. Roof uplift accommodates the required space for pluton emplacement at shallower levels. (d) Schematic rheological profile of an ideal extending crust with a high ( f 40j/km) geothermal gradient (modified after Vigneresse, 1995b), showing the increase in the stress required to flow at the brittle – ductile transition. To the right, the fields relative to the possible processes controlling pluton ascent and emplacement at different crustal levels are schematically reported.
controlled by tectonics in Tuscany. Magma intrusion in the brittle crust, even if mainly occurring through preexisting subvertical fractures, may, in its turn, further enhance fracture propagation consistently with the regional extension, as observed in various other settings; thus, magmatic and tectonic processes are linked in a feedback mechanism, which enhances the rise of magma (De Saint Blanquat et al., 1998). At shallower levels, the fact that the magmatic strain rates are larger than the tectonic ones gives a possible explanation for the emplacement of the plutons in the upper crust. In fact, the space required for pluton emplacement is
mainly accommodated here by nontectonic processes, such as roof lifting (Amiata, Campiglia and, possibly, Larderello).
6. Conclusions The case of Tuscany shows the different role of the structures responsible for regional extension on pluton ascent and emplacement at different crustal levels. In the ductile crust, the extensional structures provide a means for the ascent of granites, but constitute a main
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barrier (at the regional brittle – ductile transition) for pluton ascent. In the brittle crust, the normal faults are not efficient alone to accommodate the space required for pluton ascent and emplacement. Here, the ascent of granitic magmas mainly occurs through subvertical fractures, consistent with the regional extension. Roof uplift, observed at Amiata, Campiglia and inferred at Larderello, accommodates the space required for pluton emplacement in the upper crust. The fact that the magmatic strain rates calculated for Larderello and Amiata are much larger than the tectonic ones explains how nontectonic processes (roof uplift) control pluton emplacement in the brittle crust. The prolonged activity of the extensional structures is finally responsible for the exhumation of the deepseated plutons as cold bodies.
Acknowledgements A significant part of this study was conducted in the frame of a PhD grant (to VA) at the University of Siena (Italy). The authors wish to thank R. Funiciello and A. Lazzarotto for their encouragement and C. Faccenna for helpful discussions. J.L Vigneresse provided useful comments to an earlier version of the manuscript. The reviewers M. De Saint Blanquat and K. McCaffrey provided helpful and thoughtful revisions. This work was supported by C.N.R. funds (Responsible Prof. A. Lazzarotto) and 40% Murst funds (Responsible R. Funiciello).
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The role of extensional tectonics at different crustal levels on granite ascent and emplacement: an example from Tuscany (Italy) V. Acocella*, F. Rossetti Dipartimento di Scienze Geologiche, Universita` Roma TRE, Largo S.L. Murialdo, 1, 00146 Rome, Italy Received 27 November 2001; accepted 24 May 2002
Abstract The role of regional extension on the rise and emplacement of granites in the crust is still debated. Pluton ascent and emplacement widely occurred in Tuscany (Italy) since late Miocene during the post-orogenic collapse of the inner Apennines, and are presently occurring in the geothermal areas of Amiata and Larderello. Tuscany offers a preferred test site to study the role of regional extension on pluton ascent and emplacement at different crustal levels. Ductile extension enhanced the segregation and ascent of granitic melts in the lower crust, controlling pluton emplacement in correspondence with the brittle – ductile transition. In the brittle crust, magma ascent occurred through subvertical faults and fractures compatible with the regional extension direction; pluton emplacement mainly occurred by means of roof lifting. The case of Tuscany suggests that the extensional structures enhance melt segregation and ascent in the ductile crust, but are not efficient alone to provide a pathway for the ascent of granitic magmas in the brittle-extending crust. The estimated magmatic strain rates due to pluton emplacement in the geothermal areas are much larger than the regional tectonic strain rates. This suggests that regional tectonics did not control magma emplacement in the brittle crust and explains why nontectonic processes (roof lifting) accommodated the space required for pluton emplacement. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Magma transport; Pluton emplacement; Regional extension; Tuscany
1. Introduction The role of regional extension in the ascent and emplacement of granites has been a matter of study and debate. Extension has been commonly solicited as an efficient environment for the ascent of granitic melts at mid-crustal levels. Known examples include the Basin and Range (Gans et al., 1989; Wernicke et al., 1987; Davis et al., 1993), Sierra Nevada (Tobisch *
Corresponding author. Tel.: +39-06-54888027; fax: +39-0654888201. E-mail address: [email protected] (V. Acocella).
