Fission track analysis of the crystalline basement rocks of the calabrian arc, southern italy: evidence of oligo-miocene late-orogenic extension and erosion

TEfmNqwslcs ELSEVIER

Tectonophysics 238 (1994) 331-352

Fission track analysis of the crystalline basement rocks of the Calabrian Arc, southern Italy: evidence of Oligo-Miocene late-erogenic extension and erosion Stuart N. Thomson London Fission Track Research Group, Research School of Geological and Geophysical Sciences, Birkbeck and University College London, Cower Street, London WCZE 6BT, UK

Received 28 April 1993; accepted 7 January 1994

Abstract Fission track analysis is used to provide low temperature and time constraints on the late-erogenic cooling and exhumational history of the crystalline basement rocks of the Calabrian Arc, southern Italy. 65 samples yielded 57 apatite fission track ages, 54 zircon fission track ages and 25 apatite-confined fission track length distributions. Interpretation of this data reveals a phase of increased cooling rates related to exhumation between approximately 35 Ma (mid-Oligocene) and 15 Ma (Middle Miocene). Evidence from the sedimentary record indicates significant Oligo-Miocene erosion related to this period of exhumation. New chronological constraints are also applied to localised late-erogenic extensional tectonism that has recently been identified within the basement reeks of the Calabrian Arc. This information is used to produce a new model of the Oligo-Miocene tectonic evolution of the Calabrian Arc. It proposes that the crystalline basement rocks were part of a critical erogenic wedge between the mid-Oligocene and the Middle Miocene.

1. Introduction Late-erogenic evolution of mountain belts with the subsequent exhumation of deeply buried rocks can be attributed to two main processes: denudation or erosion associated with isostatic rebound and the more recently recognised contribution of major extensional tectonism. These ideas have largely been developed by the study of pressure

and temperature changes during metamorphism, structural geology and tectonics and high-temperature isotopic age dating techniques. This contribution aims to show how the technique of fission track analysis can be used in conjunction with these other geological fields to provide important

new chronological constraints in the study of the late-erogenic evolution of mountain belts. Fission track analysis is a particularly useful tool for this purpose. The analysis of the minerals apatite and zircon can be used to chart the low-temperature cooling history of a collisional orogen at temperatures below 250°C. This means that fission track data provides information on the post-metamorphic history of the upper parts of the crustal section of an erogenic belt. This is an area with direct relevance to the study of the late-erogenic evolution of a mountain belt. The Calabrian Arc of southern Italy (Fig. 11, is an important, but much understudied ,region of the Alpine erogenic belt. Here allodhthonous

0040-1951/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0040-1951(94)00068-K

LOW.GRADE METAbSGRFRlCKXXS -

Tonaljtcs and gramdiodtes /TJ

Leucogranites

a

Cape Vatican0 pluton

-

m -

Alpine tectonic contact

?Otigo-M&me LZGIJRIDECoMpulx Flysch and colitic

mks

Burdigalian SOUTHERNAPFNNINES

EEI

D&armed Mxaoic

-

aad ryworqcnic

flysch

.,

Fig. 1. Geological sketch map and cross-section showing the main crystalline basement rocks of the Calabrian Arc (after Messina et al.. 19Ylb).

S.N. Thomson / Tectonophysics 238 (1994) 331-352

crystalline basement rocks, metamorphosed during the Alpine Orogeny, have since been exhumed and are now exposed at the surface. Recently, several metamorphic and structural studies have shown that the basement rocks of the Calabrian Arc experienced an early phase of Alpine compressional tectcnics that buried some rocks to a considerable depth, followed by a phase of significant syn- and post-erogenic exhumation. It has been proposed that the latter phase of exhumation was achieved, in part, by major, but localised extensional tectonism. However, little detailed chronological data constraining late-erogenic exhumation of the Calabrian Arc basement rocks is available. Therefore, this study, using fission track analysis, has been undertaken to provide new information on the preNeogene, low-temperature history of the basement rocks. In conjunction with a reappraisal of high-temperature isotopic age data previously obtained from the basement rocks, new low-temperature cooling histories have been constructed from a large fission track data set collected from the crystalline basement rocks of the Calabrian Arc. These have provided important new chronological constraints on a major phase of late-erogenic cooling and exhumation from the mid-Oligocene to Early Miocene. This phase of exhumation has been linked to both late-erogenic extensional tectonic and erosional processes active at this time. This information has then been used to produce a revised tectonic model for the mid-Oligocene to Early Miocene syn- and late-erogenic evolution of the Calabrian Arc.

2. Geological background The Calabrian Arc of southern Italy is a section of the Alpine erogenic belt of western Europe where allochthonous crystalline basement rocks are now exposed at the surface. These rocks form the highest tectonic unit of the Apennine mountain chain of the Italian Peninsula. They directly overlie an allochthonous “ophiolitebearing” flysch that has been linked by Knott (1987) to similar Liguride Complex rocks found in

333

the northern Apennines. Both these units have been emplaced upon deformed Mesozoic carbonates, originally of the African and Adrian plate margin. These carbonate rocks now comprise the major part of the Apennines to the north and the Sicilian and northern African Maghrebide mountain chain to the west. There are two alternative opinions regarding the tectonic origin of the crystalline basement rocks of the Calabrian Arc. Early tectonic models developed by Alvarez et al. (1974), AmodioMorelli et al. (1976) and Scandone (1982) proposed that these rocks originally formed part of the African plate margin. These models advocated early “eo-Alpine” Cretaceous to Palaeogene south-directed, north-vergent subduction linked to compression in the western Alps that emplaced the basement rocks of the Calabrian Arc upon the European plate margin. It has been proposed that this was followed by a swbduction polarity reversal, initiated during the late Palaeogene, that affected the Alpine erogenic belt to the south and west of Corsica. This resulted in the subsequent emplacement of the Calabrian Arc basement rocks on to the margin of the African and Adrian plate during the Neogene. More recently, new structural and palaeogeographical evidence has been collated by Bouillin (1984), Bouillin et al. (1986), Knott (1987) and Dietrich (1988). This has led to a now generally accepted plate reconstruction that proposes that the allochthonous crystalline basement rocks of the Calabrian Arc originated from the European plate margin (e.g. Ogniben, 1969; Dewey et al., 1989). It is argued that these rocks migrated southeast as a result of continuous northwest-directed subduction of Neotethyan oceanic lithosphere between the Cretaceous and the Neogene. According to Bouillin (1984) and Knott (1987) this northwest-directed subduction was separated from coeval southeasterly directed subduction, recognised in the western Alps, by a transform fault margin. Southeasterly migration of the basement rocks occurred in two stages. The first stage was a result of pre-Miocene subduction, This led to the opening of the Ligurian Sea basin (Guieu and Roussel, 19901, the formation of the Liguride accretionary wedge (Knott, 1987), the rotation of

