the mechanics of the campi flegrei resurgent caldera—a model

Journal of Volcanology and Geothermal Research, 45 (1991) 161-172 161 Elsevier Science Publishers B.V., Amsterdam

The mechanics of the Campi Flegrei resurgent caldera a model

G. Luongo a'b, E. Cubellis b,c, E Obrizzo b and S.M. Petrazzuoli b

" Dipartimento di Geofisica e Vulcanologia, Universita' di Napoli, Largo S. Marcellino 10, 80138 Napoli, Italv b Ossevatorio Vesuviano, Via Manzoni 249, 80123 Napoli, Italy

c C.N.R. Gruppo Nazionale per la Vulcanologia, Via Nizza 128, Roma, Italy

(Received September 13, 1989; accepted in revised form August 30, 1990)

ABSTRACT

Luongo, G., Cubellis, E., Obrizzo, E and Petrazzuoli, S.M., 1991. The mechanics of the Campi Flegrei resurgent ca ldera--a model. J. Volcanol. Geotherm. Res., 45: 161-172. b

A model of caldera resurgence has been applied to the Campi Flegrei (southern Italy) to interpret: (a) the processes observed during the recent bradyseismic crises of 1970-1972 and 1982-1984; (b) ground movements recorded for the past 2000 years; and (c) the volcanic history of the area.

The two-dimensional mechanical model involves an area larger than the caldera itself. The crust is considered as a viscous plate overlying a viscous half-space. Geometric dimensions and mechanical parameters are the assumed variables and have been assigned a range of values of 50-100 km for length, 5-15 km for thickness, and 102h to 1022 Pa s for viscosity.

The process controlling volcanic activity in the Neapolitan area is attributed to a tensile stress field causing the opening of the Tyrrhenian Basin and the rising of the mantle to depths of about 15-25 km beneath Campi Flegrei. While regional tumescence can be related to magmatic upwelling, detumescence can be linked to the major eruptions in the area (the Green Tuff of Epomeo, Ischia, 55,000 yr B.P.; the Campanian Ignimbrite, 30-35,000 yr B.P.; and the Neapolitan Yellow Tuff, 12,000 yr B.P.), together involving more than 100 km :~ (DRE).

We propose that regional detumescence is likely to have triggered caldera resurgence. Indeed, the model suggests that detumescence may have generated an axial tectonic stress of about 1000 bar ( 100 MPa), a value which is of the correct order of magnitude for initiating caldera resurgence in the Campi Flegrei. Furthermore, we show that such a stress decreases the vertical pressure needed for uplift to occur.

Introduction

The Campi Flegrei volcanic field is a caldera complex, about 12 km in diameter, lo- cated to the west of Naples, southern Italy. It is situated in the central sector of the Plio-Quaternary Piana Campana (Campanian Plain) graben, which formed in response to the regional tensile stresses that induced the spreading of the Tyrrhenian Basin, the collapse of the western margin of the Apennine Chain and may have initiated the ascent of magmatic masses beneath several areas of the Tyrrhenian Basin itself (Scandone, 1979; Gau- diosi et al., 1984; Finetti and Del Ben, 1986;

[)377-0273/91/$03.50 © 1991 - Elsevier Science Publishers B.V.

Sartori, 1987; Locardi, 1986, 1988; Kastens, 1988; Luongo, 1988; Royden et al., 1988). Re- cent deep seismic soundings in the Neapoli- tan area suggest that the top of the mantle under Campi Flegrei rises to a minimum depth ranging from 15 to 25 km (Ferrucci et al., 1986, 1989; Nicolich, 1987).

The mantle upwelling responsible for this structure may have occurred about 2 Ma [the age of the oldest volcanic products buried in the Piana Campana (Barbieri et al., 1979)]. This may have generated regional doming, followed by volcanism and crustal thinning, thus increasing tensile stresses in the area and facilitating the formation of the graben

162 (~ [ .U()N(;O I~T AI..

- -

Carnpi Flegrei

1 2

3 4

5 8

1) Quaternary sediments ; 2) Limestones ; 3) Normal fault ; 4) Structural highs ;

5) S t r u c t u r a l lows ; 8) P h l e g r a e a n ca ldera r im .

