Heat flow and the mid-Atlantic rift volcanism of San Miguel Island, Azores

Tectonophysics, 41 (1977) 173-179 0 Elsevier Scientific Publishing Company,

173 Amsterdam

HEAT FLOW AND THE MID-ATLANTIC OF SAN MIGUEL ISLAND, AZORES

J. QUINTINO

-Printed

RIFT VOLCANISM

and F. MACHADO

Serv. Meteorol. National, Ponta Dclgada (Azores) * Junta de Inuestigac6e.s Cient. do Ultramar. Lisbon (Portugal) (Received

in The Netherlands

September

**

28, 1976)

ABSTRACT Quintino, J. and Machado, F., 1977. Heat flow and the mid-Atlantic rift volcanism Miguel Island, Azores. In: A.M. Jessop (editor), Heat Flow and Geodynamics. nophysics, 41: 173-179.

of San Tecto-

The volcanoes of San Miguel Island appear to be fed by a single elongated magma chamber at a mean depth of about 5 km. Magma probably rises from the upper mantle to that chamber through the fractures of a branch of the mid-Atlantic rift which is assumed to cross the central part of the island. The magma then flows to either side of the rift along the chamber and, as solidification proceeds, the temperature is supposed to be kept close to 12OO’C (about 1500 K), which gives a theoretical gradient of lOO--4OO’C km-l (or mK m-l), depending on position and on thickness of the magma chamber roof. At the flank of one of the active volcanoes of San Miguel, a gradient up to 3OO’C km-’ was recently measured in a borehole. This corresponds to a heat flow close to 15 /.&al cmY2 set-’ (equivalent to 0.63 W m-’ ) through the magma chamber roof which must be about 4 km thick.

INTRODUCTION

During the last few decades an increasing supply of information has been obtained about the structure of the volcanoes of the Azores. Major problems are the formation of the magma chambers and the mechanism of feeding volcanic eruptions. The solution of these problems is related to the regional tectonics and has important geothermal consequences. The purpose of this paper is to deduce the thermal gradients expected over a model of Azorean magma chamber (whose form is suggested by several geophysical surveys), and to compare these theoretical values with the temperatures and temperature gradients measured in a borehole drilled recently on San Miguel Island by scientists from Dalhousie University (Halifax, Canada).

* Now at Inst. Nat. de Meteorol. e Geofisica, Lisbon, Portugal. ** Now at Inst. Univ. dos Acores, Ponta Delgada, Azores.

TECTONIC

MODEL

OF THE AZORES

The tectonics of the Azores is determined by the local triple junction of the crustal plates (see, for example, Le Pichon, 1968). The effect of this triple junction was discussed by Krause and Watkins (1970) who postulated that, in the Azores area, the mid-Atlantic rift bifurcates into two branches, one continuing to the south, and the other merging into the Alpine fracture belt. A slightly different model has been proposed subsequently (Machado et al., 1972). In this last model it is assumed that the mid-Atlantic rift is divided into several short branches which are displaced to the east and cross all the islands with active volcanism (Fig. 1). These branches of the rift are associated with the usual transform faults. On the other hand, the contraction in the Alpine chain (which forms, in the Atlantic, the Azores-Gibraltar belt) decreases to the west and appears to have a no-strain point at a longitude close to 22” W. Thus, in the Azores, there is a slight N-S extension together with the main E-W extension of the mid-Atlantic rift. That N-S extension produces minor rifts which probably coincide with the transform faults of the E-W spreading (cf. Fig. 1). The volcanism of the Azores is certainly related to the tectonic pattern. Both seismic studies and studies of the crustal deformation associated with volcanic activity indicate the existence of magma chambers at a mean depth of 5 km. The roofs of the chambers are probably vaulted and have a minimum thickness of about 3 km (see Machado, 1974).

MINOR

I

AZORES

ISLANDS

MODEL)

( TECTONIC

L_

~~

-_c-

RIFT

~

-_f-_

~_ ~ _ ~~

SANTA

c- ~~~ ~-- -~-a+

MARIA

‘r

I 31

Fig. 1. Tectonic

10

model

29

of the Azores

28

(modified

27! from Machado

261

251

et al., 1972).

241

175

3a DC

Trachytic

pumice

associated

lavos

/

I 15 40

Fig. 2. Geological

and

25 20

map of San Miguel (from Machado

et al., 1972).

