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Palaeomagnetism of the Macaronesian Insular Region: The Canary Islands
EARTH AND PLANETARY SCIENCE LETTERS 1 (1966) 225-231. NORTH-HOLLAND PUBL. COMP., AMSTERDAM
PALAEOMAGNETISM OF THE MACARONESIAN INSULAR REGION: THE CANARY ISLANDS N. D. WATKINS Department of Geology, Florida State University, Tallahassee, Florida, USA
A. RICHARDSON and R. G. MASON Department of Geophysics, Imperial College of Science and Technology, London, S. W. 7, England Received 14 June 1966
A palaeomagnetic survey, comprising collections mostly from the oldest outcrops on five of The Canary Islands, has revealed stratigraphically valuable normal and reversed polarity lavas except on the westernmost island of Hierro, where only normal polarities were encountered. The palaeomagnetic data suggests that the Hierro samples are younger than those collected from the other islands, and that the Lanzarote outcrops are the oldest sampled.
1. ~ T R O D U C T I O N The M a c a r o n e s i a n I n s u l a r Region c o m p r i s e s the east Atlantic volcanic a r c h i p e l a g o s of The A z o r e s , Madeira, The Selvagens, The Canary I s l a n d s , and Cap Verde. In c o m m o n with other volcanic t e r r a i n , the m a j o r M a c a r o n e s i a n geological p r o b l e m s a r i s e , p a r t i c u l a r l y on a b e t w e e n - i s l a n d b a s i s , b e c a u s e of the g e n e r a l lack of s t r a t i g r a p h i c a l l y useful f o s s i l s and b e c a u s e of the lack of u n d e r s t a n d i n g of the v a r i a t i o n of volcanic petrology in t i m e and space. Due to the l a r g e n u m b e r of lavas a v a i l able, the i s l a n d s are, however, ideally suited for p a l a e o m a g n e t i c studies. As the geomagnetic field p o l a r i t y i s t i m e dependent [1] and the m a g netic p o l a r i t y of lavas is, with v e r y few exceptions [2], a function of the a m b i e n t geomagnetic field d u r i n g the i n i t i a l cooling of the lava, a p a l a e o m a g n e t i c s u r v e y can be expected to provide an indication of the potential value of such data for future i n t e n s i v e s t r a t i g r a p h i c studies. A f u r ther motive in e x a m i n i n g the p a l a e o m a g n e t i s m of oceanic i s l a n d s involves t e s t i n g the s p r e a d i n g oceanic floor hypothesis, which was f i r s t i n t r o duced by Hess [3] and i n i t i a l l y tested by Wilson [4]. As I r v i n g et al. [5] have pointed out, any longitudinal m o v e m e n t s by Atlantic T e r t i a r y i s lands would m o s t likely not be detectable by p a l a e o m a g n e t i c methods, but b e c a u s e of the fact that with i n c r e a s i n g age, the geomagnetic pole
has broadly occupied positions at i n c r e a s i n g a n g u l a r d i s t a n c e s f r o m the p r e s e n t geographic pole [6], r e l a t i v e ages between T e r t i a r y volcanic i s l a n d s may be defined u n d e r ideal p a l a e o m a g netic c i r c u m s t a n c e s . Since an i n c r e a s e in the age of an i s l a n d away f r o m the m i d - A t l a n t i c ridge is c e n t r a l to the s p r e a d i n g oceanic floor hypot h e s i s , a palaeomaguetic s u r v e y can t h e r e f o r e be expected to c o n t r i b u t e to t e s t i n g of the hypothesis as well as d e l i n e a t i n g s t r a t i g r a p h i c a l l y useful magnetic polarity v a r i a t i o n s . This c o m m u n i c a t i o n is the f i r s t of a s e r i e s which will p r e s e n t p a l a e o m a g n e t l c r e s u l t s f r o m Atlantic i s l a n d s . No other p a l a e o m a g n e t i c s u r veys have b e e n c a r r i e d out to date in the M a c a r o n e s i a n i n s u l a r region. It is hoped that p r e s e n tation will encourage u s e of the method as a s t r a t i g r a p h i c tool by geological groups c u r r e n t l y active in The Canary Islands, in suitable a r e a s where both n o r m a l and r e v e r s e l y m a g n e t i z e d lavas occur. 2. GEOLOGICAL SETTING The Canary I s l a n d s a r e a chain of seven m a jor and five m i n o r i s l a n d s totalling 7545 km 2 in a r e a , off the coast of Morocco (fig. 1). T e n e r i f e accounts for over one q u a r t e r of this a r e a , and also r e a c h e s the highest elevation in the Atlantic, in the form of the volcano Teide (3711 m). S e v e r a l E u r o p e a n geologists have v i s i t e d the
226
N.D. WATKINS, A. RICHARDSON, R. G. MASON
•
RecentVolconics(Morocco only).
Precarnbrion
o
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RABAT
zoo
Stole in Kin.
.f
~MADEIRA
MOROCCO
CANARY
v
AGADIR
o
ISLANDS o
~LANZAROTE )PALMA
TENERIFE SFUERTEVENTUR~A
GOMERA HIERRO
(~ GRANCANARIA
SAHARA
18~1oo' Fig. 1. Sketch map showing location of The Canary Islands, and adjacent African coast. i s l a n d s . Hausen has s u m m a r i z e d the state of geological knowledge c o n c e r n i n g T e n e r i f e (7), F u e r t a v e n t u r a [8], L a n z a r o t e [9], and Gran Can a r i a [10]. In these p u b l i c a t i o n s , Hausen i n cludes the r e s u l t s of a l a r g e n u m b e r of his own as well as e a r l i e r c h e m i c a l a n a l y s e s and p e t r o logical studies, the d e t a i l s of which a s s i s t in the definition of the m a j o r s t r a t i g r a p h i c units of each island. The d i v e r s i t y of the igneous petrology is large, both within and between i s l a n d s [11], r a n g ing f r o m o c e a n i t e s [12] (which p e r h a p s should be t e r m e d tholeiites [13]) to alkalic lavas, with the l a t t e r v e r y dominant. Hausen r14] has r e c e n t l y published a s u m m a r y and p r o g r e s s r e p o r t of the l i m i t e d geological i n v e s t i g a t i o n s of La Gomera. Here, as on La P a l m a [15, 16], G r a n C a n a r i a [10], and F u e r t a v e n t u r a [8], in l i m i t e d a r e a s an old " b a s e m e n t complex" is exposed, beneath a m a r k e d u n c o n f o r m i t y . This " b a s e m e n t " is c h a r a c t e r i z e d in p a r t by u l t r a b a s i c and other i n t r u sives, v a r i o u s lower grades of m e t a m o r p h i s m , s t r u c t u r a l d e f o r m a t i o n , and p a r t i c u l a r l y on La G o m e r a , by a v e r y high d e g r e e of weathering. No
such b a s e m e n t is found on H i e r r o [17], L a n z a rote [9], o r T e n e r i f e [18], which a r e n e v e r t h e l e s s b r o a d l y s i m i l a r to the other i s l a n d s , in that the d o m i n a t i n g f e a t u r e s a r e great s e r i e s of lavas, with m i n o r i n t r u s i v e s , and with the exception of G o m e r a [14], adventive volcanic cones with Recent and s o m e t i m e s H i s t o r i c [19] lavas. B l u m e n thal [20] and o t h e r s have attempted i n t e r - i s l a n d c o r r e l a t i o n s , but without quantitative methods the i n h e r e n t difficulties a r e e n o r m o u s . The r e g i o n a l s e t t i n g of the C a n a r y I s l a n d s (fig. 1) is such that a genetic r e l a t i o n s h i p with the extensive tectonic activity [21, 22] of the Atlas Mountains region in Morocco has been suggested by s e v e r a l a u t h o r s [23, 24]. The i s l a n d s may be s t r u c t u r a l l y independent of the c o n t i n e n tal geological f e a t u r e s , and in fact may be p u r e l y oceanic in c h a r a c t e r if, for example, L e M a i t r e ' s [25] o b s e r v a t i o n of tholeiitic xenoliths in volcanic ejecta on L a n z a r o t e could be shown to be c o n s i s t e n t with the ideas of Engel et al. [26] conc e r n i n g a d i s t i n g u i s h i n g c h a r a c t e r i s t i c between oceanic and continental b a s a l t s .