Fig. 1. Structural map of the Campanian area. The graben is the area covered by Piana Campana between the outcropping l imestone and the sea. The regional faults have generally N W - S E and N E - S W orientat ion [modified after Di Gi ro lamo et al.

(1 '~4) I .

(Fig. 1)(Neugebauer, 1978; Neugebauer and Braner, 1978; Bott, 1981; Illies, 1981).

Volcanism in the graben has occurred along three main directions: NW-SE (Campi Flegrei-Volturno), NE-SW (Campi Flegrei- Ischia) and west-east (Campi Flegrei-Vesu- vio). Activity along the three branches, however, has not developed evenly. The Campi Flegrei-Volturno branch now appears to be extinct, while the two others are still active, with the oldest outcropping products being dated at 150,000 yr B.P.

The most remarkable events in the volcanic history of Campi Flegrei are the caldera- forming eruptions of the Campanian Ig- nimbrite about 30-35,000 yr B.P., and the Neapolitan Yellow Tuff (or tufts) about 12,000 yr B.P. Large ash flows were produced dur-

ing both eruptions and their volumes have been evaluated at, respectively, 80 km 3 and 10 km 3 (DRE). Since the emplacement of the Neapolitan Yellow Tuff, eruptions have been much smaller (associated volume 0.01-0.5 km ~ DRE) and confined within the caldera (Rosiet al., 1983; Di Girolamo et ai., 1984; Di Vito et al., 1985; Rosi and Sbrana, 1987; Lirer et al., 1987). For at least the last 4000-8000 years (Rosi et al., 1983; Cinque et al., 1985) a remarkable ground uplift has occurred in the center of the caldera, producing the "La Starza" marine terrace, which has a 40-m-high cliff-face along the coast of the Gulf of Poz- zuoli (Fig. 1).

Evidence of both ground uplift and subsidence is also seen in other parts of the caldera and suggests that vertical movements

T H I - M I - ( H A N I ( ' S O f : T I l E ( ' A M P I F L E G R E I R E S U R G E N T ( ' A L D E R A - - A M O D E l . 163

M±,Nuovo

i

0 . . . .

- 4

=6 0 5 10 15 go

Centur ies A.D.

Fig. 2. Elevation of the Serapeo floor above sea level (dashed line) in the last two thousand years. The uplifts occurred before the eruption of Mr. Nuovo (1538) and during the bradyseismic crises of 197(/-1972 and 1982-1984 Imodified after Parascandola (1947) I.

have been common during the recent evolution of the Phlegraean area. In particular, the presence of Roman archaeological ruins submerged in the Gulf of Pozzuoli, as well as historical observations (Hamilton, 1776; Niccolini, 1846; Babbage, 1847; Lyell, 1872; Gfinther, 1903; Parascandola, 1947; Race, 1983; Fredriksen, 1984), indicate that such phenomena have been operating during the last 2000 years (Fig. 2).

Observations, started in the early 19th cen- tury, of vertical movements at the Serapeo, a Roman Market built at Pozzuoli harbour, demonstrate a sinking of the ground at an average rate of 15 mm/yr until 1968 (Fig. 3).

- 0 5 0

- t . 0 0

1.50

- 2 0 0

A.D.

o° o o ~ _ . g g g g ~ g g ~ . . . . . . . 000 , o . . . . .

o

-~0 ~ - o ~ ~ o

- 2 . 5 0

Fig. 3. Subsidence of the Serapeo floor from 1819 to 1968. The mean sinking velocity was about 1.5 ram/year [modified after Berrino et al. (1984)].

The most recent large ground deformations occurred during the two bradyseismic crises (bradyseism = slow ground movement) of 1970-1972 and 1982-1984 and resulted in an overall uplift of 3.2 m (Fig. 4).

Geophysical and geological data (Corrado et al., 1976; Berrino et al., 1984; Luongo et al., 1988) are consistent with the most recent uplift being caused by movement of magma at shallow depth. A similar explana- tion might also be valid for the short-term behaviour of the caldera floor during ear- lier periods of ground deformation• How- ever, the recurrence of ground movements in the Phlegraean area suggests that any such magma movement is the response to a longer- lived process controlling the evolution of the caldera.