et al., 1993), Papua New Guinea (Hill et al., 1995), Greenland (Strachan et al., 2001), Yemen (Geoffrey et al., 1998), the Cyclades (e.g., Gautier et al., 1993) and the Tyrrhenian Sea (Jolivet et al., 1998). In these cases, the activity of low-angle ductile extensional shear zones enhanced decompression and rise of the isotherms, inducing the ascent of granites by removing the load on the roof (e.g., Lister and Davis, 1989). In the upper crust, one (to our knowledge) case highlighted the control of extensional structures on granite emplacement (Greenland; Hutton et al., 1990); such control is, however, debated (Hutton et al., 2000), as it has been suggested that the granite
0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 0 2 ) 0 0 2 9 0 - 1
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emplaced through roof uplift and floor depression, rather than the activity of extensional structures (Grocott et al., 1999). Regional extensional structures (including high- and low-angle normal faults) in the upper crust do not seem to play an important role in solving the ‘‘space problem’’, or the creation of the space required to accommodate granite emplacement (e.g., Hutton, 1988a; Tikoff et al., 1999). The inability for regional extensional structures to accommodate pluton emplacement at shallow levels applies, in particular, to large plutons and batholiths (Paterson and Fowler, 1993; Hanson and Glazner, 1995). Rather, granite emplacement at shallow crustal levels has been often related to the activity of strike-slip faults, mainly due to strain partitioning processes in oblique convergence settings (Busby-Spera and Saleeby, 1990; Glazner, 1991; Tikoff and Teyssier, 1992; Tobisch and Cruden, 1995; Tikoff and de Saint Blanquat, 1997; de Saint Blanquat et al., 1998; Wilson and Grocott, 1999). In such a tectonic framework, the emplacement of granites along localised zones (such as relay zones, pull-apart structures or tension gashes) is the most common mechanism (e.g., Guinebertau et al., 1987; Hutton, 1988b; Schmidt et al., 1990; D’Lemos et al., 1992; McCaffrey, 1992; Morand, 1992; Moreau et al., 1994; Vigneresse, 1995a; Castro and Fernandez, 1998). All these studies suggest that regional extensional structures may control the ascent of the granitic magma, but do not control the emplacement of the plutons at shallower crustal levels. In this work, we aim to test this hypothesis, investigating the role played by extensional structures on the ascent and emplacement of the Mio – Pliocene granites of the Tuscan Magmatic Province of Italy. Tuscany provides the opportunity to study the ascent and the intrusion of granites at different structural levels within an extending crust as a consequence of the collapse of the inner Apennine chain. The mechanisms of pluton emplacement in Tuscany have been discussed in Rossetti et al. (2000) and, subordinately, in Acocella (2000). Based on these data, in this work, we first review the modalities of pluton ascent and emplacement, and then we propose a new interpretation for the role of regional extensional structures on the ascent and emplacement of the granites at different crustal levels.