PALAEOMAGNETISM OF THE MACARONESIAN INSULAR REGION T h e g e o l o g i c a l a g e of the i s l a n d s has b e e n the s u b j e c t of m u c h c o n j e c t u r e . A C r e t a c e o u s e c h i n oid Discoidea pulvinata found on H i e r r o [27] w a s l a t e r e s t a b l i s h e d [28] a s b e i n g a s s o c i a t e d with i m p o r t e d l i m e - k i l n m a t e r i a l w h i c h G a g e l [29] s p e c u l a t e d to h a v e o r i g i n a t e d f r o m F u e r t a v e n t u r a . L i m i t e d p a l a e o n t o l o g i c a l e v i d e n c e [30] and o t h e r c o n s i d e r a t i o n s [31, 32] s u c h a s r e l a t i v e d e g r e e of w e a t h e r i n g , s u g g e s t that m a n y of the l a v a s s a m p l e d m a y be a s old a s M i o c e n e . An e a r l y T e r t i a r y age h a s b e e n a s s i g n e d but not on any sound b a s i s , to s o m e p a r t s of the i s l a n d s [33], and H a u s e n [34] s u s p e c t s that the s t r u c t u r a l l y d e f o r m e d b a s e m e n t c o m p l e x on F u e r t a v e n t u r a m a y be a s s o c i a t e d with H e r c y n i a n o r o g e n i c a c t i v i t y . No i s o t o p e a g e s h a v e b e e n p u b l i s h e d , and so in t h e i r p o o r l y d e f i n e d g e o l o g i c a l age the i s l a n d s e x h i b i t one of the c l a s s i c a l p r o b l e m s of v o l c a n i c t e r r a i n .
227
3. F I E L D AND L A B O R A T O R Y M E T H O D S U s i n g a g a s o l i n e - p o w e r e d p o r t a b l e d r i l l [35], two g e o g r a p h i c a l l y o r i e n t e d c o r e s h a v e b e e n c o l l e c t e d f r o m e a c h of 115 of the o l d e r l a v a s on H i e r r o , T e n e r i f e , and L a n z a r o t e ; and f r o m e a c h of 59 l a v a s w h i c h a r e s t r a t i g r a p h i c a l l y i m m e d i a t e l y a b o v e the b a s e m e n t c o m p l e x on G r a n C a n a r i a and L a G o m e r a . T h e b a s e m e n t c o m p l e x e s w e r e not s a m p l e d s i n c e w e a t h e r i n g , i n t r u s i o n , and m e t a m o r p h i s m would i n e v i t a b l y , p e r h a p s t o t a l l y , m o d i f y the o r i g i n a l n a t u r a l r e m a n e n t m a g n e t i s m (NRM); and in any c a s e , the s t r u c t u r a l d e f o r m a t i o n which e x i s t s cannot be r e l i a b l y r e l a t e d to the o r i g i n a l a t t i t u d e of the v o l c a n i c b o d i e s , and the c o r r e s p o n d i n g o r i g i n a l NRM d i r e c t i o n s . D e t a i l e d d e s c r i p t i o n s of the s a m p l i n g s i t e s a r e not i n c l u d e d h e r e , but b r i e f m e n t i o n of the s i t e s i s i n c l u d e d in t a b l e 1, t o g e t h e r with the
Table 1 Sample locations, number of lavas sampled, and results of statistical analyses of NRM for each lava after treatment in 150 oersted alternating magnetic field. N = number of lavas sampled. D and I = mean delineation and inclination of NRM; declination angle is east of north; inclination angle is with respect to and below the horizontal. R = resultant vector assigning unit vector to each lava. k = precision parameter (= N-l/N-R). ol95 = s e m i - v e r t i c a l angle of 95% confidence cone. q~ and 0' = longitude and latitude of virtual geomagnetic pole in degrees. 6p and 5 m = semi-axes of oval of confidence about the virtual geomagnetic pole. Island
Sample locations *
Hierro
Various parts of the major scarp above E1 Golfo. Roadcut between Valverda and Alto de Malposa.
Gomera
Parts of the ~Alkali Basalt Fmt." and "Phonolite-Trachyte Fmt. ~ west and northwest of San Sebastian.
Tenerife
T h e "Ancient Basalt Formation" of Anagn north of San Andres, and in Ladera de Guimar. The "Phonolites" north of Vilaflor, and bei tween Granadilla and Ville I de Aria.
Gran Canaria
Lanzarote
(deg) 6.9
45.6 I 31.149 I 17.29
77.~ I 83.9 I 5.0
1
46
3.4
7.09
18
10.9
2
5
.
34.6
36.367 2~6.66-
* Stratigraphic t e r m s when used are those due to Hausen [7-10, 14, 17].
I
7.9
~
~135.2 I 83.4 I 4.1
"Plagioclase Basalts" between Aldea de San Nicolas and Agaete. "Olivine Basalts ~ beneath Montana Del Borno, towards Mogan. Some "Nepheline Phonolites N near Tejada. Confined to the "Basaltic : Tableland Series" on the Punta Fariones scarp and on a roadcut west of Arrieta.
6.2
(deg)
l- 6.81
9.3
10'9.4
78.5 I 6.4 ] 10.9 I
5.0
119.7
75.9
_1
3.3
5.7]
1
228
N . D . WATKINS, A. RICHARDSON, R. G. MASON N
N
i
W
E
Gomera
Hierro
Fig. 2. D i r e c t i o n of NRM in all s a m p l e d l a v a s f r o m H i e r r o a n d G o m e r a , a f t e r t r e a t m e n t in a 150 o e r s t e d a l t e r n a t i n g m a g n e t i c field. Q = u p p e r h e m i s p h e r e ~ ® = l o w e r h e m i s p h e r e . [~-- d i r e c t i o n of p r e s e n t g e o m a g n e t i c field. N = g e o graphic north. Projection is equal area.