The magma chambers appear to be elongated following a trend perpendicular to the mid-~~tlantic rift. The Island of San Miguel is a food example (Machado, 1972). It has three active volcanoes of the central type (SeteCidades, Agua-de-Pau and Furnas) and an active basaltic lava field, considered as the surface expression of the deep rift (Figs. 2 and 3); the complex forms a volcanic ridge 65 km long (if the extinct volcano of Povoac;Zo is included).

i

SOLID UPPER MANTLE

t

MAGMA CHAMBERS

t

----RIFT

\DEEP

Fig. 3. Assumed

MAGMA

magma chamber

FRACTURES

LAYER

of San Miguel Island (from Maehado,

1972)

176

Magma probably rises from the upper mantle through the fractures of that assumed branch of the mid-oceanic rift and fills a chamber at a depth of about 5 km. The chamber itself is elongated by active crustal spreading (cf. Fig. 3). In this way, volcanic activity can appear at some distance on either side of the rift, as far indeed as the molten magma can reach before solidifying. As shown previously (Machado, 1972), radiometric ages, geomagnetic anomalies and magmatic differentiztion patterns support this model. Magma intrusion related to the small N-S expansion apparently contributes very little to the volcanic activity (at least in San Miguel). THEORETICAL

TEMPERATURES

OVER

A MAGMA

CHAMBER

A thermal model of this elongated chamber can be approximated by assuming a horizontal line source of heat. From a previous discussion of similar problems (Machado, 1968) it seems that steady-state heat flow above the magma chamber is reached in less than 1 million years, whereas the presence of molten magma lasts for 2 or 3 million years (as deduced from the displacement of the oldest active volcanoes, owing to the regional crustal spreading at a rate of about 1 cm per year). We can therefore assume that this progressively extending chamber produces a near-equilibrium heat flow. The theoretical problem is then very simple (for a line source infinitely long); assuming conductive heat transfer and uniform thermal conductivity, the temperature in the crust surrounding the chamber is given by the equation: T

=

A

In

x2 + (h __ + 2)” + mz

x”+(h-2)2

where T is the temperature at depth z and at horizontal distance x on either side of the line source, m is the normal gradient in a non-volcanic crust, h is the depth of the line source, and A is a constant that can be adjusted to give a convenient size to the chamber. As accepted currently, the temperature of crystallization of the basaltic magma is probably about 1200°C. In addition, we make h = 5 km (approximate mean depth of the chambers) and m = 30°C km-‘. The constant A is chosen to give the temperature of 1200°C when x = 0 and z = 3 km (which is assumed to be the uppermost point of the chamber); the corresponding value is A = 400.35”C. With these parameters we obtained the temperatures shown in Fig. 4. The temperature at the water-table (approximately equivalent to sea-level) has been supposed to be zero; in fact it is about 20°C in the Azores. Although a crude model, it should give a reasonable picture of the temperature pattern for the roof of an Azorean magma chamber, in areas where percolation of hot fluids can be excluded.

177

AXIS

OF

VOLCANIC RIOGE SEA

LEVEL

,

I

Fig. 4. Temperatures (“C) over an idealized elongated chamber, treated as infinitely long).

The geothermal to 400°C km-i. MEASURED

gradients

THERMAL

beneath

GRADIENT

magma

BOREHOLE

chamber

(cross-section

the island would range from,

of

the

say, 100

IN SAN MIGUEL

The scientists from Dalhousie University drilled a borehole 981 m deep on San Miguel Island in 1973 (Ade-Hall et al., 1974; Muecke et al., 1974). The hole was located at the northern flank of Aguade-Pau volcano (cf. Fig. 2). This volcano, which reaches a height of 948 m and has a summit caldera with crater-lake, in 1563 suffered a large Plinian explosion producing trachytic pumice, followed a few days later by a basaltic lava flow (at the boundary of the young lava field). Permanent solfataric activity is found at various points on the outer slopes (also at sea, off the south coast). The volcano has therefore to be considered active (Agostinho, 1937; Machado, 1967; Forjaz and Weston, 1967). The temperatures measured successively at the bottom of the hole, during