2. Regional extensional tectonics in southern Tuscany Crustal thickening occurred in Tuscany at the Oligocene – Miocene boundary (Brunet et al., 2000) during the development of the northern Apennine orogenic wedge (Jolivet et al., 1998 and references therein). The post-thickening evolution was characterized by widespread extension due to the collapse of the Apennines (Carmignani et al., 1994; Jolivet et al., 1998). Extension occurred at the back of the eastward-migrating Apennine orogen, as a consequence of the progressive eastward shifting of the Apennine subduction (Jolivet et al., 1998 and references therein). In the northern Tyrrhenian area (Fig. 1), extensional tectonics developed since early Miocene in Alpine Corsica (Jolivet et al., 1990), progressively shifting eastward with an overall E –W direction of extension (Carmignani et al., 1995). Extension resulted in the development of shallowly eastward-dipping ductile extensional shear zones. These, active at the brittle– ductile transition, initially accompanied the nearly isothermal exhumation of the deep-seated metamorphic rocks and, successively (late Miocene – Pliocene), magmatic activity (Jolivet et al., 1998 and references therein) (Fig. 1). From late Miocene to late Pliocene, N – S to NW – SE normal faults were active in southern Tuscany, bordering several extensional basins (e.g., Carmignani et al., 1994; Pascucci et al., 1999) in a nonrotational setting (Mattei et al., 1996) (Fig. 1). NE – SW strike-slip faults interrupt the continuity of adjacent basins throughout southern Tuscany (Fig. 1). These faults, being coeval and kinematically compatible with the NW – SE normal faults, have been interpreted as transfer systems (Liotta, 1991). Moreover, N – S-trending dextral fault segments along the coastline of southern Tuscany have been recognized as active features during pluton emplacement (Acocella et al., 2000; Rossetti et al., 2001) (Fig. 1). These faults, being coeval and kinematically consistent with the E – W to WNW – ESE regional extension direction, have been interpreted as accommodating strain compatibility in the brittle crust during crustal-scale extensional simple shearing at depth (Rossetti et al., 2000).
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Fig. 1. Schematic map of the main structural and magmatic features of southern Tuscany (Italy). Insets show: (a) an ideal E – W section through the central part of the Tuscan Magmatic Province; (b) the eastward migration of extension and related magmatism along an ideal section. The age of the intrusion at Amiata is not radiometric and is inferred from the age of the beginning of the uplift (3.5 Ma) and the presence of an active geothermal field (0 Ma) (Acocella, 2000).
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3. Magma ascent and pluton emplacement in southern Tuscany Magmatic activity accompanied extension in southern Tuscany since late Miocene, with an overall eastward migration, following the extensional deformation front (e.g., Marinelli et al., 1993; Serri et al., 1993; Jolivet et al., 1998). The Tuscan Magmatic Province (Fig. 1) is made up of granites (7.0 Ma – present) and subordinate volcanic products (5.0 –0.3 Ma), displaying a contamination with subcrustal magmas (Innocenti et al., 1992; Serri et al., 1993). Magmatic activity is responsible for a positive thermal anomaly in southern Tuscany from late Miocene to present, culminating in the active geothermal areas of Larderello and Amiata (e.g., Mongelli et al., 1998) (Fig. 1). The exposed and buried intrusive stocks in southern Tuscany have been interpreted as apophyses of a larger deep-seated batholith (Pasquare´ et al., 1983; Marinelli et al., 1993; Baldi et al., 1994; Barberi et al., 1994; Franceschini, 1994; Bossio et al., 1995; Gianelli et al., 1997). We now review the data relative to the ascent and emplacement of the plutons outcropping in southern Tuscany, partly presented in Rossetti et al. (2000) and Acocella (2000). A main distinction is made here, grouping the plutons accordingly to their different crystallization depths: (i) greater than 2 kbar: Montecristo, Giglio, Monte Capanne and