N
N
W
E
O
O ~
W
E
~O O° J Tenerife
Gran Canaria
Fig. 3. D i r e c t i o n of NRM in a l l s a m p l e d l a v a s f r o m T e n e r i f e a n d G r a n C a n a r i a , a f t e r t r e a t m e n t in a 150 o e r s t e d a l t e r n a t i n g m a g n e t i c field. ® -- u p p e r h e m i s p h e r e . ® = l o w e r h e m i s p h e r e . [ ] = d i r e c t i o n of p r e s e n t g e o m a g n e t i c field. N -- g e o g r a p h i c n o r t h . P r o j e c t i o n is e q u a l a r e a .
PALAEOMAGNETISM OF THE MACARONESIAN INSULAR REGION n u m b e r of lavas sampled. The NRM of s p e c i m e n s from each lava were examined u s i n g an astatic m a g n e t o m e t e r and an a l t e r n a t i n g magnetic field d e m a g n e t i z i n g a p p a r a tus. 4. RESULTS One s p e c i m e n in ten was subjected to d e m a g n e t i z i n g e x p e r i m e n t s in p r o g r e s s i v e l y i n c r e a s i n g magnetic fields r a n g i n g from 25 to 300 o e r s t e d s , in o r d e r to a r r i v e at a single a l t e r n a t i n g m a g netic field value for application to all s p e c i m e n s t h e r e b y m i n i m i z i n g s e c o n d a r y magnetic compon e n t s added since the i n i t i a l cooling of the lavas. A m a g n e t i c field of 1 5 0 o e r s t e d s has been applied to s p e c i m e n s f r o m at l e a s t one core from each lava sampled, and the m e a s u r e d NRM d i r e c t i o n after this t r e a t m e n t is plotted for each lava in figs. 2-4. F i s h e r ' s [36] s t a t i s t i c a l methods have been applied to the data, and the r e s u l t s a r e p r e s e n t e d in table 1. R e v e r s e d and n o r m a l p o l a r i t y NRM were combined for the s t a t i s t i c a l a n a l y s i s , after c a l c u l a t i n g the v i r t u a l geomagnetic pole [37] for each lava. Those NRM d i r e c t i o n s yielding p a l a e o -
N
)E Lanzarote Fig. 4. Direction of NRM in all sampled lavas from Lanzarote, after treatment in a 150 oersted alternating magnetic field. (D= upper hemisphere. ®-- lower hemisphere. [~-- directionof present geomagnetic field. N -geographic north. Projection is equal area.
229
m a g n e t i c a l l y s o u t h e r n latitudes have b e e n t r a n s posed to the n o r t h e r n h e m i s p h e r e for the a n a l y sis. F o r each island, the v i r t u a l geomagnetic pole, which is the s u r f a c e e x p r e s s i o n of the hypothetical geocentric dipole giving r i s e to the m e a n NRM d i r e c t i o n for the island, is p r e s e n t e d in fig. 3, together with the a s s o c i a t e d 95% confidence ovals [36]. The g e n e r a l i z e d p o l a r - w a n d e r ing c u r v e for Europe [6, 38] is included in fig. 3. This c u r v e r e p r e s e n t s an a v e r a g e o b s e r v a t i o n of much data, and although it may be debatable whether o r not the geographic pole follows this curve, the curve does r e p r e s e n t an e x p r e s s i o n of the a n c i e n t geomagnetic field behaviour viewed from Europe. 5. DISCUSSION It can be seen that both normally and reversely magnetized lavas were encountered on each island (figs. 2-4) except Hierro. With the exception of this westernmost island, therefore, on each island surveyed there exists magnetic polarity variation of high stratigraphic value, since time dependent magnetic polarity contrasts can be obtained. In the event that isotope age determination methods could be applied, the polarity data could assist considerably in improving the quality of the results. The following five significant points emerge from examination of the data illustrated in fig. 5: (a) By inspection of the position of its virtual geomagnetic pole on the previously defined polar wandering curve, Hierro is implied to be the youngest island examined. This suggestion is strongly supported by the polarity data for Hierro (fig. 2), since only normal polarity lavas were defined despite the wide stratigraphic coverage of the collection. Normal magnetic polarity with NRM directions close to those shown in fig. 2 were obtained from Recent dikes and lavas which were also sampled. It is tentatively proposed that the exposed lavas on Hierro were extruded during the Brunhes (Nl) geomagnetic polarity epoch, as defined by Cox et al. [I]. This inferred age of less than 0.7 m.y. [39] for the exposed parts of the island corresponds to conditions on the island of Hawaii, as initially defined by the palaeomagnetic survey by Doell and Cox [40] and by later isotope dating [41]. (b) The position of the virtual geomagnetic pole for Lanzarote, shown in fig. 5, is, at the 95% probability level, different from the corresponding property for Hierro; and by association with the polar wandering curve, as discussed above, the sampled lavas on Lanzarote are much
230
N.D. WATKINS, A. RICHARDSON, R. G. MASON 180 °
90 o
0o
Fig. 5. Virtual geomagnetic poles and 95~ confidence ovals for each island. The virtual geomagnetic poles are indicated by s t a r s . The confidence ovals with solid lines are for islands in which the oldest exposed lavas were s a m pled. For islands in which the oldest lavas were not sampled (see text) the confidence oval is r e p r e s e n t e d by a dashed line. The E u r o p e a n - p o l a r - w a n d e r i n g curve (refs. [38, 39]) is shown by the heavy line, with the approximate age of various p a r t s of the curve indicated as follows: P e r m i a n (P), T r i a s s i c (Tr), Cretaceous {K), Eocene (Te). The p r e s e n t geomagnetic pole position is indicated by the heavy c r o s s .