334

S. N. Thomson / Tectonoph.vsics 238 (1994) 331-352

Corsica and Sardinia, and major talc-alkaline volcanism in Sardinia (Savelli et al., 1979). This phase of activity ceased during the Burdigalian, at around 18 Ma (Montigny et al., 1981). The second stage of migration was caused by the collision and emplacement of the Calabrian Arc basement rocks on to the African and Adrian plate margin during the Middle Miocene. This was followed by the formation of the Tyrrhenian Sea during the Tortonian (Kastens et al., 1988). This second stage of tectonism was thin-skinned in nature and disrupts much of the pre-Miocene geometry of the crystalline basement rocks Knott and Turco, 1991; Van Dijk and Okkes, 1991). A large part of the present-day outcrop in the Calabrian Arc is comprised of metamorphic and plutonic basement rocks with associated unmetamorphosed cover sediments. Their importance as the only major outcrop of crystalline basement rocks found in the Italian Peninsula was first noted by Lugeon and Argand in 1906. These rocks have since been the subject of numerous petrologic, metamorphic, structural and isotopic age studies. Important early syntheses were made by Ogniben (1969, 1973) and Dubois (1970) from detailed geological mapping over limited areas. The first regional synthesis was published by Amodio-Morelli et al. (1976). This study has provided a basis for much of the subsequent work on the basement rocks. Recently, Messina et al. (1991a) have shown through detailed new field investigation that many of the contacts used by Amodio-Morelli et al. (1976) to define tectonostratigraphy were the product of localised superficial Neogene tectonism. Instead Messina et al. (1991a), using new field and petrographic data, argue that many previously subdivided basement rock units from north and south Calabria are laterally equivalent. They show that the crystalline basement rocks can be subdivided into four main rock groups. These will be labelled in this study as the Alpine Group rocks, the highgrade Hercynian metamorphic rocks, the Hercynian plutonic rocks and the low-grade Hercynian metamorphic rocks. These units are illustrated in a geological sketch map and section in Fig. 1, based on the map produced by Messina et al. (1991a, b).

The lowest tectono-stratigraphic rocks, the Alpine Group, comprise two sub-units. The lowermost of these is the Bagni Unit. It is largely composed of Hercynian phyllites that have undergone post-mid-Cretaceous Alpine retrograde lower greenschist facies metamorphism with rare higher-pressure lawsonite (De Roever, 1972; Dietrich, 1988). The upper Castagna Unit consists largely of Hercynian amphibolite facies gneisses and granites that have been intensely deformed and metamorphosed during later Alpine orogenesis. The Alpine retrograde metamorphism was of greenschist facies with HP/LT glaucophane and lawsonite found in the lower parts of the Castagna Unit (Dubois, 1966; De Roever, 1972; Dietrich et al., 1976). Muscovite b, values obtained by Dietrich et al. (1976) from the Alpine Group rock units indicate that pressure and temperature conditions in excess of 3 kbar and 350°C (Guidotti and Sassi, 1986) occurred during Alpine retrograde metamorphism. Structural analyses of the Alpine Group rocks, largely obtained from the Castagna Unit, show that the Alpine retrograde metamorphism is strongly associated with east-west-stretching lineations (De Roever, 1972; Carrara and Zuffa, 1976; Faure, 1980; Dietrich, 1988). The majority of kinematic indicators indicate an east-directed shear sense, although Faure (1980) also shows evidence of west-directed shear. Tectonically above the Alpine Group rocks are a group of rocks, largely unaffected by Alpine metamorphism, that are known collectively as the Hercynian Group. This group comprises a lowermost high-grade metamorphic sequence and an uppermost group of low- to medium-grade metamorphic rocks with associated unmetamorphosed Mesozoic cover sediments. The contact between these two Hercynian rock groups is a thrust contact of Hercynian age, that has been examined in the Aspromonte of southern Calabria &orenzoni et al., 1980). These rocks are intruded by widespread late Hercynian granitic plutons that tend to separate the high-grade and low-grade Hercynian metamorphic rocks. The high-grade Hercynian metamorphic rocks are largely made up of amphibolite to granulite

S. N. Thomson / Tectonophysics 238 (1994) 331-352

facies meta-sedimentary and meta-igneous rocks (Schenk, 1984). In the Catena Costiera and northwestern Sila the lowermost part of the sequence shows evidence of a low-grade Alpine pumpellyite-actinolite facies metamorphic overprint (De Roever, 1972; Dietrich et al., 1976). Pumpellyite-actinolite facies metamorphism is indicative of palaeotemperatures in excess of 250°C (Yardley, 1989). The nature of the tectonic contact between the Alpine Group rocks and the high-grade Hercynian metamorphic rocks is problematical. De Roever (19721, Schenk (1980) and ZanettinLorenzoni (1982), from field evidence in various parts of the Calabrian Arc, indicate that it is a cataclastic thrust that cuts earlier mylonitic gneisses. However, the contact separates rocks that show an Alpine greenschist facies metamorphic overprint below, from rocks that were largely unaffected by an Alpine metamorphic overprint above. This would imply that some post-metamorphic extensional movement has occurred along this tectonic contact. Work by Platt and Compagnoni (1990) has revealed evidence for Alpine north-directed extensional shear within mylonitised parts of the high-grade Her&an metamorphic rocks of the Aspromonte region of southern Calabria. These shear zones juxtapose rocks showing retrograde Alpine greenschist faties metamorphism against rocks with no evidence of an Alpine metamorphic overprint. Similar metamorphic offsets implying Alpine regional extension have also been recognised in the Liguride Complex rocks of northern Calabria (Wallis et al., 1993). The low-grade Hercynian metamorphic rocks include lower Palaeozoic phyllites, meta-greywackes, limestones and meta-basalts. All these rocks show greenschist facies metamorphism that pre-dates late Hercynian plutonism (Acquafredda et al., 1988). These rocks are unconformably overlain by unmetamorphosed Mesozoic carbonates that are up to 1500 m thick in northern Calabria (Zuffa et al., 1980). The Hercynian plutonic rocks comprise a large part of the Calabrian Arc basement and outcrop over an area in excess of 2000 km2. There were two main intrusive phases, an earlier widespread

335

suite of subaluminous tonalites and granodiorites, and a later, more localised suite of peraluminous monzogranites and leucogranites (Atzori et al., 1984; Messina et al., 1991a, b). Tectono-stratigraphically below the Bagni Unit of the Alpine Group are the Liguride Complex rocks. These have been discussed in detail elsewhere by De Roever (1972), Knott (1987, 1988) and Wallis et al. (1993). In summary, this group of rocks comprise Cretaceous to Eocene deep water flysch deposits. Intercalated ophialitic fragments are found in the northern part of the Calabrian Arc. The flysch deposits vary in metamorphic grade from unmetamorphosed to greenschist facies with localised high-pressure lawsonite and glaucophane blueschist facies metamorphism. These rocks have been interpreted as being part of a Cretaceous to Palaeogene accretionary wedge produced during northwestwarddirected subduction of Neotethys (Knott, 1987). Comparable Neotethyan units, with no evidence of ophiolitic fragments, are also found in northern Sicily and are usually referred to as the Sicilide Complex (Ogniben, 1973). 3. A reappraisal of previous isotopic age data collected from the basement rocks of the Calabrian Arc A large amount of readily available isotopic age data has been obtained from the basement rocks of the Calabrian Arc during the last two decades. This includes studies by Borsi and Dubois (19681, Civetta et al. (1973), Borsi et al. (19761, Schenk (19801, Del Moro et al. (1982), Nicoletti and Ardanese (19841, Zuppetta et al. (19841, Rottura (1985) and Atzori et al. (1990). These authors have interpreted these data sets on an individual basis. However, no attempt has been made to combine all the data together. Therefore, as a part of this study this data has been collated and, where necessary, recalculated (Thomson, 19921, with the aim of producing detailed high-temperature cooling historles of the various basement rock groups of the Calabrian Arc. These can then be used to supplement the lower-temperature cooling histories derived from fission track analysis.