Here we propose a model for describing

62

6.0

• . ,%. •

5.5 ":

K

50"

E" 4.5.

• : \:%-~..:

•t" 40 - •"

I 35 /

/ / •HIGH PRECISION LEVELLING LINES

• TIDE-GAUGE 3 0 /

/ 2 7 - I /

68"69 ' 70 ' 71 ' 72 ' 73 ' 74 ' 75 ' 76 ' 77 ' 78 ' 79 ' 80 ' 81 ' 88 83 '84 ' 85 ' 86 ' 87 ' 88 '

yeors

Fig. 4. Vertical ground movements at Pozzuoli during 1968- 1988, according to tide gauge and high precision levelling data [modified after Luongo et al. (1988)].

164 G. LUONGO ET AL.

such long-lived behaviour in terms of crustal response to a large initial uplift associated with the ascent of mantle and hence magmatic material.

As has been suggested for other caldera complexes (Smith and Bailey, 1968; Diet- rich and Decker, 1975; Marsh, 1984; Ko- muro, 1987), the movement of magma beneath Campi Flegrei, especially the large masses involved before the caldera-forming eruptions, may have significantly affected the stress patterns in an area much larger than of the caldera itself.

Evidence of such large-scale subsidence throughout the Piana Campana is provided by the sinking of a carbonate platform which outcrops on the margin of the graben (Ip- polito et al., 1973; Carrara et al., 1973, 1974; Finetti and Morelli, 1974; Rapolla, 1986; Cas- sano and La Torre, 1987). The carbonate platform is not found at depths greater than 3000 m, near the mouth of the Volturno river (Fig. 1), while the graben itself is filled with Qua- ternary sedimentary deposits (alluvial and py- roclastic material). Such an intense rate of subsidence is likely to be connected with major eruptions in the Neapolitan volcanic area.

Indeed, conditions may favour a long-term self-feeding process whereby a major eruption triggers regional subsidence which, in turn, forces new magma into the volcanic system. This initiates a local cycle of crustal doming, eruption and crustal collapse, leading to con- tinued regional subsidence. Accordingly, an intimate relation might be expected between subsidence of the Piana Campana graben and volcanic activity in Campi Flegrei and, probably, also on the neighbouring island of Is- chia [for example, the Green Tuff eruption of Epomeo, on Ischia, dated at c. 55,000 yr B.P. (Gillot et al., 1982)]. In a similar vein, Smith and Bailey (1968) and Marsh (1984) have suggested regional detumescence as a driving mechanism for caldera resurgence.

We therefore propose that the dynamical behaviour of the Campi Flegrei may be

quantitatively examined in terms of a caldera resurgence model, based on the interaction of regional and local crustal deformation with the ascent and the eruption of magma. In this analysis, we focus attention on the evolution of the Campi Flegrei following the eruption of the Campanian Ignimbrite (Barberi et al., 1978), because this represents the main volcanic event for the Campi Flegrei magmatic system.

Ground deformation model: previous studies

Most of the models proposed for analyzing ground deformation in volcanic areas assume the presence of a near-surface pressure source warping the crust upward.

The model proposed by Mogi (1958) as- sumes a spherical source of dilation, whose radius R is small compared with its depth below the surface, in a homogeneous, isotropic and linear elastic half-space (crust). Walsh and Decker (1971), also considering elastic crustal behaviour, assume that the shape of the source is linear.

Application of these two models to the deformations recorded in the Campi Fle- grei during the two latest bradyseismic crises (Corrado et al., 1976; Berrino et al., 1984) leads to unreasonably high source pressures. This difficulty was overcome by Bianchi et al. (1984, 1987) who used a finite element model to examine the influence of pressure changes in an elliptical magma chamber within a crust with temperature-dependent elastic proper- ties. However, although they obtained a good fit to the observed ground deformations with a pressure source of only 30 MPa, their assumed value for Young's modulus of crustal rocks is probably too small (1-4.8 GPa instead of a more reasonable value of 30 GPa).