o l d e r t h a n t h o s e s a m p l e d on H i e r r o . Fig. 5 a l s o i l l u s t r a t e s t h a t it i s r e a s o n a b l e to a s s i g n a n a g e to t h e l a v a s s a m p l e d on T e n e r i f e w h i c h i s i n t e r m e d i a t e b e t w e e n t h e a g e of t h e l a v a s s a m p l e d on H i e r r o a n d L a n z a r o t e . T h e d o m i n a n c e of r e v e r s e l y m a g n e t i z e d l a v a s in T e n e r i f e , r e f l e c t i n g a reversed geomagnetic polarity epoch, supports t h i s i n f e r r e d r e l a t i v e age. In L a n z a r o t e , the dominantly normal polarity lavas underlie reversely magnetized lavas at the highest point in the island. (c) W i d e s t a t i s t i c a l l i m i t s to t h e c a l c u l a t e d virtual geomagnetic poles for the sampled lavas on La Gomera and Gran Canaria prevent any rel a t i v e a g e s b e i n g a s s i g n e d to t h e s e i s l a n d s o t h e r
than that they are older than Hierro, this conclus i o n b e i n g b a s e d on t h e e x i s t e n c e of l a v a s of r e v e r s e d p o l a r i t y on L a G o m e r a a n d G r a n C a n a r i a , a s s h o w n in f i g s . 3 a n d 4, r e s p e c t i v e l y . (d) By c o m p a r i s o n w i t h d a t a f r o m p a l a e o m a g n e t i c s u r v e y s of T e r t i a r y r o c k s e l s e w h e r e [42] (and s p e c i f i c a l l y f r o m E o c e n e r o c k s , w h i c h y i e l d an average virtual geomagnetic pole as indicated in fig. 5), it s e e m ~ r e a s o n a b l e to p r o p o s e t h a t t h e l a v a s s a m p l e d a r e n o t on a v e r a g e o l d e r t h a n E o c e n e s i n c e it i s h i g h l y l i k e l y t h a t E o c e n e p a l a e o m a g n e t i c d i r e c t i o n s w o u l d not h a v e g o n e u n . d e t e c t e d , to r e s u l t in a t l e a s t s o m e a g r e e m e n t with independent data. (e) T h e c l o s e c o r r e s p o n d e n c e between the
PALAEOMAGNETISM OF THE MACARONESIAN INSULAR REGION European polar wandering curve and the Canary I s l a n d s v i r t u a l g e o m a g n e t i c p o l e s (fig. 3) s u g g e s t s t h a t no s i g n i f i c a n t m o v e m e n t h a s o c c u r r e d between The Canary Islands and Europe since the sampled lavas were extruded. A s d i s c u s s e d a b o v e , i n t h e c a s e of t h e t h r e e islands on which the oldest lavas were sampled (Hierro, Tenerife, and Lanzarote), an increase i n a g e of o u t c r o p f r o m w e s t to e a s t i s i n f e r r e d by the palaeomagnetic data. The La Gomera and Gran Canaria samples are older than those coll e c t e d f r o m H i e r r o , b u t t h e i r a g e r e l a t i v e to t h e s a m p l e s f r o m t h e o t h e r i s l a n d s i s not known. From these results, it can be stated that the data a c q u i r e d s o f a r i s not i n c o n f l i c t w i t h H e s s ' s [3] spreading oceanic floor hypothesis, in which the Atlantic floor is envisaged as spreading away f r o m t h e m i d - A t l a n t i c r i d g e . It i s s t r e s s e d , of c o u r s e , t h a t n e i t h e r do t h e d a t a s u b s t a n t i a l l y c o n t r i b u t e to r e s o l u t i o n of t h e h y p o t h e s i s s i n c e other conditions can be envisaged by which an i n c r e a s e i n a g e of v o l c a n i c a c t i v i t y in a g i v e n d i rection could occur without invoking an oceanic s p r e a d i n g of t h e t y p e a c c o m m o d a t i n g H e s s ' s m o d e l . T h i s p a r t i c u l a r a s p e c t of t h e d a t a p r e s e n t e d h e r e i s m e r e l y i n t e n d e d to b e i n t e g r a t e d with results from appropriate independent methods which may be applied in the future.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
ACKNOWLEDGEMENTS [31] T h e f i e l d w o r k w a s s u p p o r t e d by t h e D e p a r t m e n t of G e o p h y s i c s , I m p e r i a l C o l l e g e of S c i e n c e a n d T e c h n o l o g y , L o n d o n , w h i l e o n e of u s (N.D.W.) was avisiting Leverhulme fellow at the Chadwick L a b o r a t o r y , U n i v e r s i t y of L i v e r p o o l , E n g l a n d . D.S.I.R. a r e g r a t e f u l l y a c k n o w l e d g e d f o r t h e s u p p o r t of A.R.
[32] [33] [34] [35] [36] [37] [38]
REFERENCES [39] [1] R.R. Doell, G. B. Dalrymple and A. Cox, J. Geophys. Res. 71 (1966) 531. [2] S. Uyeda, Japan, J. Geophys. 2 (1958) 1. [3] H. H. Hess, History of Ocean Basins in Petrologic Studies: A Volume to Honor A. F. Buddington (Geol. Soc. America, 1962) p. 599.
[40] [41] [42]
231
J. T. Wilson, Nature 197 (1963) 536. E. Irving and A. Major. Sedimentology 3 (1964) 135. R.R.Doell and A.Cox, Advan. Geophys. 8 (1961) 298. H. Hausen, Soc. Scien. Fenn. Comm. Phys. Math. 18 (1956) 270. H. Hausen, ibid. 22 (1958) 211. H. Hausen, ibid. 23 (1959) 116. H. Hausen. ibid. 27 (1962) 418. H. Hausen, ibid. 27 (1962) 349. H. Hausen, ibid. 18 (1956) 200. G. A. MacDonald, J. Pet. 5 (1964) 88. H. Hausen, Acta Geog. (Helsinki) 18 {1965) 15. H. Hausen. ref. [8], p. 197. C. Gagel, Z. Deutsch. Geol. Gesel. 78 (1926) 556. H.Hausen, Ragos geologicos generales de la isla del Hierro. Patronato de la Casa de Colon (Madrid) 10 (1964) 547. H. Hausen, ref. [7], p. 37. H.Hausen, ref. [7]. p. 182. M. Blumenthal. Bol. Inst. Geol. Min. Espai~a 72 (1961) 1. C. Vidal Box. Las rocas eruptivas del Sahara E s panol. Tomo extraord. Real. Soc. Esp. de Hist. Nat. (1946). C. B u r r i and J. M a r c a i s , Reg. Mono. Ser. 3. Maroc. (6). XIX Int. Geol. Cong. Alger. Rabat. 1952. H. Hausen (summary), ref. [7], p. 14. H. Hausen (summary), ref. [9], p. 101. R.W. LeMaitre, Proc. Geol. Soc. London 1626 (1965) 143. A. E. J. Engel, C. G. Engel and R. G. Havens. Geol. Soc. Am. Bull. 76 (1965) 719. J. Cotteau and C. Lemoine, Bull. Soc. Geol. F r a n c e (1910) 267. L. F. Navarro, Compt. Rend. (1917) 165. C. Gagel, ref. [16], p. 564. H.Hausen, ref. [8], p. 27; ref.[9], p. 180; ref. [10], p. 30. H.Hausen, ref. [7], p. 30; ref. [8], pp. 193, 204-207: ref. [9], p. 101; ref. [10], pp. 27, 383. F. Macau Vilar, 20th Int. Geol. Cong. (Mexico), Sect. I (1957) 411. H. Hausen, ref. [8], p. 204. H. Hausen, ref. [9], p. 101. R. R. Doell and A. Cox, Geol. Surv. Bull. 1023-A (1960) 664. R.A. Fishe}~, Proc. Roy. Soc. London A217 (1953) 295. A. Cox and R.R. Doell, Bull. Geol. Soc. Am. 71 (1960) 664. K. M. C r e e r , E. Irving and S. K. Runcorn, Roy. Soc. London Phil. T r a n s . A250 (1957) 144. R. R. Doell and G. B. Dalrymple. Science 152 (1966) 1060. R. R. Doell and A. Cox. Nature 192 {1961) 645. I. McDougall, Bull. Geol. Soc. Am. 75 (1964) 107. E . I r v i n g , P a l e o m a g ~ e t i s m and its application to geological and geophysical p r o b l e m s (John Wiley and Sons, New York. 1964) p. 310.
