RE CENT VOLCANISM

Volcanism is one of the most spectacular expressions of the dynamic activity of the lithosphere. Petrological and geochemical studies are now able to characterise volcanism in such a way that mechanisms responsible for volcanism can be deduced. While recent volcanism is absent in the Baltic Shield, a moderate amount is found in central Europe and substantial activity characterises the Mediterranean area. In central Europe, volcanism is mostly alkaline in nature and related to a rift system that includes the Rhenish massif (lower Rhine and Leine grabens) and the upper Rhine graben (Figure 5-14) with its main volcanic

I

Figure 5-14. Recent volcanic activity in central Europe, after Wilson and Downes (1991).

activity during the Neogene. In the upper Rhine graben, Neogene volcanism has been dated as 19-13 Ma in age and is found along the southern margin of the graben. Quaternary volcanism is restricted to the west and east Eiffel region, with the main activity between 0,7 and 0.01 Ma.

Most of the volcanic expressions are located not within grabens themselves but on the adjoining horst blocks or near the main structural dislocations (see Figure 5-14). This suggests that the volcanism is structurally controlled, at least at upper crustallevels where the dominant mechanism of extension is simple shear (Wilson and Downes 1991). The alternation of periods of subsidence with compressional events (Ziegler 1982) indicate that magmatism mayaiso be related to lithospheric flexuring caused by Alpine activity, resulting in adiabatic decompression and partial melting of the upper mantle (Wilson and Downes 1991). On the basis of detailed geochemical studies, these authors suggest involvement of both lithospheric and asthenospheric mantle source components without extensive crustal contamination, finding no need to invoke the existence of mantle plumes.

Figure 5-15 shows the known volcanism in the Tyrrhenian sea and adjacent regions according to a classification of magma sources by Serri (1990). The evolution of the area, including Corsica and Sardinia, can be constrained by existing studies ofNeogene-Quaternary volcanism. The latter, however, embraces such a wide variety of magma types that it is impossible to relate it to a single geodynamic process.

Volcanism related to ocean island basalt-type sources is only found south of 41 oN latitude, with large amounts being erupted during the Plio-Quaternary in Sardinia. Geochemical studies show that this volcanism may have been contaminated by subduction and/or crustal effects. Magma related to partial melting ofMORB-type sources is limited but present in the Tyrrhenian area (e.g. Vavilov basin).

Volcanism typical of island ares is common, although it has only been found south of 41°N. This volcanism developed in two clearly separated cycles. The first was Oligocene-Miocene (32-13 Ma) in age and is found in Sardinia. This type of volcanism can be explained by a roughly NNW -dipping subduction of oceanic lithosphere beneath Sardinia. The second cycle was Quaternary in age and is mostly localised around the Marsili basin in the

1134

Acid Tuscan Magmatism

• •

•

A CONTINENT REVEALED

I

Anatectic Magmas

W (Dominant Upper Crustal Sourees)

a

rn

NW Roman Province

~ lAB-Type Sources _ OIB-Type Sources

@J

Central Campania Province

Figure 5-15. Recent volcanism in the Tyrrhenian Sea and adjacent regions, simplifiedfrom a classification of magma sources by Serri (1990).

Tyrrhenian Sea. Data obtained from ODP leg 107 indicates that oceanic crust drilled at site 651 in the Vavilov basin and at site 650 in the Marsili basin is consistent with calc-alkaline convergent plate margin basalts. A detailed discussion by Serri (1990) Ied to the recognition that the subducted plate und er the Tyrrhenian region, which is still seismically active, is oceanic in nature.

Widespread volcanism from Tuscany to Campania covers a time span from Tortonian to Recent. Detailed studies of this vo1canism have helped to define the roles played by upper crustal and mantle magma sources and by subduction-derived components. As an example, the potassic vo1canism of the NW Roman province displays a marked contamination of mantle sources through hybridization with melts derived by subducted upper crustal materiaL Serri (1990) has argued that intermediate to acid rocks were formed by melting ofthe upper crustal material subducted within the uppermost mantle during continent-continent colli-sion. In the Tuscan region, granitic and rhyolitic vo1canism appears to have been derived by melting of crustal material with typical features of an upper crustal reservoir, although melting may have occurred in the uppermost mantle. This, and the upper crust hybridised mantle sources of potassic vo1canism, requires the subduction of a continentallithosphere during collision related to Apennine orogenesis.

Taking all the available data, Serri (1990) found that Neogene-Quatemary volcanism in the Tyrrhenian and adjacent areas could be explained consistently. A lithospheric boundary along 41°N and a transform fault oriented NE-SW which marks a second lithospheric boundary separating the NW Roman province from the central Campania vo1canism appears to be related to subduction of the continental Adriatic lithosphere. This subduction carried large amounts of upper crustal rocks to depths of 50-1 00 km or more below the NW Romani Tuscany region. In contrast, an old oceanic (Ionian) lithosphere, still seismically active,

I

N _{Po Plain Corsica}

Alps Ligurian Sardinia

Sardinia

Channel Tunisia

s

200~---.

175 150 125

mWm-² ?

100 75 50 • 25

(a)

0

0 500 1000 1500

Distance (km)

0 -50 mGal

-100 -150

(b)

-200

0 500 1000 1500

Distance (km)

135

Figure 5-16. Heatflowalong the southern segment of EGT showing signijlcant short wave-lengfh variations, after Delta Vedova et al. (1990).

(a) Average heatflow density (dots) with standard deviations calculatedfor areas of JOO km width; regional heat flow (continuous line).

(b) Bouguer gravity anomaly. Areas ofunusualty high heatflow probably represent transient heatflow and correlate with marked positive Bouguer gravity anomalies.

below the Calabrian are and southern Tyrrhenian Sea is required to explain volcanism south of the lithospherie boundaries just deseribed.

The reeent age of volcanism indieates that some of the geodynamie proeesses responsible for it are still aetive. This is shown, for instanee, by the deep seismieity related with the subduetion of the Ionian lithosphere. Therefore it may weIl be that the currently active processes produce perturbations in the steady state heat flow in the form of transient pulses.