the 1973 drilling, are shown in Fig. 5. The thermal gradients are rather irregular. The average value to total depth is close to 200°C km-r, although the gradients above a depth of 550 m reach 300°C km-‘l. Muecke et al. (1974) postulated that this increase in the gradients at depths of 100-550 m is due to heat carried horizontally by hot fluids originating at the central part of the volcano, where the overall average gradient must be above 300°C km-‘. In the theoretical model, a gradient of about 200°C km-’ is predicted at a distance of some 4 km from the axis of the volcanic ridge (a distance that coincides with the borehole); however, a gradient of 300°C km-’ would exist at points closer to the axis, where the thickness of the magma chamber roof is a little less than 4 km (cf. Fig. 4). Using a thermal conductivity of 5 - lo3 peal cm-’ set-r (“C)-‘, such high gradients would correspond to heat flows of about 15 peal crnm2 see-i. This is analogous to other measurements in geothermal areas (see Lee and Clark, 1966, p. 510). A recent drilling programme in Hawaii (Zablocki et al., 1974) encountered somewhat lower temperatures, probably because hot fluid circulation is less important in that island, which may be discouraging for commercial

TEMPERATURE

(‘C) 3oc

, --

1 7

\ \ \ \

i --f

Fig. 5. Bottom-hole temperatures (from Muecke et al., 1974).

measured

during

drilling

near

Agua-de-Pau

volcano

179

geothermal development. San Miguel, on the contrary, is a very promising area for geothermal exploration (Quintino, 1969; Ade-Hall et al., 1974). ACKNOWLEDGEMENT

The writers are indebted to J.M. Ade-Hall offering important suggestions.

for reading the manuscript

and

REFERENCES Ade-Hall, J.M., Aumento, F., Mueeke, G.K., MacDonald, A., Hyndman, R.D., Quintino, J., Opdyke, N. and Lowrie, W., 1974. Deep drilling on Sao Miguel, Azores: Preliminary results. Trans. Am. Geophys. Union, 55: 454 (abstract). Agostinho, J., 1937, Volcanic activity in the Azores, Report for 1933-1936. Bull. Volcanol., Ser. II, 2: 183-192. Forjaz, V.H. and Weston, F.S., 1967. Volcanic activity in the Azores, Report for 19591964. Bull. Volcanol., 31: 261-266. Krause, D.C. and Watkins, N.D., 1970. North Atlantic crustal genesis in the vicinity of the Azores. Geophys. J., 19: 261-283. Lee, W.H.K. and Clark Jr., S.P., 1966. Heat flow and volcanic temperatures. In: S.P. Clark, Jr. (editor), Handbook of Physical Constants. Geol. Sot. Am., New York, pp. 483511. Le Pichon, X., 1968. Sea-floor spreading and continental drift. J. Geophys. Rex, 73: 3661-3697. Machado, F., 1967. Active volcanoes of the Azores. In: M. Neumann van Padan (editor), Catalogue of Active Volcanoes of the World, Part 21. Int. Assoc. Volcanol., Rome, pp. 7-52. Machado, F., 1968. Evolu~~o t&mica de regiiies vulcdnicas. Garcie de Orta, 16: 502-512. Machado, F., 1972. Acid volcanoes of San Miguel, Azores. Bull. Volcanol., 36: 319-327. Machado, F., 1974. The search for magmatic reservoirs. In: L. Civetta, P. Gasparini, G. Luongo and A. Rapolla (editors), Physical Volcanology. Elsevier, Amsterdam, pp. 255-273. Machado, F., Quintino, J. and Monteiro, J.H., 1972. Geology of the Azores and the midAtlantic rift. Proc. 24th Int. Geol. Congr., Montreal, 3: 134-142. Muecke, G.K., Ade-Hall, J.M., Aumento, F., MacDonald, A., Hyndman, R.D., Quintino, J., Opdyke, N. and Lowrie, W., 1974. Deep drilling in an active geothermal area in the Azores. Nature, 252: 281-285. Quintino, J., 1969. Sistemas hidrotermais associados ao vulcanismo e sua prospeqao corn fins eeonbmicos, Aplieaqao i regiao das Furnas (s. Miguel, Acores). T&mica, 44 (389): 449-472. Zablocki, C.J., Tilling, R.I., Peterson, D.W., Christiansen, R.W., Keller, G.V. and Murray,