Porto Azzurro of Elba island (off-shore granites); (ii) less than 2 kbar: Campiglia, Gavorrano, Amiata and Larderello (on-shore granites). Table 1 summarizes the published data concerning the conditions for pluton emplacement and the associated tectonic features. (i) The Montecristo monzogranite (Fig. 1) has a Rb/Sr cooling age of 7.0– 7.3 Ma (Ferrara and Tonarini, 1985). Using the ternary plot of normative composition (quartz, albite and orthoclase, recalculated to 100%), a pressure of crystallization starting at about 5 kbar was calculated (Innocenti et al., 1997). The Giglio monzogranite (Fig. 1) has a Rb/Sr cooling age of 5.0 Ma; according to the mineral assemblages (ternary plot of normative quartz, albite and orthoclase), a minimum crystallization pressure of approximately 4 kbar was calculated (Westerman et al., 1993). The Elba monzogranites (Fig. 1) have a cooling age between 6.8 and 5.5 Ma (K/Ar and Rb/Sr methods: Ferrara and Tonarini, 1985); thermobarometric data in the aureole of Porto Azzurro pluton indicate an emplacement depth slightly larger than 2 kbar (Duranti et al., 1992). All these plutons preserve magmatic concentric foliation, with solid-state recrystallisation processes increasing towards their boundaries; such a fabric developed as a consequence of a flattening-type deformation (Boccaletti and Papini, 1989; Burrelli and Papini, 1992; Westerman et al., 1993; Rossetti et al., 1999). Top-to-the-east exten-
Table 1 Main features associated with the emplacement of the monzogranite intrusions of southern Tuscany Pluton
Location
Minimum age (Ma)
Emplacement pressure (kbar)
Main structural features associated
Montecristo
off-shore
7.0 – 7.3 (1)
5 (2)
Giglio
off-shore
5.1 (3)
>4 (3)
Elba
off-shore
6.8 – 5.5 (1)
2 – 3 (5)
Campiglia Gavorrano Amiata Larderello
on-shore on-shore on-shore on-shore
5.7 (13, 14) 4.9 – 4.4 (14) 3.5 – 0 (17) 3.8 – 0 (18)
V 1 (13) ? 1 – 1.7 (17) f 1 (18)
shallow-dipping extensional shear zones (E – W extension) (3) shallow-dipping extensional shear zones (E – W extension) (6, 8, 9, 12) shallow-dipping extensional shear zones (E – W extension) (4, 7, 10, 11) N – S right-lateral faults; domed roof (15) N – S right-lateral faults (16) NE – SW structures (?); domed roof (19, 20) NE – SW structures (?); domed roof (?) (20, 21, 22, 23)
Related references: 1 = Ferrara and Tonarini, 1985; 2 = Innocenti et al., 1997; 3 = Westerman et al., 1993; 4 = Boccaletti and Papini, 1989; 5 = Duranti et al., 1992; 6 = Burrelli and Papini, 1992; 7 = Daniel and Jolivet, 1995; 8 = Rossetti et al., 1999; 9 = Rossetti et al., 1998; 10 = Keller and Pialli, 1990; 11 = Bouillin et al., 1994; 12 = Lazzarotto et al., 1964; 13 = Barberi et al., 1967; 14 = Borsi et al., 1967; 15 = Acocella et al., 2000; 16 = Rossetti et al., 2001; 17 = Gianelli et al., 1988; 18 = Del Moro et al., 1982; 19 = Acocella, 2000; 20 = Gianelli et al., 1997; 21 = Batini et al., 1983; 22 = Mongelli et al., 1998; 23 = Dallmeyer and Liotta, 1998; ? = unknown data (see text for further details).
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sional shearing is observed at the boundaries of the plutons, overprinting the previous magmatic foliation (Daniel and Jolivet, 1995; Jolivet et al., 1998; Rossetti et al., 1999) (Fig. 2). The shear zones belong to a set of low-angle, eastward-dipping extensional systems, mostly active at the depth of the regional brittle– ductile transition (Jolivet et al., 1998 and references therein). Shearing affected the intrusions during their final stage of cooling (Rossetti et al., 1999) and continued after pluton solidification, forming lowgrade mylonites and cataclasites along the boundary of the intrusions (Daniel and Jolivet, 1995; Rossetti et al., 1999) (Fig. 2). Fission tracks data on the Capanne intrusion at Elba (Bouillin et al., 1994) indicate that the final cooling and exhumation of the pluton occurred in the last 2 Ma. At Giglio island, brittle faulting controls the contact between the monzogranite and the country rocks, where no evidence of thermal metamorphism is observed (Lazzarotto et al., 1964; Rossetti et al., 1999). The Elba and Giglio examples thus indicate that the final exhumation of the plutons occurred after their cooling.
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(ii) The quartzmonzonite pluton outcropping at Campiglia (Fig. 1) has a K/Ar cooling age of 5.7 Ma and, according to the mineral assemblages, was emplaced at a depth f 1 kbar (Barberi et al., 1967). The Gavorrano monzogranite (Fig. 1) has K/Ar and Rb/Sr cooling ages of 4.9 and 4.4 Ma, respectively (Borsi et al., 1967; Ferrara and Tonarini, 1985). No data exist on its depth of crystallization, due also to the fact that faulting disrupted the original contacts with the host rocks. Both at Campiglia and Gavorrano, it has been recognized that pluton ascent occurred in correspondence with releasing bends formed along N – S dextral faults (Fig. 3; Acocella et al., 2000; Rossetti et al., 2001). At Campiglia, a subhorizontal schistosity in the thermal aureole, acquired during a flattening-type deformation, is folded forming a dome bordered by a rim synform, with an axis parallel to the main fault trace (Fig. 3). This feature is interpreted as due to the ascent of the cooling intrusion, suggesting roof lifting as the main mechanism of emplacement (Acocella et al., 2000).