33

S.N. Thomson / Tectonophysics

This reappraisal uses the concept of closure temperature, originally conceived by Dodson (19731, and reviewed by Cliff (1985) and Zeitler (1989). The closure temperature is defined as “the temperature of an isotopic system at the time represented by its apparent age”. The relevant closure temperature values for the isotopic systems that have been used to determine ages from the basement rocks of the Calabrian Arc are summarised in Table 1. At least 140 isotopic age dates are used in this reappraisal and these are incorporated into time-temperature (T-t) plots illustrated in Fig. 2. The isotopic data obtained from the Alpine Group rocks is shown in Fig. 2a. It has been demonstrated by Harrison et al. (1979) that such a plot is open to two interpretations. First, the data may represent simple gradual cooling of the Alpine Group rocks from a temperature maximum during the Hercynian Orogeny. The alternative hypothesis is that a heating event occurred at about 50 Ma that caused differential resetting of the various mineral ages. In this case, the more stable isotopic systems, such as Rb-Sr white mica ages, are significantly less affected than more easily reset minerals such as biotite. Such a heating event would mask any rapid cooling that occurred subsequent to the Hercynian Orogeny. The Alpine Group rocks show petrological evidence of a post-Cretaceous or Alpine greenschist facies metamorphic overprint. This indicates that these rocks experienced reheating to temperatures in excess of 350°C. Schenk (1980) correlates a young Rb-Sr biotite age of 43 f 1 Ma obtained from the Castagna Unit of the Alpine Group with Table 1 Closure temperatures

used for interpretation

238 (1994) 331-352

the pervasive greenschist facies Alpine metamorphic overprint recognised in the sample. Therefore it is evident that an “Alpine resetting” time-temperature history is applicable to these rocks. The isotopic age data obtained from the highgrade Hercynian metamorphic rocks (Fig. 2b) may also be interpreted in two ways. Schenk (1980, 1989) suggests that the data is the result of gradual cooling, whereas Del Moro et al. (1982), Rottura (1985) and Atzori et al. (1990) propose that the biotite and white mica ages were partially reset by an “Alpine” phase of heating. Older, low-temperature ages have been obtained from high-grade Hercynian metamorphic rocks from the Aspromonte of southern Calabrian and northeast Sicily. This indicates that the high-grade Hercynian metamorphic rocks in these areas underwent rapid post-Hercynian cooling followed by a long pause in cooling during the Mesozoic. In contrast, Bonardi et al. (1987) have obtained Rb-Sr biotite and white mica ages between 30 Ma and 25 Ma from the Aspromonte region. Platt and Compagnoni (1990) relate these ages to local Alpine extensional mylonite zones. Isotopic ages from the Hercynian plutonic and low-grade Hercynian metamorphic rocks are combined in Fig. 2c. The vast majority of ages are indicative of rapid post-Hercynian cooling following intrusion of the plutonic rocks. Such a cooling curve is typical of the emplacement of plutonic rocks into cool host rocks at a high crustal level (Harrison and Clarke, 1979). A long pause in significant cooling between about 250 Ma and 50 Ma is confirmed by the presence of unmetamorphosed Mesozoic carbonates that unconformably

of isotopic age data

Isotopic system

Temperature (“C)

Reference

K-Ar K-Ar Rb-Sr Rb-Sr Rb-Sr

3OOf50 350 f 50 300f.50 SOOf 6.50400 > 650 >700

(Cliff, 1985) (Purdy and Jiiger, 1976) UZIiff, 1985) (Purdy and Rger, 1976) (Harland et al., 1990) (Searle and Tirrul, 1991) (Searle and Tirrul, 1991)

(biotite) (white mica) (biotite) (white mica) (whole rock) U-Pb (monazite) U-Pb (zircon)

S. N. Thomson / Tectomphysics 238 (I 994) 331-352

overlie the low-grade Hercynian metamorphic rocks throughout the Calabrian Arc. No petrological evidence of a post-Hercynian heating phase is

a) 600_

apparent in these rocks. In contrast, the plutonic rocks of the Capo Vatican0 intrusion show younger K-Ar biotite and white mica ages. Therefore these rocks have either undergone slower post-Hercynian cooling than the other plutonic rocks of the region or have been partially reset by a post-Hercynian heating event. Either interpretation would require that these rocks lay tectono-stratigraphically below the other Hercynian plutonic rocks of the Calabrian Arc.

Alpine Group rocks

600Gradual cooling hypothOSiS

g, ?? % 400' !i. c" 200-

0

b)

100

4. Fission track data from the basement rocks of the Calabrian Arc

Alpine msefflng hypNwais

,I------

4.1. Sampling procedure

300

200

MeWa)

Hercynlan 8w-High-grade metamorphic rocks 600'

g

2

si a g 400' P. If 200'

U’

0 C) 600

100

MeWa)

200

300

Hercynian plutonic and lrw+&rade metamorphic

1

1 2

OJ 0

loo Age(W) 2w

337

300

Fig. 2. Time-temperature plots incorporating previously obtained isotopic age data from the crystalline basement rocks of the Calabrian Arc. (a) Alpine Group samples. (b) Highgrade Hercynian metamorphic rock samples. (c) Hercynian plutonic and low-grade metamorphic rock samples. I = Rb-Sr (whole rock); 2 = Rb-Sr (white mica); 3 = K-Ar (white mica); 4 = K-Ar and Rb-Sr (biotite); 5 = U-Pb (zircon); 6 = U-Pb (monazite).

A total of 80 samples of 3-5 kg were collected from the crystalline basement rocks of the Calabrian Arc. Localities were chosen either from descriptions given from previous isotopic age dating, or alternatively, only when the sample could definitely be ascribed to a particular basement rock unit. Representative samples from each major basement rock group were collected, with consideration taken of potential apatite and zircon yield, elevation, geographical spread and accessibility. 4.2. Analytical details Separates of the minerals apatite and zircon were obtained via conventional crushing, “‘wilfley” shaking table, magnetic and heavy liquid techniques. Apatites were mounted in AralditeB, polished and etched in 5 M HNO, for 20 s. Zircons were mounted in FEP Teflon@, polished and etched in a molten KOH-NaOH eutectic at - 220°C for between 10 and 60 hours. All samples were analysed using the external detector method (Hurford and Green, 19831, with a lowuranium mica detector attached to each sample mount. Samples were irradiated at the X-7 facility at the Australian Energy Commission HIFAR research reactor, Lucas Heights, N.S.W. A fluence of - 1.2 x 1016 thermal neutronshcm2 was used for apatite and a fluence of * 1 X 10” thermal neutrons/cm2 for zircon. Fluences were

S.N. Thomson / Tectonophysics 238 (19941 331-352

338

Table 2 Summary of fission track results from the Calabrian Arc crystalline basement units Location

Sample name and altitude

U.T.M.