PALAEOMAGNETISM OF THE MACARONESIAN INSULAR REGION: THE CANARY ISLANDS N. D. WATKINS Department of Geology, Florida State University, Tallahassee, Florida, USA
A. RICHARDSON and R. G. MASON Department of Geophysics, Imperial College of Science and Technology, London, S. W. 7, England Received 14 June 1966
A palaeomagnetic survey, comprising collections mostly from the oldest outcrops on five of The Canary Islands, has revealed stratigraphically valuable normal and reversed polarity lavas except on the westernmost island of Hierro, where only normal polarities were encountered. The palaeomagnetic data suggests that the Hierro samples are younger than those collected from the other islands, and that the Lanzarote outcrops are the oldest sampled.
1. ~ T R O D U C T I O N The M a c a r o n e s i a n I n s u l a r Region c o m p r i s e s the east Atlantic volcanic a r c h i p e l a g o s of The A z o r e s , Madeira, The Selvagens, The Canary I s l a n d s , and Cap Verde. In c o m m o n with other volcanic t e r r a i n , the m a j o r M a c a r o n e s i a n geological p r o b l e m s a r i s e , p a r t i c u l a r l y on a b e t w e e n - i s l a n d b a s i s , b e c a u s e of the g e n e r a l lack of s t r a t i g r a p h i c a l l y useful f o s s i l s and b e c a u s e of the lack of u n d e r s t a n d i n g of the v a r i a t i o n of volcanic petrology in t i m e and space. Due to the l a r g e n u m b e r of lavas a v a i l able, the i s l a n d s are, however, ideally suited for p a l a e o m a g n e t i c studies. As the geomagnetic field p o l a r i t y i s t i m e dependent [1] and the m a g netic p o l a r i t y of lavas is, with v e r y few exceptions [2], a function of the a m b i e n t geomagnetic field d u r i n g the i n i t i a l cooling of the lava, a p a l a e o m a g n e t i c s u r v e y can be expected to provide an indication of the potential value of such data for future i n t e n s i v e s t r a t i g r a p h i c studies. A f u r ther motive in e x a m i n i n g the p a l a e o m a g n e t i s m of oceanic i s l a n d s involves t e s t i n g the s p r e a d i n g oceanic floor hypothesis, which was f i r s t i n t r o duced by Hess [3] and i n i t i a l l y tested by Wilson [4]. As I r v i n g et al. [5] have pointed out, any longitudinal m o v e m e n t s by Atlantic T e r t i a r y i s lands would m o s t likely not be detectable by p a l a e o m a g n e t i c methods, but b e c a u s e of the fact that with i n c r e a s i n g age, the geomagnetic pole
has broadly occupied positions at i n c r e a s i n g a n g u l a r d i s t a n c e s f r o m the p r e s e n t geographic pole [6], r e l a t i v e ages between T e r t i a r y volcanic i s l a n d s may be defined u n d e r ideal p a l a e o m a g netic c i r c u m s t a n c e s . Since an i n c r e a s e in the age of an i s l a n d away f r o m the m i d - A t l a n t i c ridge is c e n t r a l to the s p r e a d i n g oceanic floor hypot h e s i s , a palaeomaguetic s u r v e y can t h e r e f o r e be expected to c o n t r i b u t e to t e s t i n g of the hypothesis as well as d e l i n e a t i n g s t r a t i g r a p h i c a l l y useful magnetic polarity v a r i a t i o n s . This c o m m u n i c a t i o n is the f i r s t of a s e r i e s which will p r e s e n t p a l a e o m a g n e t l c r e s u l t s f r o m Atlantic i s l a n d s . No other p a l a e o m a g n e t i c s u r veys have b e e n c a r r i e d out to date in the M a c a r o n e s i a n i n s u l a r region. It is hoped that p r e s e n tation will encourage u s e of the method as a s t r a t i g r a p h i c tool by geological groups c u r r e n t l y active in The Canary Islands, in suitable a r e a s where both n o r m a l and r e v e r s e l y m a g n e t i z e d lavas occur. 2. GEOLOGICAL SETTING The Canary I s l a n d s a r e a chain of seven m a jor and five m i n o r i s l a n d s totalling 7545 km 2 in a r e a , off the coast of Morocco (fig. 1). T e n e r i f e accounts for over one q u a r t e r of this a r e a , and also r e a c h e s the highest elevation in the Atlantic, in the form of the volcano Teide (3711 m). S e v e r a l E u r o p e a n geologists have v i s i t e d the
226
N.D. WATKINS, A. RICHARDSON, R. G. MASON
•
RecentVolconics(Morocco only).
Precarnbrion
o
IOO
RABAT
zoo
Stole in Kin.
.f
~MADEIRA
MOROCCO
CANARY
v
AGADIR
o
ISLANDS o
~LANZAROTE )PALMA
TENERIFE SFUERTEVENTUR~A
GOMERA HIERRO
(~ GRANCANARIA
SAHARA
18~1oo' Fig. 1. Sketch map showing location of The Canary Islands, and adjacent African coast. i s l a n d s . Hausen has s u m m a r i z e d the state of geological knowledge c o n c e r n i n g T e n e r i f e (7), F u e r t a v e n t u r a [8], L a n z a r o t e [9], and Gran Can a r i a [10]. In these p u b l i c a t i o n s , Hausen i n cludes the r e s u l t s of a l a r g e n u m b e r of his own as well as e a r l i e r c h e m i c a l a n a l y s e s and p e t r o logical studies, the d e t a i l s of which a s s i s t in the definition of the m a j o r s t r a t i g r a p h i c units of each island. The d i v e r s i t y of the igneous petrology is large, both within and between i s l a n d s [11], r a n g ing f r o m o c e a n i t e s [12] (which p e r h a p s should be t e r m e d tholeiites [13]) to alkalic lavas, with the l a t t e r v e r y dominant. Hausen r14] has r e c e n t l y published a s u m m a r y and p r o g r e s s r e p o r t of the l i m i t e d geological i n v e s t i g a t i o n s of La Gomera. Here, as on La P a l m a [15, 16], G r a n C a n a r i a [10], and F u e r t a v e n t u r a [8], in l i m i t e d a r e a s an old " b a s e m e n t complex" is exposed, beneath a m a r k e d u n c o n f o r m i t y . This " b a s e m e n t " is c h a r a c t e r i z e d in p a r t by u l t r a b a s i c and other i n t r u sives, v a r i o u s lower grades of m e t a m o r p h i s m , s t r u c t u r a l d e f o r m a t i o n , and p a r t i c u l a r l y on La G o m e r a , by a v e r y high d e g r e e of weathering. No
such b a s e m e n t is found on H i e r r o [17], L a n z a rote [9], o r T e n e r i f e [18], which a r e n e v e r t h e l e s s b r o a d l y s i m i l a r to the other i s l a n d s , in that the d o m i n a t i n g f e a t u r e s a r e great s e r i e s of lavas, with m i n o r i n t r u s i v e s , and with the exception of G o m e r a [14], adventive volcanic cones with Recent and s o m e t i m e s H i s t o r i c [19] lavas. B l u m e n thal [20] and o t h e r s have attempted i n t e r - i s l a n d c o r r e l a t i o n s , but without quantitative methods the i n h e r e n t difficulties a r e e n o r m o u s . The r e g i o n a l s e t t i n g of the C a n a r y I s l a n d s (fig. 1) is such that a genetic r e l a t i o n s h i p with the extensive tectonic activity [21, 22] of the Atlas Mountains region in Morocco has been suggested by s e v e r a l a u t h o r s [23, 24]. The i s l a n d s may be s t r u c t u r a l l y independent of the c o n t i n e n tal geological f e a t u r e s , and in fact may be p u r e l y oceanic in c h a r a c t e r if, for example, L e M a i t r e ' s [25] o b s e r v a t i o n of tholeiitic xenoliths in volcanic ejecta on L a n z a r o t e could be shown to be c o n s i s t e n t with the ideas of Engel et al. [26] conc e r n i n g a d i s t i n g u i s h i n g c h a r a c t e r i s t i c between oceanic and continental b a s a l t s .