Bonafede et al. (1986), moving away from linear elasticity, assumed a viscoelastic crust and showed that, with a pressure source of 100 bar (variable with time) at 3 km depth, it is possible to obtain close agreement to the

T H E M E C H A N I C S O F T H E CAMPI F L E G R E I R E S U R G E N T C A L D E R A - - A M O D F L 165

variation of displacements with time recorded during the 1970-1972 bradyseismic crisis.

Although some of the previous models can describe aspects of recent deformation behaviour in Campi Flegrei, they have the po- tential weakness of not accounting for the interaction of the regional external field with that developing locally inside the caldera.

In particular, regional detumescence may increase the tectonic stresses acting on the caldera block, so encouraging its uplift and increasing the chances of eruptive phenomena. It is the nature of such interaction between regional and local stresses that the present work will address.

Mechanical model

In order to obtain a parametric analysis, and hence a better understanding of ground deformation in Campi Flegrei, we have uti- lized a simplified model in which the crust is regarded as a thin two-dimensional plate, of thickness h and width b, overlying magma represented as a viscous half-space (Fig. 5) (Biot, 1961; Johnson, 1970; Turcotte and Schubert, 1982; Marsh, 1984).

The model is based on that used by Marsh (1984) for analyzing styles of ground deformation in volcanic areas, with particular ref-

erence to caldera resurgence. Comparing observed detumescence times with those pre- dicted by the model, Marsh found that better agreement is obtained by assuming a viscous behaviour for the crust; he also concluded that caldera resurgence is most easily ex- plained in terms of a detumescence process.

The general differential equation governing the deformation w in a detumescence process is (Biot, 1961; Marsh, 1984):

Ow 02w l& 3 05w - 2 # ~ k - ~ = Phlox2 + 3 0 t O x 4 + Apgw (1)

The first term represents the magmatic resistance, the second term the effect of tectonic stress (P), the third the effects of bending moment, and the last the action of gravity. Ap is the density difference between the crust and the atmosphere, # and ~ are the viscosity of crust and magma respectively.

The analytical solution of eq. (1) can be found by assuming, as already done implicitly in writing the first term of eq. (1), that the deformation has a sinusoidal shape w = s(t). sin(kx), obtaining:

w = woexp [ - - e h k 2 ) , /

b i

Fig. 5. The model used to analyze the dynamical behaviour of Campi Flegrei. The crust is regarded as a thin two dimensional plate overlying a viscous half-space [see text for definitions of ,u, #l , b, h, w, P and Pro(x)] [modified after Marsh (1984)].

106 (;. LUONGO ETAI .

+ b +

s(t)*sin(kx)

Fig. 6. Sketch showing the sinusoidal shape of the deformed crust, where the wave number is equal to 7r/b (b is defined in the text).

in which Wo represents the initial value of s(t). The wave number k is given by: k = 7r/ b (b = width of the plate) (Fig. 6). Since the magmatic viscosity coefficient is about 10 7 Pa s while the viscosity of crust ranges f r o m 10 j7 to 10 23 Pa s, the term 2#jk is much smaller than (tzh3/3)k 4. Assuming at first that the term Phk 2 may be neglected, by eq. (2) we can evaluate the sinking velocity of the point of maximum displacement [sin(kx) = 1]. Differentiating with respect to time and putting A = l~h3/b 4 (Pa s/m), we obtain:

1" - - Atog/(/7T4A). exp [-Apgt/('Tr4A'~]k5 ]3(3) Wo

In Figure 7 the ratio between sinking velocity (v) and Wo is plotted against time and the parameter A. As A increases, the process becomes slower and lasts longer. Figure 7 also

200

50 0t4

~ k ' x , ~ . ~ 10~4 15 Ap=2500 Kg/m3

15 s a*s/rn]

1 ,'--S-'-7 30 50 i00 150 200

t ime(xl0 a yrs)

Fig. 7. v/wo ratio versus time for various values ofA = izh3/b 4. The dashed line shows the values of V/Wo ratio today, 30 x 103 years after the eruption of the Campanian Ignimbrite (/~, h, b, v and Wo are defined in the text).

shows that the sinking velocities following the eruption of the Campanian Ignimbrite (about 30,000 yr B.P.) should range from about 13 mm/yr to 1 mm/yr for Wo = 1 km or one half of these values for Wo = 500 m. Such values are near to those recorded at Serapeo from 1819 to 1968 (Fig. 3).