Rock type

grid reference

Cm)

Apatite FT

Apatite mean track

Zircon FT

central

length (pm + lo)

central

age a

age ”

ct Itr)

(*la)

Alpine Group: Bagni Unit

COL-1180 CUR-380b PAO-750 PAO-850

Colosimi Curinga Paola Paola

Castagna Unit CAS-880 Castagna

COR-390 COR-410 CUR-380a PAO-600 PAS-l 140

Cortale Cortale Curinga Paola Lago di Passante

XD230328 xc155995 WD917547 WD945541

Phyllite Biotite schist Phyllite Chloritic phyllite

18.1 12.9 13.2 12.7

XD250246 XC218988 XC225993 xc155995 WD903553 XD314287

Retrograde proto-mylonite Retrograde ntylonitic gneiss Retrograde augen gneiss Garnetiferous gneiss Proto-mylonite Proto-mylonitic granite

18.7 + 14.7+ 15.5 f 17.1 f 14.7 f 19.9 +

WD923649

Meta-porphyrite

+ f f rtr

32.x +

2.7 2.3 1.2 2.3

3.8 1.0 0.8 0.7 1.5 2.1

1.6

28.3 It 1.4 23.4 + 1.5

_ 14.98 f 0.28 (0.881 15.16 f 0.17 (1.36) 14.34 f 0.15 (0.901

27.8 f 17.8 f 14.4 f

1.3 0.6 0.5

23.1 f 1.2 24.5 k 1.2

Liguride Complex

FUS-1100

Fuscaldo

136.0 f 19.7

Hercynian Group: High-grade Hercynian metarrwtphic rocks

ALT-19OOa AMEARV-1300 ATT-1124 BOT-1850 BOT-1920 CAP-545 CAR-610 CEL-490 COP-50 FUM-340 GAM-1650 GAR-1500 ONO-400 PAL-750 SCI-30 SOF-450 TIR-700 VAL-590

Montalto Gambarie Lago Arvo Mt. Attenamare Mt. Botte Donato Mt. Botte Donato Terre Capona Cardinale Celia Copanello Fossa Umbrita Gambarie Mt. Gariglione San Onofrio Palazzello Scilla San Sofia d’Epiro Tiriolo Vallelonga

Hercynian &tonic rocks ALE-600 San AIessio

AMP-1300 AND-480 CAL-100 CAR-50 CEC-1150 CEC-1250 CHI-810 CIC-480 CIT-460 CIT-930

Lago Ampollino Andali Capo di Calava Capo Rascalmo Lag0 Cecita Lag0 Cecita Pzo. Chiarino Mt. Ciccia Cittanova Cittanova

WC813241 WC786212 XD264411 WC409240 XD279497 XD257498 XC159923 xc206777 xc141903 xc360905 XC133876 WC768240 XD363335 WC973825 WD944657 WC670352 XDll3780 XD313140 XC120783

Proto-mylonitic gneiss Retrograde amphiholite Biotite, chlorite schist Bioti?e schist Silhmanite gneiss Sillimanite gneiss Metabasite Meta-tonalite Granulite paragneiss Biotite orthogneiss Granulite (kinzigite) Protomylonitic Gt gneiss Gneiss Garnet sillimanite gneiss Biotite gneiss Granodioritic gneiss Gametiferous gneiss Garnetiferous leucogneiss Meta-tonalite

WC660262 XD449427 XD549201 VC92%?66 WC452384 XD330611 XD364626 WC420270 WC467331 WC918449 WC989427

Leucogranite Leucogranite Quartz monzonite Leucogranite Leucogranite Granite Granite Leucogranite Alkali granite Leucogranite Leucogranite

22.7 25.2 17.5 25.1 19.3 _

f f Jo f f

1.7 5.2 2.4 2.0 1.9

24.9 21.1 22.4 18.8 23.7

f f f + f

3.3 0.8 1.7 1.5 3.7

17.5 f 20.5 it 9.5 +

1.6 1.9

1.1 1.2

18.1 f _ 11.2* 23.2 f

1.3 2.8

23.2 rfr 34.4 f 25.5 f 7.4 + 24.8 f 14.6 f 21.5 f

1.4 2.5 1.5 1.0 1.1 1.3 2.8

35.1 f 21.0 f 17.9 f

1.3 2.0 1.3

12.49 f fI.24 (1.611 14.69 f 0.30 (1.18)

14.34 f 0.13 (1.28)

15.35 + 0.22 (0.96) _ 13.94 f 0.13 (1.25) _ 14.32 f 0.12 (0.72) 15.70 f 0.39 (0.96)

14.06 &-0.13 (1.281 14.12 f 0.16 (1.14) 14% f 0.21(1.181

13.20 f 0.18 (1.76) _

_ 24.1 + 28.1 f 45.0 f 29.3 + 25.0 f 24.7 f 27.9 + 23.1 + 30.0 f 17.5 f 21.3 f 29.2 f 90.2 f 38.8 f 99.6 It 22.6 It 26.7 f

114.9 f 98.5 + mt 112.5 f 185.7& 128.9 f 26.5 f 152.1 f

1.2 1.3 1.9 1.3 1.6 1.0 1.1 1.3 1.3 1.5 2.0 1.3 4.6 1.6 4.3 0.8 1.0

4.6 3.1 4.0 8.8 9.5 1.1 5.0

193.8 f 11.4

S. N. Thomson / Tectonophysics 238 (I 994) 331-352

339

Table 2 (Continued) Sample name and altitude (m)

Location

U.T.M. grid reference

Rock type

Apatite FT central age a (flu)

CRO-200 FER-1148 GER-1200 INF-1050 LIM-130 MAN-550

Cropani Ferdinandea German0 Infantino Limbadi Mandatorricio

XD543142 XC193706 XD446505 XD505463 XC839662 XD610704

25.9 f 28.5 f 19.4 + 31.4 f 18.8 f 42.5 +

1.6 1.0 0.8 2.3 1.0 20.1

PAO-940 POT-505 SAV-920 SAV-990 SGF-840

Paola Fiumara Potamo Savelli Savelli San Giovanni in Fiore San Giovanni in Fiore San Sofia d’Epiro Trepido Cap0 Vatican0 Zambrone

WD954524 XD547257 XD523534 XD530551 X1)459469

Granite Granite Granite Granite Granite Garnetiferous granite Granite Leucogranite Granite Granite Granite

12.5 ? 22.4 k 30.4 f 40.3 f 29.0 f

2.6 1.7 3.2 4.9 2.5

XD456462

Alkali granite

24.1 f

5.2

171.0 j: 10.3

XD154784 XD503374 WC725752 WC853854

Leucogranite Granite Granite Foliated granite

23.6 + 1.5 22.6 + 1.3 20.9 f 0.7

14.45 f 0.24 (1.98) 13.77 + 0.13 (1.27)

137.5 i 6.3 166.7 f 8.0 34.0 * 1.2 31.4 z 1.0

26.5 f

2.4

12.06 f 0.25 (2.47)

12.7 f

1.4

SGF-1000 SOF-750 TRE-1050 VAT-118 ZAM-125

Apatite mean track length (pm f lu)

12.13 f 13.90 f 13.98 f 14.09 f

0.24 (2.42) 0.17 (1.67) 0.23 (1.59) 0.15 (1.45)