PALAEOMAGNETISM OF THE MACARONESIAN INSULAR REGION T h e g e o l o g i c a l a g e of the i s l a n d s has b e e n the s u b j e c t of m u c h c o n j e c t u r e . A C r e t a c e o u s e c h i n oid Discoidea pulvinata found on H i e r r o [27] w a s l a t e r e s t a b l i s h e d [28] a s b e i n g a s s o c i a t e d with i m p o r t e d l i m e - k i l n m a t e r i a l w h i c h G a g e l [29] s p e c u l a t e d to h a v e o r i g i n a t e d f r o m F u e r t a v e n t u r a . L i m i t e d p a l a e o n t o l o g i c a l e v i d e n c e [30] and o t h e r c o n s i d e r a t i o n s [31, 32] s u c h a s r e l a t i v e d e g r e e of w e a t h e r i n g , s u g g e s t that m a n y of the l a v a s s a m p l e d m a y be a s old a s M i o c e n e . An e a r l y T e r t i a r y age h a s b e e n a s s i g n e d but not on any sound b a s i s , to s o m e p a r t s of the i s l a n d s [33], and H a u s e n [34] s u s p e c t s that the s t r u c t u r a l l y d e f o r m e d b a s e m e n t c o m p l e x on F u e r t a v e n t u r a m a y be a s s o c i a t e d with H e r c y n i a n o r o g e n i c a c t i v i t y . No i s o t o p e a g e s h a v e b e e n p u b l i s h e d , and so in t h e i r p o o r l y d e f i n e d g e o l o g i c a l age the i s l a n d s e x h i b i t one of the c l a s s i c a l p r o b l e m s of v o l c a n i c t e r r a i n .
227
3. F I E L D AND L A B O R A T O R Y M E T H O D S U s i n g a g a s o l i n e - p o w e r e d p o r t a b l e d r i l l [35], two g e o g r a p h i c a l l y o r i e n t e d c o r e s h a v e b e e n c o l l e c t e d f r o m e a c h of 115 of the o l d e r l a v a s on H i e r r o , T e n e r i f e , and L a n z a r o t e ; and f r o m e a c h of 59 l a v a s w h i c h a r e s t r a t i g r a p h i c a l l y i m m e d i a t e l y a b o v e the b a s e m e n t c o m p l e x on G r a n C a n a r i a and L a G o m e r a . T h e b a s e m e n t c o m p l e x e s w e r e not s a m p l e d s i n c e w e a t h e r i n g , i n t r u s i o n , and m e t a m o r p h i s m would i n e v i t a b l y , p e r h a p s t o t a l l y , m o d i f y the o r i g i n a l n a t u r a l r e m a n e n t m a g n e t i s m (NRM); and in any c a s e , the s t r u c t u r a l d e f o r m a t i o n which e x i s t s cannot be r e l i a b l y r e l a t e d to the o r i g i n a l a t t i t u d e of the v o l c a n i c b o d i e s , and the c o r r e s p o n d i n g o r i g i n a l NRM d i r e c t i o n s . D e t a i l e d d e s c r i p t i o n s of the s a m p l i n g s i t e s a r e not i n c l u d e d h e r e , but b r i e f m e n t i o n of the s i t e s i s i n c l u d e d in t a b l e 1, t o g e t h e r with the
Table 1 Sample locations, number of lavas sampled, and results of statistical analyses of NRM for each lava after treatment in 150 oersted alternating magnetic field. N = number of lavas sampled. D and I = mean delineation and inclination of NRM; declination angle is east of north; inclination angle is with respect to and below the horizontal. R = resultant vector assigning unit vector to each lava. k = precision parameter (= N-l/N-R). ol95 = s e m i - v e r t i c a l angle of 95% confidence cone. q~ and 0' = longitude and latitude of virtual geomagnetic pole in degrees. 6p and 5 m = semi-axes of oval of confidence about the virtual geomagnetic pole. Island
Sample locations *
Hierro
Various parts of the major scarp above E1 Golfo. Roadcut between Valverda and Alto de Malposa.
Gomera
Parts of the ~Alkali Basalt Fmt." and "Phonolite-Trachyte Fmt. ~ west and northwest of San Sebastian.
Tenerife
T h e "Ancient Basalt Formation" of Anagn north of San Andres, and in Ladera de Guimar. The "Phonolites" north of Vilaflor, and bei tween Granadilla and Ville I de Aria.
Gran Canaria
Lanzarote
(deg) 6.9
45.6 I 31.149 I 17.29
77.~ I 83.9 I 5.0
1
46
3.4
7.09
18
10.9
2
5
.
34.6
36.367 2~6.66-
* Stratigraphic t e r m s when used are those due to Hausen [7-10, 14, 17].
I
7.9
~
~135.2 I 83.4 I 4.1
"Plagioclase Basalts" between Aldea de San Nicolas and Agaete. "Olivine Basalts ~ beneath Montana Del Borno, towards Mogan. Some "Nepheline Phonolites N near Tejada. Confined to the "Basaltic : Tableland Series" on the Punta Fariones scarp and on a roadcut west of Arrieta.
6.2
(deg)
l- 6.81
9.3
10'9.4
78.5 I 6.4 ] 10.9 I
5.0
119.7
75.9
_1
3.3
5.7]
1
228
N . D . WATKINS, A. RICHARDSON, R. G. MASON N
N
i
W
E
Gomera
Hierro
Fig. 2. D i r e c t i o n of NRM in all s a m p l e d l a v a s f r o m H i e r r o a n d G o m e r a , a f t e r t r e a t m e n t in a 150 o e r s t e d a l t e r n a t i n g m a g n e t i c field. Q = u p p e r h e m i s p h e r e ~ ® = l o w e r h e m i s p h e r e . [~-- d i r e c t i o n of p r e s e n t g e o m a g n e t i c field. N = g e o graphic north. Projection is equal area.