Fig. 2. Schematic cartoon showing the tectonic framework (from ductile to brittle) affecting the intrusions in offshore Tuscany. Even though field examples are from the Giglio intrusion, solid-state, eastward-directed extensional shearing is documented throughout all the intrusions (Rossetti et al., 2000). (a) Milonitic fabric from the roof of the intrusion with fractured feldspar in a quartzitic matrix; shear sense sinistral. (b) Spaced S – C foliation at the border of the intrusion; shear sense dextral (c) Eastward-directed, low-angle extensional faults cutting across the cooled intrusion.
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Fig. 3. Block diagram showing the ascent of the Campiglia pluton within the releasing bend induced by dextral faults at the Miocene – Pliocene boundary and the roof uplift induced by pluton emplacement. Inset A shows N – S-trending fracture cleavage associated with dextral fault strands in the thermal aureole; the granite lies few hundreds of meters at depth. Inset B shows the domed roof of the intrusion within the hostrock. The subhorizontal schistosity and boudinage induced by the flattening due to the emplacement of the granite is also shown.
The Amiata and Larderello areas show gravity minima, positive heat flow, uplift of hundreds of meters of early Pliocene marine deposits and the culmination of the ‘‘K-horizon’’ seismic reflector, interpreted as the present brittle – ductile transition (Pasquare´ et al., 1983; Cameli et al., 1993; Marinelli et al., 1993; Baldi et al., 1994; Carmignani et al., 1994; Mongelli et al., 1998; Gianelli et al., 1997; Liotta and Ranalli, 1999). These features have been interpreted as due to the emplacement, up to 5 km deep, of buried granitoids since early Pliocene (Del Moro et al., 1982; Barberi et al., 1994; Gianelli et al.,
1997; Manzella et al., 1998; Mongelli et al., 1998). At Amiata, petrologic data from the host rock around the pluton and from the xenoliths in the erupted lavas suggest emplacement at a depth of 1 – 1.7 kbar (Gianelli et al., 1988). At Larderello, the mineral assemblages suggest a depth of crystallization of 1 kbar (Del Moro et al., 1982). The space required for the emplacement of the Amiata pluton (Fig. 1) was mainly accommodated by roof lifting, as shown by the domed attitude of the pre-uplift Pliocene marine deposits (Acocella, 2000) (Fig. 4). The NE – SW elongation of the pluton and its roots, shown by
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Fig. 4. Crustal section showing the shape of the Amiata pluton (from the interpretation of gravity data) and the roof uplift induced by its emplacement (from field data). Inset shows a portion of the dome, formed as a consequence of pluton emplacement, as revealed by the attitude of the Pliocene marine deposits.
seismic lines, gravity data, heat flow data and the elongation of the uplifted overburden (Calamai et al., 1970; Pasquare´ et al., 1983; Gianelli et al., 1988; Orlando et al., 1994; Gianelli et al., 1997) is parallel to the NE –SW structures (fractures and faults) found in the area (Fig. 1). Despite the large amount of both surface and subsurface data and modelling, no mechanisms of emplacement have been proposed so far for the Larderello intrusion. The intrusion induced a broad uplift (>600 m) of Pliocene deposits, as shown by surface (Baldi et al., 1994; Bossio et al., 1995) and seismic data (Pascucci et al., 1999). Even though a link between extensional shear and the rise of the intrusion has been suggested (Cameli et al., 1993; Dallmeyer and Liotta, 1998), seismic lines, gravity and heat flow data point out N – S and NE – SW elongations for significant portions of the Larderello pluton (Batini et al., 1983; Gianelli et al., 1997; Mongelli et al., 1998). These trends are parallel to the N – S and NE – SW structures (fractures and faults) found in the area (Fig. 1).