In both cases, the associated maximum and minimum values for v occur whenA = 5 × 1014

Pa s/m and A = 2 × 1016 Pa s/m, respectively. To estimate the crustal viscosities involved in the sinking process, we have used two pair of b-h values: b = 100 km, h = 15 km and b = 50 km, h = 5 km. For A = 5 × 1014 Pa s/m, the first pair of values yields a viscosity of 1.5 x 1022 Pa s and the second a viscosity of 2.5 × 1022 Pa s, while f o r A = 2 × 10 ~6 Pa s/m, the corresponding viscosities are 6 × 102~ Pa s (b = 100 km, h = 15 kin) and 1024 Pa s (b = 50 km, h = 5 kin). Of these viscosity estimates, the first two are nearer to values considered reasonable for the crust. Sinking therefore takes place at measurable rates over a period of the order of years.

Besides being a measurable quantity that can be monitored in the field, the sinking velocity is also a very important parameter for analysing the dynamics of a volcanic area. In addition, these velocities can be used to estimate the magma resistance pressure. Thus, referring to the first term of eq. (1), putting Ow/Ot = 10 mm/yr, izj = 10 7 Pa s, b = 50 km, we obtain a resisting pressure Pm = 4.0 × 10 7 Pa. Consequently, as ment ioned above, magmatic resistance is too small to have a signif- icant influence on deformation of the crust. Such a low resistance also indicates that sub- stantial crustal subsidence could occur with- out first creating a discrete void in the magma chamber.

Effects of tectonic stresses on uplift phenomena

Considering the exponential coefficient of eq. (2), if (Apg -- Phk 2) > 0 the function w

THY MECHANICS OF THE CAMPI FLEGREI RESURGENT CALDERA--A MODEL 167

decreases with time, but if (Apg - Phk 2) < 0 it increases. Consequently the value of tectonic stress causing resurgence must satisfy the condition:

P > Apg/ (hk 2) (4)

However, eq. (4) does not provide the time taken for resurgence. In order to overcome this problem, we impose, like Marsh (1984), that the initial amplitude of w increases by a factor e 2, obtaining from eq. (2):

p = 4#~k + 2/31~h3k 4 + Apgt k2ht (5)

We can now determine the tectonic stress required to trigger resurgence in Campi Fle- grei. Since the viscosity coefficient plays a very important role it has been evaluated by using ground deformation data from 1982 to 1988.

A subsidence of about 50 cm has been recorded during the four years following the phase of maximum uplift of 1.80 m. Accord- ing to Bonafede et al. (1986) the observed deformation time sequence could be related to viscoelastic crustal behaviour; we can therefore use eq. (2) to describe the sinking phase. Assuming further that h = 4 km [maximum depth for seismic loci (De Natale et al., 1984; Luongo et al., 1988)], b = 14 km and At) = 2 x 103 kg/m 3, substitution in eq. (2) gives:/~ = 10 ~6 Pa s.

If the Neapolitan Yellow Tuff eruption(s) are linked to resurgence, we can set t = 20,000 years (the interval between eruption of the Campanian Ignimbrite and Neapolitan Yellow Tuff) which, when used in eq. (5), indicates a minimum stress for caldera uplift of 1000 bar (100 MPa).

This value represents the total horizontal stress and not the variation of stress at rest. Taking P from eq. (5) we find that, since the viscosity coefficient is relatively small on the numerator of eq. (5), the term &pgt domi- nates and we may write:

P = Apg/ (hk 2) (6)

which is independent of time. If doming is caused by a coupling of the

increases in magmatic pressure and tectonic stress, the general equation governing deformation becomes:

p. 0 2W izh 3 05w e m ( x ) = h~x 2 + 3 0 t ON 4 "1- ADgw ( 7 )

Assuming that the magmatic pressure is distributed under the base of the cauldron block as Pm(x) = Po sin(kx), we obtain:

Po W - -

A pg -- Phk 2

x {1 - exp

× sin(kx) (8)

We can see that the tectonic stress P decreases the value of Po required to reach the same value of w, by a factor:

&pg r - (9)

Apg -- Phk 2

Applied to the Campi Flegrei caldera (b = 14 kin, h = 4 kin, &p = 2000 kg/m3), with

10

9 -

8-

6 -

5-

4 -

2-

1-

0 o 260 460 660 860 ~ooo

P(bars) Fig. 8. Reduction factor (r) of magmatic pressure needed to induce resurgence because of the effect of tectonic stress P.

