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Control of granite emplacement by regional deformation
TECTONOPHYSICS Tectonophysics 249 (1995) 173-186
ELSEVIER
Control of granite emplacement by regional deformation Jean Louis Vigneresse
1
CREGU, 54501 Vandoeuvre / Nancy Cedex. France
Received 14 February 1994; accepted 11 January 1995
Abstract A combination of structural measurements and gravity data is used to investigate the mode of granite emplacement in various tectonic settings. The deepest zones revealed by gravity data, provided those zones also show vertical lineations in outcrop, are interpreted as magma feeder channels. These structures, or root zones, reflect the flow pattern of the upwelling magma while being emplaced. They can be interpreted within the regional stress field active during that time. Granites intruded during transcurrent shearing have only a few feeder channels. These roots are not connected to the main shear zone. All of them lie within locally extensional areas of the regional stress field. This situation is also observed in granites emplaced during a compressional phase of deformation. When emplaced during a regionally extensional phase, the granitic plutons are very thin, with many small deeper zones in which variously evolved granite types may be intrusive. These regions, vertically differentiated, are also interpreted as feeder channels. The different morphology at depth, as well as the difference in the number of root zones suggest that deformation controls the emplacement of granitic plutons. The root zones are oriented at a high angle to the direction of maximum compression when magma is emplaced in a plastic crust. If the pluton is emplaced higher in a brittle crust, then the roots are aligned with the maximum compression, according to the Andersonian theory of fracture. The orientation of the root zone along the direction of extension in a plastic crust may be explained by shearing which acts as a valve mechanism for segregating magma. In all the examples, the geometry of the root zone relative to the major fault plane implies that the shear zone does not induce magma generation and that it does not serve as a guide for magma emplacement.
1. Introduction C u r r e n t observations o f granites and s u r r o u n d i n g rocks describe the structure and shape of granitic bodies as observed in outcrop. Petrographic variations, structures or m i n e r a l i z a t i o n s are described, which m a i n l y reflect the very late history of the granitic pluton. E x a m i n a t i o n o f the s u r r o u n d i n g rocks
Also at ENS G6ologie, Nancy, France.
shows the t h e r m o - m e c h a n i c a l c o n s e q u e n c e s o f m a g m a emplacement. On a larger scale, the schistosity displays the influence o f m a g m a intrusion on the regional pattern o f deformation. Field information is restricted to the final stage of e m p l a c e m e n t and processes taking place at depth must be extrapolated. C o m b i n i n g structural and gravity data indicates how the m a g m a was intruded. Structural studies on granites are related to deformation caused by the m a g m a as it intrudes the surrounding crust. T h e y are c o m m o n l y restricted to surface observations, or in a few cases to structures eroded d o w n to 1 or 2 kin,
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still limited compared to the inferred thickness (approximately 5 kin) of granitic plutons (Vigneresse, 1988). Information on deeper features is derived from gravity data. The resolution of the gravity survey must be geared to a detailed investigation of the magma feeder channels. Both sets of data provide information only on the final stages of magma emplacement. They do not record the successive conditions of intruding magma, though the host rocks provide information about the regional stress field. This paper examines the morphology at depth as well as the position, shape and number of the root zones due to the stress field active at the time of magma emplacement. Most of the technical information on individual plutons has already been published and these discussions relate to the specific conditions of emplacement (Guillet et al., 1985; Guineberteau et al., 1987; Vigneresse, 1988; Audrain et al., 1989a,b; B r u n e t al., 1990; Aranguren et al., 1992). They concern mainly biotite or biotite-cordierite granites, the most evolved being two-mica granites. The conclusions are therefore restricted to magmas derived from the middle or lower crust and do not apply to alkaline granites, ring-dyke complexes, nor to mantle-derived magmas.
maps, granitic facies are contoured according to the total iron content of the magma which is deduced from the intensity of the magnetic susceptibility (Bouchez et al., 1990). It shows the chronology of each phase of emplacement. To detect deeper structures, geophysical data are needed (Vigneresse, 1990). Gravity surveys provide evidence for the shape at depth of the granitic bodies. Readings are taken at the same density (approximately 1 point/km 2) as that of the geological observations, in order that both sets of data can be correlated. Subsequent data treatment differentiates the anomaly generated by the granitic body from larger anomalies which could obscure its effects. Geological variations such as igneous facies changes must also be taken into account as they cause changes in density contrasts (Vigneresse, 1990). The precision of depth and shape ineasurements depends on the density of stations, as well as the intrinsic precision of the measurements and the accuracy of the density readings at depth. The estimated precision is about 15%, because of the loss of confidence in estimating the density values at depth. The floor is modeled with better precision, about 5%, since it reflects the homogeneity and the density of the readings.