13.33 f 0.35 (1.94) 9.81 f 0.29 (2.88)

Zircon ‘FT central age a (flu) 60.0 f 99.6 + mt 146.9 k 36.0 + 192.2 +

2.6 3.5 5.9 1.3 20.2

18.1 3: 0.9 176.8 + 24.7 269.4 f 12.6 197.6 f 14.3 170.3 + 10.6

Low-grade Hercynian metamorphic rocks

ARI-580

Arietta

XD551246

BOC-780 BOC-1300

Bocchigliero Bocchigliero

XD508661 XD477594

BOV-30 MAN-450 MAN-650

Bova Marina Mandanici Mandatorricio

WB879973 WC267070 XD591686

MAZ-180

Mazzara

WC118151

Chlorite, muscovite schist Conglomerate Bi, MS, Chl augen gneiss Augen gneiss Phyllite Biotite, muscovite schist Phyllite

32.8 f 1.4 21.1 + 5.2 14.6 f 1.3 7.8 f

0.6

289.0 f 25.6 178.2 t 8.6 12.83 f 0.19 (1.90) _ 13.75 f 0.19 (1.79)

299.3 f 21.4 162.0 .+ 9.4 134.6 f

4.2

mt = metamict zircon (probably > 100 m.y old). a Detailed IUGS standard data set available from the author on request.

monitored by using mica detectors attached to CN-5 and SRM-612 glasses with apatite and CN-2 glass with zircon (Hurford, 1990). After irradiation the mica detectors were etched in 40% HF for 45 min. Mounts were counted using a Zeiss Axioplan@ microscope at X 1250 magnification with an automated stage. Only prismatic mineral grains with well etched surfaces were selected for counting. The subsequent calculation of all fission track ages was done using the zeta-calibration approach (Hurford and Green, 1983), using IUGS-recommended age standards (Hurford, 1990) and a value of 0.5 for the 4~/2~ geometry correction factor. All ages are quoted as a “central

age” (Galbraith, 1992) with standard errors quoted to zf la. The central age is a weighted modal age that uses an iterative algorithm to provide an estimate of the modal age of the sample. The spread in ages about the mode is given as an “age dispersion” or relative error and is quoted as a percentage. As a generalisation a value of around 20% marks the division between a single sample age (below) and a mixed grain age (above). Apatite-confined FT lengths were measured using a Zeiss AxioplanB microscope at X 1250 magnification with a Houston Instruments@ digitising tablet projected onto the field of view using an attached drawing tube. The

340

S. N. Thomson / Teclonophysics 238 (1994) 331-352

system was calibrated using a 100 pm graticule. Horizontal-confined tracks were then measured from prismatic grains. These were identified by using a fine focus control. As many tracks mount as possible were counted for each sample mount, up to a maximum of 100. 4.3. Results and interpretation From the 80 samples collected from the crystalline basement rocks of the Calabrian Arc, 6.5 yielded either apatite or zircon, producing 57 apatite FT ages, 55 zircon FT analyses and 25 confined track length analyses in apatite. The

data are shown in Table 2, with the geographical distribution of the data points shown in Figs. 3 and 4. The apatite FT ages show a relatively narrow scatter of ages between 40 k 5 Ma and 7 IL-1 Ma. A general trend is observed, with younger apatite FT ages in the lower, more internal basement units, and older apatite FT ages in the tectonically higher, external basement units, The zircon FT ages show a much wider age scatter between 299 + 21 Ma and 14 rt 1 Ma. The tectonically lower, internal Alpine Group and high-grade Hercynian metamorphic rocks give low zircon FT ages of less than 45 of:2 Ma. The tectonically

iW.1 a2.7

Fig. 3. Location of apatite and zircon fission track ages from northern Calabria (the apatite fission track ages arc shown in bold, the zircon fission track ages are shown in itaIic4.

S.N. Thomson / Tectonophysics 238 Cl994) 331-352

higher, external Hercynian plutonic and low-grade metamorphic rocks give older zircon FT ages greater than 60 ? 3 Ma. Low-temperature cooling histories are derived from fission track ages by using the concept of closure temperature, summarised earlier. With a cooling rate between 5°C and 30°C per million years, a closure temperature of 100 5 20°C is applied to apatite FT ages and a value of 225 f 25°C is applied to zircon FT ages (Hurford, 1991). Low-temperature, time-temperature plots incorporating the fission track data obtained in this study combined with low-temperature isotopic age data are illustrated in Fig. 5. The FT data obtained from the Alpine Group and high-grade Hercynian metamorphic rocks are combined in Fig. 5a. It reveals that both these rock groups experienced increased cooling from above the zircon FT closure temperature

341

(> 250°C) to below the apatite FT closure temperature (< 80°C) between N 35 Ma and 10 Ma. An additional constraint on these plots is provided by Tortonian sedimentary rocks, with a maximum age of 11 Ma (Van Dijk and Okkes, 1990, that directly overlie the crystalline basement rocks throughout most of the Calabrian Arc. This implies that cooling had ceased in the majority of the basement rocks by this time. A general cooling rate of between 10°C and lS”C/m.y is indicated by most of the samples collected from these two rock groups. However, several samples from the Alpine Group rocks reveal cooling rates in excess of 30”C/m.y. The zircon FT data show that the majority of high-grade Hercynian metamorphic rocks were at temperatures in excess of 250°C at the time of Alpine metamorphism recognised in the Alpine Group rocks. However, no Alpine metamorphic COW390 14.7 f 1.0 17.82 0.6 ZFZF 18.8 *1.5 .ql*

1.3

CHI-810 26.5* 1.1 El

y 23.7

MAZ-180

El 7.8 z 0.6

134.6~~4.2

Fj

y 23.2 t2.8

I

,I !6.7* 1.0

1

IF mq ER'

,21.1,5.2,

/25.2,5.2 1 I,,,-.5)

122.7~1.7)

I;;;Wd;f,l

Fig. 4. Location of apatite and zircon fission track ages from southern Calabria ages are shown in bold, the zircon fission track ages are shown in italics).

)21.0+2.0)

and northeastern

);;;",:;;:',II;",W.",I

Sicily (the apatite

fission

track

S. N. Thomson / Tectonophysics 238 (1994) 331-352

Alpine Group and High-Grade Hercynian Metamorphic Rocks

a)

Alpine M&morphism and Re-Heating

4Otl

(Previous

Biotite K-Ar & Rb-Sr Ages)

Alpine Resetting Hypothesis

100 Alpine Grwp n

I

0 0

Rocks

High-Grnde Hercynian Metamorphic Rocks

Tortonian Sedimentation

20

40

60

8Q

100

120

140

160

Age(W

b)

rade ks @revious Biotitc K-Ar & Rb-Sr A&Y)

1SOJ

0

Apatite Fission Track Data

IOOQ SO-

Oi 0

10

b

20

30

lb0

40

1 50

2bO

,

Hercynion Plutonic Rocks

0

Capo Vatican0 Pluton

q

Low-Grade Hercydan Metamorphic rocks

Age (Ma)