168 (;. I . U O N G O ET AI..

values for P ranging from 500 to 750 bar, the value of r ranges from 2 to 4 (Fig. 8).

Tectonic s tress i n d u c e d by d e t u m e s c e n c e

The effects linked to detumescence are represented by both an increase in magmatic resistance due to squeezing of the magma chamber and also an increasing tectonic stress inside the sinking crust which presses the caldera block and pushes it upward.

In the previous section, however, we de- termined that magmatic resistance may be neglected. Regional detumescence causing caldera resurgence may thus be attributed to the growth of tectonic stress alone.

Because of sinking and the constraint of rocks surrounding the deformed crust, the length of the crustal arc decreases.

The average axial strain in the arc is:

L - L,, e m - (10)

Lo

in which L is the arc length at an arbitrary time, and Lois the initial length.

The length function w(x) is given by:

b

L=/,,. vq+w'2dx (11 t Since w = s(t) sin(kx), we have:

b

L = [ ~1 + s2k 2 cos 2 k.x dx (12) / U

Developing the argument into a Taylor se- ries and ignoring second order and higher terms, we have:

L = 1 + 2 dx

s2k2b (13) = b + 4

and hence: "~ 2 s2k 2 . s;k .

b + --4--b - b - - ~ - b Qn = s k:

b + - ~ - b

s2k 2 2 2 -~ b - ~ b

= (14) 2 2

b + 4/9 since: b + (s2ok2/4)b ~ b, then:

s2k2 s2°k2 (15) tTm-- 4 4

from which:

O(m ssk 2 - (16)

Ot 2 The axial stress produced by a strain im-

pulse &m/Ot, is (Blot, 1961):

P = - 4 ~ 0 % _ 2kZs~# (17) Ot

Eq. (1) thus becomes:

Ow -2p,~k Ot

OZw Ith 3 05w = -2k'sk#ho~Sx2 + 3 0 t O x 4 + 2xpgw (18)

Assuming w = s(t) sin(kx) and neglecting the first term, we obtainffom eq. (18):

2k4s2~#h + k4;a + Apgs = 0 (19)

dSdt k(hka2PSApg + 3A~Jjh3k4 ) = - -1 (20)

Integration of eq. (20) and insertion of the initial condition s = Wo at t = 0, leads to:

h k 4 # ( h21n(s) , h2 ln(wo)) s 2 + w; = - 1

Apg 3 3

(21)

The solution has been found in implicit form, meaning that we have obtained the function t(s) instead of s(t).

The term s 2 represents the axial stress con- tribution which tends to increase detumescence time but its effect is negligible. By sub- stituting eq. (20) into eq. (17) we find:

p = 6Apg b2 rc2(6h + h3/s2) (22)

F i l e M E ( ' H A N I C S OF T H E CAMPI F L E G R E I R E S U R G E N T C A L D E R A A M O D E l , 169

90- • . ~ . ~ ' " ~%

8 o - o / "

. i / ,

10-

0 160 ' ' '5dO' ' '1'0100 ' 20'00 ' 30'00 ' 4d00'50'00 P(bclrs)

Fig. 9. Axial stress induced by regional detumescence as a function of crustal length b and R = 6h + h3/wo (b, h. w,, and P are defined in the text).

which shows that P is independent of viscosity, increases with b and s, and decreases with h. In Figure 9 the initial value of P is plotted as a function of b and R = 6h + h3/w2o, and we can see that the value of tectonic stress sufficient to trigger caldera resurgence, previ- ously computed at 1000 bar, is related to several pairs of values for geometric dimensions, b and h, of area involved in the detumescence process and several values for the initial uplift Wo.

We conclude, therefore, that tectonic stresses accumulating as a result of detumescence could be the cause of caldera resurgence in Campi Flegrei.