2. Structure and shape of granites
3. Root zones in granites
Structural studies describe the present geometry of granitic bodies, from which conclusions are drawn on their emplacement into the upper crust, thus influencing the regional field of deformation. The stress field is recorded in the schistosity of the surrounding rocks (Hanmer and Vigneresse, 1980; Paterson et al., 1989) and in the relationship between internal granitic structures and regional deformation. Within the pluton structures are mapped by conventional means (Guillet et al., 1985; Guineberteau et al., 1987) or by measuring the anisotropy of the magnetic susceptibility (Guillet et al., 1983; Bouchez et al., 1990). These techniques are used to map the foliation and the lineation recorded by the first minerals crystallizing while the magma was still viscous. They determine the pattern of the magma flow, during the final stages of emplacement. On such
The combination of both structural and geophysical data aid the modeling of the mode of pluton emplacement (Guillet et al., 1985; Guineberteau et al., 1987; Audrain et al., 1989a,b; Brunct al., 1990). When erosion only affects the uppermost part of a pluton, the magmatic structures reflect the interaction between pluton emplacement and its surrounding rocks (Hanmer and Vigneresse, 1980). For a more deeply eroded pluton, zones where magmatic lineations are vertical, often correlate with the deepest zones computed from gravity data. Conversely, zones where lineations are gently dipping, often denote areas of shallow to medium depth. The deeper zones, if they show vertical lineations, are interpreted as magma feeder zones. They are apparently active during the time the pluton is being emplaccd and they are often reflected in outcrop by chemically
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m o r e e v o l v e d m a g m a or late intrusive facies. For instance in the C a b e z a de A r a y a c o m p l e x , Spain, a m o r e leucocratic m a g m a crops out o v e r the root zones, w h e r e a s the r e m a i n i n g part o f the pluton is mainly biotitic ( A m i c e and B o u c h e z , 1989; Audrain et al., 1989a). The location and g e o m e t r y o f the root zones are not i n f l u e n c e d by the nature o f the specific m a g m a associated with that area since changes in density induced by facies variations are taken into account during the inversion process (Vigneresse, 1990). T h e s e zones can be d e t e r m i n e d in cross sec-
tions drawn through a pluton, because the slope o f the floor o f the granitic pluton strongly increases when approaching the root zone. R o o t zones are located b e t w e e n 5 and 14 km depth and h a v e a cross-sectional area o f 1 to 10 km 2, c o m p a r e d to several hundreds o f square kilometers for the surface area o f a massif. The g e o m e t r y and orientation o f the roots p r o v i d e direct information on the t h e o l o g i c a l properties o f the crust during intrusion. T h e y reflect the lateral spreading o f the m a g m a and its e v o l u t i o n within the regional stress field.
°c,
0-1
Po ~
\.O"F_,~-~./.,. /
Gu
L_g Pab
0 :
5 0 km i
Fig. 1. Depth (in kin) of the floor of the granitic bodies in Brittany deduced from gravity data (modified from Vigneresse, 1983). The root zones trend (dashed line) at high angles with the major stress component (o-j) responsible for the main dextral fault zone. Granitic plutons quoted in the text: Lo = Locronan: Po = Pontivy-Rostrenen; Gu = Gu6henno; Lg = La Gacilly; Qu = Questembert; Pab = Pont I'Abb6; PIo = Ploeumeur; Gue = Gu6rande. The Mortagne (Mo) massif, quoted in the text and in Fig. 2, is 20 km to the southeast of this map. The cross section across the Pontivy pluton is shown in Fig. 7.
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J.L. Vigneresse / Tectonophysics 249 (1995) 173 186
!