3io

S. N. Thomson / Tectonophysics 238 (1994) 331-352

overprint has been recognised in these samples. This implies that if any Alpine reheating affected these rocks it was probably short-lived. That is, short enough to prevent any pervasive retrograde metamorphism taking place, but long enough, or with temperatures high enough, to totally reset or anneal fission tracks in zircon. An alternative hypothesis is that at around 35 Ma these rocks underwent a sudden increase in cooling rate following a period of slow cooling between 250°C and 350°C. Two high-grade Hercynian metamorphic rock samples from the northwestern Sila (SOF-450) and the Catena Costiera (PAL-750) reveal older zircon ages. The favoured interpretation for these ages is that the zircons were partially reset or annealed during Alpine reheating. Although the systematics of zircon annealing are not fully understood, partial resetting over geological time scales would indicate a temperature in excess of 200°C. Indeed, De Roever (1972) has shown that the high-grade Hercynian rocks from the Catena Costiera reveal a pumpellyite-actinolite Alpine metamorphic overprint, implying temperatures in excess of 250°C. One zircon FT age has been obtained from an undeformed meta-porphyrite from the Liguride Complex. It gives an age of 136 f 20 Ma. This is close to the suggested original Late Jurassic-Early Cretaceous age of these rocks (De Roever, 1972; Knott, 1987). This implies that either the highpressure/ low-temperature metamorphism recognised in these rocks occurred at temperatures not greater than 225 f 25°C (the closure temperature of fission tracks in zircon) or that heating in excess of 250°C during high-pressure metamorphism was of insufficient duration to affect the zircon fission track age. Fig. 5b illustrates the combined FT data collected from the Hercynian plutonic and low-grade Hercynian metamorphic rocks. Immediately noticeable is that all, bar five, zircon ages are above 60 Ma. Three of the five anomalously young ages

343

are obtained from the Capo Vatican0 pluton. These ages are similar to the zircon fission track ages of the high-grade Hercynian metamorphic rocks. K-Ar ages from the Capo Vatican0 pluton (Civetta et al., 1973) also correlate with K-Ar ages obtained from the high-grade Hercynian metamorphic rocks of the region. This is further evidence that the Capo Vatican0 plutonic rocks were tectono-stratigraphically lower than other Hercynian plutonic rocks of the region. The other two young zircon ages include sample CHI-810 from northeast Sicily and sample PAO-940 from the Catena Costiera. Sample CHI-810 is in a region affected by Alpine mylonitisation and retrograde metamorphism (Messina et al,, 1990). Sample PAO-940 is in the vicinity of high-grade Hercynian metamorphic rocks that show a pumpellyite-actinolite metamorphic overprint (De Roever, 1972). The lack of any evidence of an Alpine reheating in these rocks favours a slow cooling hypothesis followed by an increase in the cooling rate at N 35 Ma. The zircon FT ages show low age dispersions indicative of single sample ages. This implies that the zircons have not been partially reset by Alpine reheating, and represent cooling ages. Overall the FT data shows that the samples cooled from above the zircon closure temperature (250°C) to somewhere above the apatite closure temperature (N lSWC> between the end of the Hercynian Orogeny and - 35 Ma at a cooling rate of between 0.5”C and 2”C/m.y. Three samples show Hercynian zircon FT ages. This shows that parts of the low-grade Hercynian metamorphic rocks and Hercynian plutonic rocks have not experienced palaeotemperatures in excess of 200°C since the end of the Hercynian Orogeny. This is confirmed by the presence of Mesozoic carbonates unconformably overlying much of the low-grade Her&an metamorphic rocks of the Calabrian Arc. All the samples from these rock units have experienced more rapid cooling from above the apatite FT closure temperature

Fig. 5. Low-temperature time-temperature plots incorporating the apatite and zircon fission track ages obtained in tht study (see text for explanation). (a) Data from Alpine Group and high-grade Hercynian metamorphic rock samples. (b) Data froti Hercynian plutonic and low-grade Hercynian metamorphic rock samples.

344

5. N. Thomson / Tectonophysics

( - 150°C) to surface temperatures ( - 1000 since 35 Ma at rates of between 5°C and 10”C/m.y.

4.4. Apatite-confined fission track length analysis 25 apatite-confined track length distributions have been obtained from the basement rocks of the Calabrian Arc. The data is included in Table 2 and shown graphically in Fig. 6. The Alpine Group and high-grade Hercynian metamorphic rocks give long mean track lengths with low standard deviations. This is indicative of rapid passage of the rock sample through the apatite annealing zone, usually defined as being between 120°C and 7o”C, at the time indicated by the apatite fission track age of the same sample. The shorter mean track lengths and higher standard deviations obtained from the Hercynian plutonic and low-grade Hercynian metamorphic rocks imply a slower cooling rate between 12O’C and 70°C. The higher apatite FT ages obtained from these samples also suggest that cooling between 120°C and 70°C occurred earlier in these rocks when compared with the Alpine Group and high-grade Hercynian metamorphic rocks.

yy+m*~b&-+++ n

0,.

9

High-Grade

0

Hrrcynian

0

Low-Grade

Hercynian Plubmic Hercynian

I.

t.

10

11

I

_ I.

1

12 13 14 15 Mean Track Length (pm)

I

-..

I

-

16

17

Fig. 6. Apatite-confined fission track length data. Axes show mean track length of each individual sample track length distribution vs. the standard deviation (1~) of that distribution.

238 (1994) 331-352

5. Geological interpretation Analysis of FT data has revealed that the basement rocks of the Calabrian Arc experienced a phase of rapid low-temperature cooling between - 35 Ma and - 15 Ma. The relative position in the crustal section of the different rock groups prior to the initiation of this phase of cooling, based on the FT data, is illustrated in Fig. 7a. Above 25O”C, any zircon or apatite fission tracks that form are immediately annealed. Therefore any samples collected today from rocks that resided at these temperatures before 35 Ma will give apatite and zircon FT ages younger than 35 Ma. From the FT data collected in this study it follows that the Alpine Group rocks, high-grade Hercynian metamorphic rocks and the rocks of the Capo Vatican0 pluton must have resided at temperatures in excess of 250°C before 35 Ma. Rocks that reside at temperatures between - 225°C and 150°C will retain zircon fission tracks, but not apatite fission tracks. Hence, rocks that were at these temperature before 35 Ma will give pre-35 Ma zircon FT ages and post-35 Ma apatite FT ages. Such ages are obtained from the Hercynian plutonic and low-grade Hercynian metamorphic rocks of the Calabrian Arc. No rock samples from basement rocks of the Calabrian Arc have yielded apatite FT ages older than 40 Ma. This indicates that any rocks that resided at temperatures below - 120°C before - 35 Ma have now been removed from the crustal section. Fig. 7b shows a hypothetical interpretation of the exhumation of the basement rocks of the Calabrian Arc between - 35 Ma and - 15 Ma. This model equates cooling of the rock column with exhumation of the rock column. This assumption relies on the crustal geotherm remaining constant through time. This assumption can usually be applied to fission track data because of the relatively low temperatures involved (< 250°C). Also the basement rocks of the Calabrian Arc show evidence of an early Alpine high-pressure/ low-temperature metamorphism, followed by a later lower-pressure/ hightemperature greenschist facies metamorphism. Platt and Compagnoni (1990) propose that this is indicative of a P-T-t path typical of a collisional