Discussion and conclusions

The dynamical behaviour of Campi Flegrei is related to the interaction of the regional stress field with that induced by the ascent of magmatic masses. The model we have pre- sented to describe this interaction emphasizes the dynamical importance of areas surrounding Campi Flegrei.

In particular, we have shown that during regional detumescence a large enough axial (tectonic) stress may develop. Such a stress makes the occurrence of uplift in a small area very likely, which may of course be within the caldera itself. In fact this stress may induce resurgence either by itself or together with an increase in magmatic pressure. However, the stress field linked to detumescence is likely to be strengthened by regional tectonics.

The calculated sinking velocities are large enough to be measurable and suggest that the formation of the thick sedimentary sequence in the Piana del Volturno, which belongs to the same depositional sedimentary environ- ments, are closely linked to deep-seated dynamical processes. In other words, the continuous subsidence of the Campanian Plain balanced by sedimentation, is a consequence of detumescence.

The model also predicts that the zone of deformation will decrease in area with time, since, although the axial stress generated by regional detumescence cannot alter the course of deformation of the area, it can pro-

170 G LUON(;O ET AL.

gressively reduce the area in which uplift occurs. Such a reduction could occur even when resurgence is attributed to the rising of magmatic masses from a closed feeding system. In this case, after each eruption, the initial magmatic mass becomes smaller, but it continues to rise, initiating a sequence of eruptions of generally decreasing intensity. Magma ascent eventually halts when the dimension of the rising body falls below a critical level and the magma solidifies before being able to reach the surface.

For "closed system" resurgence, therefore, detumescence must occur on a timescale t shorter than that needed t~ for the magma to solidify. Marsh (1984) has shown that this occurs when the caldera diameter is greater than 10 km. It follows, therefore, that if the feeding system is closed the probability of new activity may tend to diminish with time.

The resurgence model proposed here can in principle be applied to events in Campi Flegrei for the last 50,000 years. In this case, the overall evolution of Campi Flegrei may be considered in terms of a chain reaction, each event being the cause of the following one. Hence the link between the Campa- nian Ignimbrite and Neapolitan Yellow Tuff may be the same, but on a different scale, as that between the Neapolitan Yellow Tuff and more recent phenomena, such as the uplift of "La Starza" marine terrace. Indeed, the latest bradyseismic events (ground uplift and sinking) may represent the final part of such a sequence, which can only be reversed by dynamical reactivation from deep levels, i.e., if the magmatic feeding system is open. If this should be the case, the arrival of new magma from depth might interrupt the process of regional detumescence, and instead cause crustal bulging over an area whose ex- tent depends on the volume of magma involved. Since such a hypothesis cannot be neglected, it is necessary to continue long-term monitoring of vertical unrest in the Campi Flegrei caldera and surrounding areas.

Because the resurgence mechanisms oper- ate over a long time scale (103-105 years), it is unlikely to be directly responsible for the recent bradyseismic crises which developed much more rapidly over a period of two decades. However, it probably has an indi- rect influence, since, as shown, the large horizontal stresses generated by detumescence reduce the vertical pressure needed for local uplift to occur.

Another important implication of this study is that it does not require the empty- ing of shallow magma chambers for initiating caldera collapse, as demonstrated by the low resistance of magma during ground sinking.

The major uncertainty of the model is that it predicts too high a rate of release of tectonic stress. This is probably a consequence of using an oversimplified rheological model (Maxwell body) for crustal deformation. On the other hand, the assumption of sinusoidal crustal deformation with wave number k = 7r/ b does not impose undue restrictions on the model results, since calculations using different wave numbers (k = 27r/b) yield results of the same order of magnitude.

Acknowledgements

We want to thank the staff of the Osser- vatorio Vesuviano for the continuous work on the collection of data and C. Kilburn (Osservatorio Vesuviano) for discussing the manuscript.

References

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Barberi, E, Innocenti, E, Lirer, L., Munno, R., Pescatore, T. and Santacroce, R., 1978. The Campania Ignimbrite: a major prehistoric eruption in the neapolitean area (Italy). Bull. Volcanol., 41(1): 10-31.

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the mechanics of the campi flegrei resurgent caldera—a model

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