N
N
W
E
O
O ~
W
E
~O O° J Tenerife
Gran Canaria
Fig. 3. D i r e c t i o n of NRM in a l l s a m p l e d l a v a s f r o m T e n e r i f e a n d G r a n C a n a r i a , a f t e r t r e a t m e n t in a 150 o e r s t e d a l t e r n a t i n g m a g n e t i c field. ® -- u p p e r h e m i s p h e r e . ® = l o w e r h e m i s p h e r e . [ ] = d i r e c t i o n of p r e s e n t g e o m a g n e t i c field. N -- g e o g r a p h i c n o r t h . P r o j e c t i o n is e q u a l a r e a .
PALAEOMAGNETISM OF THE MACARONESIAN INSULAR REGION n u m b e r of lavas sampled. The NRM of s p e c i m e n s from each lava were examined u s i n g an astatic m a g n e t o m e t e r and an a l t e r n a t i n g magnetic field d e m a g n e t i z i n g a p p a r a tus. 4. RESULTS One s p e c i m e n in ten was subjected to d e m a g n e t i z i n g e x p e r i m e n t s in p r o g r e s s i v e l y i n c r e a s i n g magnetic fields r a n g i n g from 25 to 300 o e r s t e d s , in o r d e r to a r r i v e at a single a l t e r n a t i n g m a g netic field value for application to all s p e c i m e n s t h e r e b y m i n i m i z i n g s e c o n d a r y magnetic compon e n t s added since the i n i t i a l cooling of the lavas. A m a g n e t i c field of 1 5 0 o e r s t e d s has been applied to s p e c i m e n s f r o m at l e a s t one core from each lava sampled, and the m e a s u r e d NRM d i r e c t i o n after this t r e a t m e n t is plotted for each lava in figs. 2-4. F i s h e r ' s [36] s t a t i s t i c a l methods have been applied to the data, and the r e s u l t s a r e p r e s e n t e d in table 1. R e v e r s e d and n o r m a l p o l a r i t y NRM were combined for the s t a t i s t i c a l a n a l y s i s , after c a l c u l a t i n g the v i r t u a l geomagnetic pole [37] for each lava. Those NRM d i r e c t i o n s yielding p a l a e o -
N
)E Lanzarote Fig. 4. Direction of NRM in all sampled lavas from Lanzarote, after treatment in a 150 oersted alternating magnetic field. (D= upper hemisphere. ®-- lower hemisphere. [~-- directionof present geomagnetic field. N -geographic north. Projection is equal area.
229
m a g n e t i c a l l y s o u t h e r n latitudes have b e e n t r a n s posed to the n o r t h e r n h e m i s p h e r e for the a n a l y sis. F o r each island, the v i r t u a l geomagnetic pole, which is the s u r f a c e e x p r e s s i o n of the hypothetical geocentric dipole giving r i s e to the m e a n NRM d i r e c t i o n for the island, is p r e s e n t e d in fig. 3, together with the a s s o c i a t e d 95% confidence ovals [36]. The g e n e r a l i z e d p o l a r - w a n d e r ing c u r v e for Europe [6, 38] is included in fig. 3. This c u r v e r e p r e s e n t s an a v e r a g e o b s e r v a t i o n of much data, and although it may be debatable whether o r not the geographic pole follows this curve, the curve does r e p r e s e n t an e x p r e s s i o n of the a n c i e n t geomagnetic field behaviour viewed from Europe. 5. DISCUSSION It can be seen that both normally and reversely magnetized lavas were encountered on each island (figs. 2-4) except Hierro. With the exception of this westernmost island, therefore, on each island surveyed there exists magnetic polarity variation of high stratigraphic value, since time dependent magnetic polarity contrasts can be obtained. In the event that isotope age determination methods could be applied, the polarity data could assist considerably in improving the quality of the results. The following five significant points emerge from examination of the data illustrated in fig. 5: (a) By inspection of the position of its virtual geomagnetic pole on the previously defined polar wandering curve, Hierro is implied to be the youngest island examined. This suggestion is strongly supported by the polarity data for Hierro (fig. 2), since only normal polarity lavas were defined despite the wide stratigraphic coverage of the collection. Normal magnetic polarity with NRM directions close to those shown in fig. 2 were obtained from Recent dikes and lavas which were also sampled. It is tentatively proposed that the exposed lavas on Hierro were extruded during the Brunhes (Nl) geomagnetic polarity epoch, as defined by Cox et al. [I]. This inferred age of less than 0.7 m.y. [39] for the exposed parts of the island corresponds to conditions on the island of Hawaii, as initially defined by the palaeomagnetic survey by Doell and Cox [40] and by later isotope dating [41]. (b) The position of the virtual geomagnetic pole for Lanzarote, shown in fig. 5, is, at the 95% probability level, different from the corresponding property for Hierro; and by association with the polar wandering curve, as discussed above, the sampled lavas on Lanzarote are much
230
N.D. WATKINS, A. RICHARDSON, R. G. MASON 180 °
90 o
0o
Fig. 5. Virtual geomagnetic poles and 95~ confidence ovals for each island. The virtual geomagnetic poles are indicated by s t a r s . The confidence ovals with solid lines are for islands in which the oldest exposed lavas were s a m pled. For islands in which the oldest lavas were not sampled (see text) the confidence oval is r e p r e s e n t e d by a dashed line. The E u r o p e a n - p o l a r - w a n d e r i n g curve (refs. [38, 39]) is shown by the heavy line, with the approximate age of various p a r t s of the curve indicated as follows: P e r m i a n (P), T r i a s s i c (Tr), Cretaceous {K), Eocene (Te). The p r e s e n t geomagnetic pole position is indicated by the heavy c r o s s .