345

S. N. Thomson / Tectonophysics 238 (1994) 331-352

orogen as suggested by England and Thompson (1984). Cessation of greenschist facies metamorphism indicates that the crustal geotherm has re-equibrilated and become stable. Greenschist facies metamorphism is indicative of temperatures well above 250°C. Therefore FT data collected in this study must post-date this metamor-

phism. This is therefore further evidence to support the assumption that a stable crustal geotherm existed at the times represented by the FT data. The zircon and apatite FT age data reveal that the Alpine Group and high-grade Hercynian metamorphic rocks must have cooled from temperatures greater than 250°C to temperatures be-

a) Relative positions of basement units prior to 30Ma NW

SE

Mesozoic sedimentary cover and Low-grade metamorphic rocks (LGM)

-120°C

C__--

No registration of apatile fission tracks

Hercynian plutonic ioccrk~(&%) _______________~__________________________-250°C

-------._..___, //

Cap0 Vatican0 /---Plutonic rocks &gh-grade /

/ presen< day erosion surface

metamorphic rocks (HGM)

Alpine Group rocks (AG)

No registration of apatite or zircon fission tracks

b) Exhumation between 30Ma and 15Ma (Diagram represents position of basement units at -25Ma)

SE

AG

present dav I erosion surface

I c)

Present day position of basement units

15.7 Ma 14.8um 24.OMa

\ i i

20.0 Ma 14.4um 27.5Ma

/ 24.OMa : 13.3um i 146Ma

i 19.3Ma [ 12.9um i 213Ma

Mean Apatite Age ApatiteTrack

Length

Mean Zircon Age

SE

Fig. 7. Schematic model to explain the obtained fission track data from the crystalline basement rocks of the Calabrian Arc in terms of differential exhumation (see text for details).

S.N. Thomson / Tectonophysics 238 (1994) 33I -352

low - 70°C between - 35 Ma and - 15 Ma. In contrast, the Hercynian plutonic and low-grade Hercynian metamorphic rocks only cooled from temperatures of - 150°C to below - 70°C during the same time interval. Apatite fission track length data also confirms that cooling between - 120°C and - 70°C was more rapid in the Alpine Group rocks and high-grade Hercynian metamorphic rocks in comparison to cooling in the Hercynian plutonic and low-grade Hercynian metamorphic rocks. The present-day outcrop of the relevant basement rock groups (Fig. 7’~) suggests that greater and more rapid exhumation of the Alpine Group and high-grade Hercynian Group rocks occurred in the present northern and western parts of the Calabrian Arc. The assumption that the period of increased cooling during the Oligo-Miocene was the result of exhumation can also be tested by examining the evidence of exhumational processes active at this time. There are two main exhumational processes: extensional tectonism and erosion or denudation.

6. Evidence for Oligo-Miocene

extension

Extensional tectonism is now regarded as a major contributor to syn- and post-erogenic exhumation in many erogenic belts around the world. Evidence of its existence is usually achieved by using metamorphic petrology and structural geology. In Calabria such techniques have recently been used to provide evidence of major syn- and post-erogenic extension within the Liguride Complex rocks (Wallis et al., 1993) and within the crystalline basement rocks of the Aspromonte region of southern Calabria (Platt and Compagnoni, 1990). This section will aim to show how fission track analysis can be used in conjunction with this information to provide new constraints on the locality, and more importantly, the timing of late-erogenic extensional tectonism. The lack of suitable rock types for fission track analysis has ruled out an extensive study of the Liguride Complex rocks. However, a number of fission track analyses have been obtained from the rocks of the Aspromonte, southern Calabria

and from northeastern Sicily (Fig. 8). Here work by Bonardi et al. (1984, 1987), Platt and Compagnoni (1990) and Messina et al. (1990) has provided extensive petrological and structural evidence of “Alpine” late-erogenic extension within the high-grade Hercynian metamorphic rocks of the region. It was discovered by Bonardi et al. (1984) that some of the high-grade Hercynian rocks of the Aspromonte had undergone post-Hercynian or “Alpine” retrograde greenschist facies metamorphism associated with intense ductile deformation. Similar observations were also made in the Peloritani Mountains of northeastern Sicily after detailed petrographic investigations by Messina et al. (1990). This metamorphism and the related ductile deformation has been discussed in detail by Platt and Compagnoni et al. (1990). Two stages of metamorphic regression have been recognised within the affected rocks of the Aspromonte. An early moderately high-pressure stage (500°C + 30°C 5 + 1 kbar) was followed by later lower-pressure greenschist facies metamorphism accompanied by intense ductile deformation. Platt and Compagnoni (1990) reveal that ductile mylonitic shear zones separate rocks with no Alpine metamorphic overprint above from thoroughly re-equibrilated rocks below. Metamorphic isograd offsets combined with kinematic indicators within the mylonites that indicate a north-directed extensional shear have led the above authors to interpret Alpine shear zones as low-angle, late-erogenic ductile normal faults that have juxtaposed rocks from different parts of the metamorphic pile. Rb-Sr ages obtained from the retrograde metamorphic rocks reveal ages of between 30 Ma and 25 Ma (Bonardi et al., 1987). This data provides a minimum age constraint on the timing of the Alpine retrograde metamorphism. The fission track data acquired in this study can be used to further constrain the timing and cessation of the extensional tectonism recognised in the Aspromonte and in northeast Sicily. The locations of the fission track data obtained from this region are illustrated in Fig. 8. Unfortunately, only one sample collected from rocks that show an Alpine metamorphic overprint yielded both apatite and zircon. This was due to

S. N. Thomson / Tectonophysics 238 (I 994) 331-352

t

347

CIC-480 152*5 (35+1)

CHI-8 10 27*1 (no apatite)

ATT-I 124 28*1 (2522)

Fig. 8. Map of the basement rocks of the Aspromonte region of southern Calabria and the Peloritani Mountains of northeast Sicily. Young zircon fission track ages coinciding with areas of Alpine retrograde metamorphism are highlighted. (Apatite ages are shown in brackets)

unsuitable rock types and the destruction of apatite and zircon crystals caused by the intense mylonitisation seen in many of these rocks. Nevertheless it is seen that the zircon FT ages show abrupt differences. Those taken from the Alpine retrograde rocks recognised by Bonardi et al. (1984), Messina et al. (1990) and Platt and Compagnoni (1990) show zircon FT ages less than 30 Ma. In contrast, zircon FT ages > 112 Ma occur in the unretrogressed rocks of the region. However, two samples from outside the previously recognised Alpine retrograde rocks (ATT- 1124 and X1-30) give young zircon ages of 28 f 1 Ma and 39 k 2 Ma, respectively, indicating that these

rocks have been subjected to Alpine palaeotemperatures in excess of 200°C. This suggests that the thermal effects of Alpine retrograde metamorphism may have been more widespread than the petrological effects. The offset in these zircon ages also indicates that the extensional faults that separated the areas of retrograde and mon-retrograde rocks were active between N 30 Ma and N 20 Ma. In contrast to the zircon ET ages, the apatite FT ages reveal no significant age offset across the metamorphic isograd offsets of the A$romonte and northeast Sicily. This means that any relative movement between the Alpine retrograde meta-