4. Magmatic rates vs. tectonic rates in the brittle crust We try to estimate the magmatic and tectonic strain rates occurred during early Pliocene in Tuscany. Based on the previously calculated velocity ( f 2 cm/year) of migration of the extensional axis in the inner part of the northern Apennines (Faccenna et al., 1997 and references therein), a regional tectonic strain rate e˙t f 10 15 s 1 is estimated. Calculating a magmatic strain rate is more challenging and several assumptions have to be made. Amiata is probably the best pluton for these calculations, as its overall geometry is relatively known: the pluton has a laccolith shape and is fed by a NE –SWtrending conduit f 10 km long (Gianelli et al., 1988, 1997; Orlando et al., 1994; Acocella, 2000) (Fig. 4). We assume a density contrast with the host rock Dq = 400 kg m 3 (Orlando et al., 1994), a mean granite effective viscosity l = 108 Pa s (Talbot, 1999), g (acceleration due to gravity) = 9.8 m s 2, a length of the feeder conduit L = 104 m and a width w = 101 m.
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The latter value is purely indicative and is based on the fact that the mean aspect ratio (L/w) of large feeder dikes on earth is 103 – 104 (Ernst et al., 1995). These values permit to calculate the rate of pluton filling Q (Cruden, 1998) as: Q ¼ gLw3 Dq=12l
ð1Þ
From Eq. (1), Q is f 33 m3 s 1 at Amiata. Given the possible volume (v f 2000 km3; Acocella, 2000) of the intrusion, the pluton infilling time is t f 2000 years. The strain rates required to emplace the 2.5km-thick tabular intrusion at Amiata in 2000 years are ˙ f 10 11 s 1. This value is the same as the one em ˙ = 10 11 s 1) we obtain applying numerical mod(em els (Fig. 7b in Cruden, 1998), based on the infilling rate and assuming a radius for the tabular intrusion r f 15 km (Acocella, 2000 and references therein). These calculations are consistent with the magmatic strain rates deduced from cores of deep wells at Larderello and Amiata. In these studies, the fact that hydrothermal quartz in the aureole of the plutons is associated to fluid inclusions showing a temperature of 550 jC is interpreted as due to high (10 10 – 10 12 s 1) strain rates related to pluton emplacement (Gianelli, 1994). We do not have any information on the magmatic strain rates associated with the emplacement of the other plutons in Tuscany. Assuming the magmatic strain rate values for Amiata and Larderello as representative for pluton emplacement above the regional brittle – ductile transition (we refer to the one active during the Miocene– Pliocene boundary), it appears that the magmatic strain rates in the brittle crust of Tuscany are larger than the tectonic ones.
5. Discussion The depicted scenario allows us to outline the role of the extensional structures on the mechanisms of magma ascent and emplacement at different crustal levels. The deeper crystallized (offshore) intrusions give insights on the role of extensional shear zones on (i) melt segregation and magma ascent within the ductile crust and (ii) pluton emplacement at the Miocene – Pliocene brittle– ductile transition. The absence
of the K-horizon (present brittle –ductile transition) in the offshore domain indicates that extension re-equilibrated the previous crustal portion, driving exhumation of the deeper crustal levels. The shallower crystallized (onshore) intrusions give insights on the ascent and emplacement of the plutons in the brittle crust. Here, the K-horizon is steeply rising in correspondence with the geothermal areas of Amiata and Larderello, confirming that shallower crustal levels are exposed. The deeper crystallized off-shore granites (Montecristo, Giglio and Elba) rose within an extensional context, characterized by extensional shear zones active at the regional brittle– ductile transition. This suggests that decompression induced by the shear zones enhanced melting at deeper crustal levels and the segregation and ascent of the Montecristo, Giglio and Elba granitic melts in the ductile crust. We have no clear indication for the mechanism (diapirism or fracturing) of ascent through the ductile crust, but we can put constraints on the emplacement of the plutons. The magmatic concentric foliation pattern in the offshore plutons can be interpreted as due to the stopping of the ascending plutons. In addition, the increasing solid-state character of the foliation towards the pluton boundaries suggests that the cooling plutons behaved as expanding, solid-state diapiric bodies within more resistant host rock (England, 1990; Weinberg and Podladchikov, 1994). Eastwarddirected extensional shearing then affected the intrusions as solid-state bodies. The fact that the eastward-directed shear occurred at low-grade conditions suggests that shearing