o l d e r t h a n t h o s e s a m p l e d on H i e r r o . Fig. 5 a l s o i l l u s t r a t e s t h a t it i s r e a s o n a b l e to a s s i g n a n a g e to t h e l a v a s s a m p l e d on T e n e r i f e w h i c h i s i n t e r m e d i a t e b e t w e e n t h e a g e of t h e l a v a s s a m p l e d on H i e r r o a n d L a n z a r o t e . T h e d o m i n a n c e of r e v e r s e l y m a g n e t i z e d l a v a s in T e n e r i f e , r e f l e c t i n g a reversed geomagnetic polarity epoch, supports t h i s i n f e r r e d r e l a t i v e age. In L a n z a r o t e , the dominantly normal polarity lavas underlie reversely magnetized lavas at the highest point in the island. (c) W i d e s t a t i s t i c a l l i m i t s to t h e c a l c u l a t e d virtual geomagnetic poles for the sampled lavas on La Gomera and Gran Canaria prevent any rel a t i v e a g e s b e i n g a s s i g n e d to t h e s e i s l a n d s o t h e r
than that they are older than Hierro, this conclus i o n b e i n g b a s e d on t h e e x i s t e n c e of l a v a s of r e v e r s e d p o l a r i t y on L a G o m e r a a n d G r a n C a n a r i a , a s s h o w n in f i g s . 3 a n d 4, r e s p e c t i v e l y . (d) By c o m p a r i s o n w i t h d a t a f r o m p a l a e o m a g n e t i c s u r v e y s of T e r t i a r y r o c k s e l s e w h e r e [42] (and s p e c i f i c a l l y f r o m E o c e n e r o c k s , w h i c h y i e l d an average virtual geomagnetic pole as indicated in fig. 5), it s e e m ~ r e a s o n a b l e to p r o p o s e t h a t t h e l a v a s s a m p l e d a r e n o t on a v e r a g e o l d e r t h a n E o c e n e s i n c e it i s h i g h l y l i k e l y t h a t E o c e n e p a l a e o m a g n e t i c d i r e c t i o n s w o u l d not h a v e g o n e u n . d e t e c t e d , to r e s u l t in a t l e a s t s o m e a g r e e m e n t with independent data. (e) T h e c l o s e c o r r e s p o n d e n c e between the
PALAEOMAGNETISM OF THE MACARONESIAN INSULAR REGION European polar wandering curve and the Canary I s l a n d s v i r t u a l g e o m a g n e t i c p o l e s (fig. 3) s u g g e s t s t h a t no s i g n i f i c a n t m o v e m e n t h a s o c c u r r e d between The Canary Islands and Europe since the sampled lavas were extruded. A s d i s c u s s e d a b o v e , i n t h e c a s e of t h e t h r e e islands on which the oldest lavas were sampled (Hierro, Tenerife, and Lanzarote), an increase i n a g e of o u t c r o p f r o m w e s t to e a s t i s i n f e r r e d by the palaeomagnetic data. The La Gomera and Gran Canaria samples are older than those coll e c t e d f r o m H i e r r o , b u t t h e i r a g e r e l a t i v e to t h e s a m p l e s f r o m t h e o t h e r i s l a n d s i s not known. From these results, it can be stated that the data a c q u i r e d s o f a r i s not i n c o n f l i c t w i t h H e s s ' s [3] spreading oceanic floor hypothesis, in which the Atlantic floor is envisaged as spreading away f r o m t h e m i d - A t l a n t i c r i d g e . It i s s t r e s s e d , of c o u r s e , t h a t n e i t h e r do t h e d a t a s u b s t a n t i a l l y c o n t r i b u t e to r e s o l u t i o n of t h e h y p o t h e s i s s i n c e other conditions can be envisaged by which an i n c r e a s e i n a g e of v o l c a n i c a c t i v i t y in a g i v e n d i rection could occur without invoking an oceanic s p r e a d i n g of t h e t y p e a c c o m m o d a t i n g H e s s ' s m o d e l . T h i s p a r t i c u l a r a s p e c t of t h e d a t a p r e s e n t e d h e r e i s m e r e l y i n t e n d e d to b e i n t e g r a t e d with results from appropriate independent methods which may be applied in the future.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
ACKNOWLEDGEMENTS [31] T h e f i e l d w o r k w a s s u p p o r t e d by t h e D e p a r t m e n t of G e o p h y s i c s , I m p e r i a l C o l l e g e of S c i e n c e a n d T e c h n o l o g y , L o n d o n , w h i l e o n e of u s (N.D.W.) was avisiting Leverhulme fellow at the Chadwick L a b o r a t o r y , U n i v e r s i t y of L i v e r p o o l , E n g l a n d . D.S.I.R. a r e g r a t e f u l l y a c k n o w l e d g e d f o r t h e s u p p o r t of A.R.
[32] [33] [34] [35] [36] [37] [38]
REFERENCES [39] [1] R.R. Doell, G. B. Dalrymple and A. Cox, J. Geophys. Res. 71 (1966) 531. [2] S. Uyeda, Japan, J. Geophys. 2 (1958) 1. [3] H. H. Hess, History of Ocean Basins in Petrologic Studies: A Volume to Honor A. F. Buddington (Geol. Soc. America, 1962) p. 599.
[40] [41] [42]
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J. T. Wilson, Nature 197 (1963) 536. E. Irving and A. Major. Sedimentology 3 (1964) 135. R.R.Doell and A.Cox, Advan. Geophys. 8 (1961) 298. H. Hausen, Soc. Scien. Fenn. Comm. Phys. Math. 18 (1956) 270. H. Hausen, ibid. 22 (1958) 211. H. Hausen, ibid. 23 (1959) 116. H. Hausen. ibid. 27 (1962) 418. H. Hausen, ibid. 27 (1962) 349. H. Hausen, ibid. 18 (1956) 200. G. A. MacDonald, J. Pet. 5 (1964) 88. H. Hausen, Acta Geog. (Helsinki) 18 {1965) 15. H. Hausen. ref. [8], p. 197. C. Gagel, Z. Deutsch. Geol. Gesel. 78 (1926) 556. H.Hausen, Ragos geologicos generales de la isla del Hierro. Patronato de la Casa de Colon (Madrid) 10 (1964) 547. H. Hausen, ref. [7], p. 37. H.Hausen, ref. [7]. p. 182. M. Blumenthal. Bol. Inst. Geol. Min. Espai~a 72 (1961) 1. C. Vidal Box. Las rocas eruptivas del Sahara E s panol. Tomo extraord. Real. Soc. Esp. de Hist. Nat. (1946). C. B u r r i and J. M a r c a i s , Reg. Mono. Ser. 3. Maroc. (6). XIX Int. Geol. Cong. Alger. Rabat. 1952. H. Hausen (summary), ref. [7], p. 14. H. Hausen (summary), ref. [9], p. 101. R.W. LeMaitre, Proc. Geol. Soc. London 1626 (1965) 143. A. E. J. Engel, C. G. Engel and R. G. Havens. Geol. Soc. Am. Bull. 76 (1965) 719. J. Cotteau and C. Lemoine, Bull. Soc. Geol. F r a n c e (1910) 267. L. F. Navarro, Compt. Rend. (1917) 165. C. Gagel, ref. [16], p. 564. H.Hausen, ref. [8], p. 27; ref.[9], p. 180; ref. [10], p. 30. H.Hausen, ref. [7], p. 30; ref. [8], pp. 193, 204-207: ref. [9], p. 101; ref. [10], pp. 27, 383. F. Macau Vilar, 20th Int. Geol. Cong. (Mexico), Sect. I (1957) 411. H. Hausen, ref. [8], p. 204. H. Hausen, ref. [9], p. 101. R. R. Doell and A. Cox, Geol. Surv. Bull. 1023-A (1960) 664. R.A. Fishe}~, Proc. Roy. Soc. London A217 (1953) 295. A. Cox and R.R. Doell, Bull. Geol. Soc. Am. 71 (1960) 664. K. M. C r e e r , E. Irving and S. K. Runcorn, Roy. Soc. London Phil. T r a n s . A250 (1957) 144. R. R. Doell and G. B. Dalrymple. Science 152 (1966) 1060. R. R. Doell and A. Cox. Nature 192 {1961) 645. I. McDougall, Bull. Geol. Soc. Am. 75 (1964) 107. E . I r v i n g , P a l e o m a g ~ e t i s m and its application to geological and geophysical p r o b l e m s (John Wiley and Sons, New York. 1964) p. 310.