348

S.N. Thomson / Tectonophysics 238 (1994) 331-352

morphic rocks and the undeformed high-grade Hercynian metamorphic rocks had ceased by the time represented by the apatite FT ages. Therefore late-erogenic extensional tectonic activity must have ceased sometime between 23 Ma and - 18 Ma. A similar geological situation to that seen in the Aspromonte occurs in the Catena Costiera region of northwestern Calabria. Here high-grade Hercynian metamorphic rocks with no deformation and little recognised retrograde metamorphism overlie Alpine Group and Liguride Cornplex rocks that have experienced widespread pervasive Alpine metamorphism and deformation. The contact between the two rock groups has been documented by Dubois (19701, De Roever (1972) and Colonna and Compagnoni (1982). It is marked by a zone of intense cataclasis and mylonitisation. However, the kinematics of the deformation have not yet been investigated in detail, although WNW-ENE-stretching lineations have been recognised by De Roever (19721. The zircon FT ages show an abrupt age differential across this tectonic contact. Rocks showing pervasive Alpine retrograde metamorphism give zircon FT ages between 29 Ma and 18 Ma. However, sample PAL-700 (see Fig. 31, an undeformed Hercynian plutonic rock sample, gives a zircon FT age of 90 or 5 Ma. The apatite FT data from all the previous samples show no age offset, with ages ranging between 15 Ma and 10 Ma. The fission track data therefore indicates that the tectonic contact was active between 29 Ma and 18 Ma, but activity had ceased by 15 Ma.

tion (Bonardi et al., 19801 is found that is coeval with the increased cooling rates recognised from FT analysis. The formation comprises of major basal fan delta conglomerates, dated as midOligocene or 28 + 1 Ma (Harland et al., 1990; Weltje, 19921, overlain by a thick sequence of coarse terrigenous turbidites and sand bodies. Provenance studies by Ferla and Alaimo (1976) and Cavazza (1989) confirm that the detritus of the Stilo-Capo d’orlando Formation is derived from the local crystalline basement rocks of the Calabrian Arc. It can therefore be assumed that the Stilo-Capo d’orlando Formation represents the sedimentary signature of a major increase in the erosion rate of the crystalline basement rocks. It should be noted that no detritus from the high-grade Hercynian metamorphic or Alpine Group rocks has been recognised within the sedimentary rocks of the Stilo-Capo d’orlando Formation. This is consistent with the IT data obtained from these rocks that indicates that they were not exposed at the surface at this time, and therefore could not act as a sediment source. Work is in progress (Thomson, 1994) that aims to apply FT analysis to the rocks of the Stilo-Capo d’orlando Formation to constrain the extent and timing of this erosion. However, what is not in doubt, is that major erosion did take place in conjunction with extensional tectonism in the crystalline basement rocks of the Calabrian Arc between - 35 Ma and - 15 Ma.

8. Conclusions 7. Evidence of Oligo-Miocene

erosion

Evidence of increased rates of erosion linked to increased rates of exhumation can be obtained by looking for coeval evidence of increased deposition in the local sedimentary record. Within collisional orogens major phases of erosion are usually linked to the deposition of syn-erogenic terrigenous flysch or molasse within intramontane or foreland basins. In the Calabrian Arc, a major Oligo-Miocene flysch, labelled the Stilo-Capo d’orlando Forma-

Detailed analysis of the apat$e and zircon FT data obtained from the crystalline basement rocks of the Calabrian Arc has identified a major period of increased exhumation rates between _ 35 Ma and N 15 Ma. This has important imp&ations for the Oligo-Miocene tectonic evohttion of the Calabrian Arc. It is now generally accepted that prior to 35 Ma the evolution of the Calabtian Arc was dominated by northwest subduction of part of the Neotethyan oceanic crust that resulted in the formation of the Liguride accretionary complex

I LT Metamorphism

9. HypothetIcal

tectonic reconstructions

of the Calabrian Arc: (a) prior to 35 Ma (modified after

Ccntbmd undqtatlng ot N4otethycnaadtmMa

DepoMtlan of the Stllc-Capcd’Orlando and AlbtdoncFormctlona(SW Wctt)e,1922)

FrontalAccratbn ct Ncotcthyanaadlmcnta (L@alda Complex- Uppar Ophblltlc Napps)

Wallis

et

S @I

al., lY93); (b) between 35 Ma and 15 Ma,

A&la I African continental plats margin

phic rocks.

LGM = low-grade Hercynian metamorphic rocks; HPR = Hereynian plutonic rocks; CVP = Capo Vatican0 plutonic rocks; HGM = high-grade Hercynian metamor-

Fig.

N*dtnctrd~bfdtlbdItha *sHomontrmglcndaoMtcmtMabrla (act Ptattend Compagncnl,19930)

NW

b) 35 Ma to 15Ma

Formationot AtpineGroup rocka

a) Prior to 35Ma Am cryatallino NOH)ColabrIan baaamant rccka

350

S.N. Thomson / Tectonophysics 238 (1994) X31-352

and the development of an erogenic wedge (Knott, 1987; Dewey et al., 1989; Wallis et al., 1993). This early phase of subduction was dominated by frontal accretion and internal shortening, probably in the form of sedimentary underplating. A cross-section showing a hypothetical reconstruction of the tectonic environment prior to 35 Ma is shown in Fig. 9a. It has also been proposed that the retrograde metamorphism of the Alpine Group rocks occurred during deformation at the decollement between the high-grade Hercynian metamorphic rocks and the underlying, underplating Neotethyan sediments of the subducting plate (Dietrich, 1988). The presence of coeval extensional tectonism and erosion during the mid-Oligocene to Early Miocene can be explained by proposing that the erogenic wedge of the Calabrian Arc maintained a critical taper during this time. According to models of the dynamics of a critical erogenic wedge developed by Platt (1986), sufficient underplating of an erogenic wedge will eventually lead to the wedge becoming gravitationally unstable. This will cause the erogenic wedge to extend to maintain a stable configuration. Continued underplating leads to continued extension. This will eventually result in large-scale exhumation of the rock column, especially in the more internal parts of an orogen, towards the rear of the wedge. Similarly, underplating will also cause an increase in the surface height of an orogen producing an increase in the erosion rate. Such an explanation is used to produce the hypothetical model for the Oligo-Miocene tectonic evolution of the basement rocks of the Calabrian Arc illustrated in Fig. 9b. A similar model has been proposed by Wallis et al. (1993) to explain extensional tectonism found within the Liguride Complex rocks of northern Calabria. The subsequent Miocene to Recent tectonic evolution of the Calabrian Arc is dominated by the collision of the Calabrian erogenic wedge with the rifted continental margin of the African/Adrian plate. This took place during the Burdigalian (Dewey et al., 1989). This resulted in the subsequent opening of the Tyrrhenian Sea basin and has caused large-scale disruption of the original geometry of the Oligo-Miocene Cal-

abrian erogenic wedge. This deformation does not affect the majority of the FT data, indicating that this latter phase of tectonism was largely thin-skinned in nature.

Ackwwledgements This work has been supported by a NERCfunded studentship (GT 4/88/ GS/121) and NERC research grants GR3/7068 and GR3/8291 awarded to the London Fission Track Research Group. Thanks are due to Michel SCranne and two anonymous reviewers who provided critical reviews of the final draft of this manuscript.

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