was active (at the brittle– ductile transition) at the final cooling of the plutons. Two major factors contributed to the emplacement of the plutons at the brittle– ductile transition: (i) the change in rheology between the ductile and the brittle crust and (ii) the localised noncoaxial shear deformation. (i) At the brittle –ductile transition, in fact, the differential stress required for flow at a given strain rate increases dramatically (e.g., Ranalli, 1995). As a result, the further rise of magma by density contrast alone is highly inhibited, unless the brittle crust is fractured (Vigneresse, 1995b). (ii) Eastward-directed extensional shearing localised the emplacement of the intrusions, as the noncoaxial fabric of the shear deformation operated in catalysing deformation (e.g., Passchier and Trouw, 1998). The Miocene –Pliocene brittle– ductile transition thus acted as a major boun-
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dary for the plutons, as also confirmed by the fact that their final exhumation (mainly controlled by normal faulting) occurred later, after their complete cooling. The onshore granites (Campiglia, Gavorrano, Amiata and Larderello) were emplaced at shallower crustal levels, in a brittle-extending upper crust. The releasing bends formed by the subvertical N – S dextral faults constitute the pathways for the ascent of granitic magma in the brittle crust (Campiglia and Gavorrano). Field data show that, where the original contacts with the host rock are preserved (Campiglia), roof uplift accommodates the space required for pluton emplacement. At Amiata, pluton emplacement occurred through roof lifting (Acocella, 2000); a similar mechanism is likely to have occurred also at Larderello, as suggested by the significant (>600 m) uplift of the Pliocene marine deposits in the area. The ascent of granitic magma at Amiata and Larderello possibly occurred through NE – SW-trending structures, as suggested by the shape of the pluton and of its roots (Calamai et al., 1970; Batini et al., 1983; Pasquare´ et al., 1983; Gianelli et al., 1988, 1997; Orlando et al., 1994; Mongelli et al., 1998). These may have constituted long-lasting pathways for magma rise, as at Larderello, where anomalous geothermal activity has been occurring in a nonrotational setting in the last 4 Ma (Del Moro et al., 1982; Mattei et al., 1996). The interpretation of the NE – SW structures possibly associated with the Amiata and Larderello plutons is not straightforward in such an extensional framework. We interpret these as subvertical transfer systems of the regional normal faults, accommodating portions with different extension modalities (i.e. Liotta, 1991). Independently from the interpretation, it appears that the N – S and NE – SW steeply dipping fracture systems constitute a preferred pathway for the ascent of granitic magmas in the extending brittle crust in southern Tuscany. As a consequence, pluton ascent and emplacement at shallow levels in Tuscany were not directly controlled by the activity of the normal faults responsible for regional extension. The review of the ascent and emplacement mechanisms at deeper and shallower crustal levels in southern Tuscany thus suggests the following points. (i) The activity of the extensional shear zones in correspondence with the regional brittle – ductile transition constitutes an efficient mechanism for the
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segregation and rise of granitic magmas in the ductile crust. The granites (Montecristo, Giglio and Elba) cooled in correspondence with the regional brittle – ductile transition, which acts as a rheological boundary during their ascent. (ii) Above the regional brittle –ductile transition, pluton ascent occurs along subvertical faults, consistent with regional extension, but not along the normal faults responsible for regional extension. In fact, as suggested by the lack of intrusions within the normal faults, these structures did not provide significant pathways for magma ascent. This suggests the inability of these extensional structures to play a major role for the transfer of magma in the brittle crust. (iii) Roof uplift is the main mechanism to accommodate the space required for pluton emplacement in the brittle crust, as shown at Amiata and Campiglia and supposed at Larderello. The reviewed data thus permit to propose a model for the ascent and emplacement of granites in an extending crust, consisting of concurrent shear processes active at different crustal levels (Fig. 5). Decompression triggered by ductile extension is an efficient mechanism for the segregation and rise of granites in the ductile crust (Fig. 5a). Most of the granitic melts stop their ascent at the regional brittle– ductile transition, which constitutes a major rheological boundary (Fig. 5b). In the brittle crust, the further ascent of granitic melts occurs through subvertical fractures and not along normal faults. Localised opening (i.e. releasing bends) along the deep-reaching subvertical fractures provides a preferred pathway for magma ascent in the brittle crust (Fig. 5c). Granites then mainly emplace by lifting the overburden, solving the space problem at the shallowest levels (Fig. 5c). As a result of the prolonged activity of the extensional regime, the deep-seated plutons are later exhumed as cold bodies. Fig. 5d shows a rheological profile of an ideal extending crust with a high ( f 40j/km) geothermal gradient and summarizes, to the right, the fields of compatibility for the possible processes controlling pluton ascent and emplacement at different crustal levels. The fact that the calculated magmatic strain rates in the upper crust (Larderello and Amiata) are larger than the tectonic ones implies that, analogously to what was pointed out in other settings (Petford et al., 2000; de Saint Blanquat et al., 2001), the ascent and the emplacement of the plutons cannot be adequately
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Fig. 5. Schematic model (not to scale) proposed for granite rise and emplacement in southern Tuscany (Miocene – Pliocene boundary). (a) The decompression induced by the activity of low-angle extensional shear zones enhances the rise of granites in the ductile crust. (b) The granites resided in correspondence with the regional brittle – ductile transition and are locally sheared. (c) The ascent of further magma above the regional brittle – ductile transition occurs along subvertical fractures compatible with the regional extension direction. Roof uplift accommodates the required space for pluton emplacement at shallower levels. (d) Schematic rheological profile of an ideal extending crust with a high ( f 40j/km) geothermal gradient (modified after Vigneresse, 1995b), showing the increase in the stress required to flow at the brittle – ductile transition. To the right, the fields relative to the possible processes controlling pluton ascent and emplacement at different crustal levels are schematically reported.
controlled by tectonics in Tuscany. Magma intrusion in the brittle crust, even if mainly occurring through preexisting subvertical fractures, may, in its turn, further enhance fracture propagation consistently with the regional extension, as observed in various other settings; thus, magmatic and tectonic processes are linked in a feedback mechanism, which enhances the rise of magma (De Saint Blanquat et al., 1998). At shallower levels, the fact that the magmatic strain rates are larger than the tectonic ones gives a possible explanation for the emplacement of the plutons in the upper crust. In fact, the space required for pluton emplacement is
mainly accommodated here by nontectonic processes, such as roof lifting (Amiata, Campiglia and, possibly, Larderello).
6. Conclusions The case of Tuscany shows the different role of the structures responsible for regional extension on pluton ascent and emplacement at different crustal levels. In the ductile crust, the extensional structures provide a means for the ascent of granites, but constitute a main
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barrier (at the regional brittle – ductile transition) for pluton ascent. In the brittle crust, the normal faults are not efficient alone to accommodate the space required for pluton ascent and emplacement. Here, the ascent of granitic magmas mainly occurs through subvertical fractures, consistent with the regional extension. Roof uplift, observed at Amiata, Campiglia and inferred at Larderello, accommodates the space required for pluton emplacement in the upper crust. The fact that the magmatic strain rates calculated for Larderello and Amiata are much larger than the tectonic ones explains how nontectonic processes (roof uplift) control pluton emplacement in the brittle crust. The prolonged activity of the extensional structures is finally responsible for the exhumation of the deepseated plutons as cold bodies.
Acknowledgements A significant part of this study was conducted in the frame of a PhD grant (to VA) at the University of Siena (Italy). The authors wish to thank R. Funiciello and A. Lazzarotto for their encouragement and C. Faccenna for helpful discussions. J.L Vigneresse provided useful comments to an earlier version of the manuscript. The reviewers M. De Saint Blanquat and K. McCaffrey provided helpful and thoughtful revisions. This work was supported by C.N.R. funds (Responsible Prof. A. Lazzarotto) and 40% Murst funds (Responsible R. Funiciello).
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