Mo Qu Lg Gu
4. Granites emplaced during shear deformation Some granitic intrusions may be spatially associated with major strike-slip shear zones as in Brittany, western France (Vigneresse, 1983) or in Galicia, northwestern Spain (Courrioux, 1983; Aranguren and Tubia, 1989). In such cases, the shear plane is subvertical and the lineations associated with the shear are horizontal with the same strike as the fault zone. Such plutons have a moderate volume, less than 1500 km ~, and crop out at surface over a few hundred square kilometers (Vigneresse, 1988) (Fig. I). At outcrop level, their overall shape is elliptical, reflecting shear deformation. Their ellipticity, defined as the ratio of the minor to the major axis, is less than 0.8 in Brittany, with a small component of shortening ( 5 - 1 0 % ) superimposed on a dextral shear (Vigneresse and Brun, 1983). In Galicia, the shear is sinistral. Greater stretching yields a lower (0.4) ellipticity ratio lk~r the plutons. At the present surface level, the ellipticity ratio varies according to the shear intensity on a regional scale. Within granitic bodies, it decreases with depth (Fig. 2), down to 0.3 at 6 km in Brittany, but its gradient remains constant with depth on a regional scale. In those plutons emplaced during wrench-faulting, only one single root is observed, two in larger plutons. The floor plunges gently to 5 + 2 km, after which a sharp increase in the dip of the floor indicates the root zone which extends down to 6 - 1 2 km. The root zone is very small in size compared to the pluton itself. Its cross-sectional area represents only 1 to 5% of that of the whole pluton. Several geometrical patterns of the root relative to the shear zone are observed, depending if granites have either been emplaced within overlapping shear zones, or are adjacent to the shear zone. When magma intrudes anastomosing shear planes, the volume of granite is generally small and deformation may be enhanced (Hollister and Crawford, 1986). When the shear zone is wider or discontinuous, with en 6chelon relay faults, or under tensional stress, magma fills the space created by the opening of pull-apart-like structures (Guineberteau et al., 1987; Hutton et al., 1990; Tikoff and Teyssier, 1992). In this case the granitic body has steep walls controlled by the shear zone. The morphology has been detailed for the Mortagne massif in Brittany, France
/ /,s 2v o
/
8-
Z(km)
Po Lo
II II /,I /I
I
=t
o
O5
I
to B/A
Fig. 2. Diagram of the ellipticity ratio versus depth of granitic plutons in Brittany. The ellipticity ratio is defined as the ratio of minor axis B to major axis A. The pluton trends with an angle /3 relative to the shear direction. Depth contours have been obtained from gravity data. Captions referring to granitic plutons are simi lar to those for Fig. I. Generally plutons (Locronan to Mortagne) show a W-E-oriented decrease in their ellipticity ratio (B/A), but the gradient with depth is constant with an average value of 0.055 km l showing an increase of deformation with depth.
(Guineberteau et al., 1987). There, magma intruded while overlapping faults were active (Fig. 3). This situation is limited to transtensive environments which provide simultaneous shear and space due to the extensional component of the deformation. In the remaining cases, which are the majority, the root zone is always located off the major deformation plane, and no connection exists between them. The distance of the root from the maior shear plane does not correlate with any other parameter, such as the volume of erupted magma or shear intensity. Two situations exist, depending on the orientation of the root zone relative to the major stress component (o-1). In the first, which is more fl'equent, the roots are at a high angle with %. For instance in Brittany (Fig. 1), the roots strike northeast relative to a dextral shear zone oriented N I l 0 °. According to the regional stress field, the roots are at a high angle to the major stress component, and are aligned with the direction of tensional stress. In the second case, the roots are aligned with o-I, as to be expected in the case of brittle fracture (Delaney et al., 1986). This is particularly obvious in the Guitiriz massif, Galicia,
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Spain (Aranguren and Tubia, 1989; Aranguren et al., 1992). There (Fig. 4), two disconnected diverging shear zones lead to the northward expulsion of a piece of crust into which the granitic pluton of Guitiriz intruded. According to the shear direction along both faults, the major stress component is E - W oriented. In spite of the pronounced N-S-oriented structural pattern (Aranguren and Tubia, 1989) within the Guitiriz massif, gravity data reveal two root zones trending N100 ° (Aranguren et al., 1992). The granitic root zones are always situated in locally extensional areas relative to the regional field of detbrmation. This is observed in single plutons (Brittany, Galicia) or even at a larger scale. For instance, the intrusion pattern of Proterozoic granites in northeastern Brazil clearly relates to E-W-oriented dextral shear zones (Archanjo et al., 1992). Though some of these granites show an intense
deformation and reorientation due to shearing, all plutons are oriented along N045°-trending structures, within the extensional direction as to be expected in a plastic crust. This does not conform to the Andersonian theory of fracture which would predict N135°-oriented tension gashes. On a larger scale, plutons were intruded within local extensional areas of the stress field. In most plutons emplaced during shearing, conjugate shear zones affect the granitic pluton along its borders. Gravity data do not indicate a connection between the root zones and the deformation planes, as shown, for instance, in Brittany or in Galicia (Figs. 1 and 3). This precludes a direct relationship between the zone of intense deformation and the root zones. However, those fracture planes show well-developed mylonites with low-temperature C-S structures and broken quartz attesting that intense defor-
granite outcrop and faults- - %.._ -. . ~ ~ - ...
Lineations ..... > ®
0-45* 45-90 o
Depth
contours
,x
I L ~
10
km
.
in km
,
~~
Fig. 3. Map of the granitic floor and corresponding lineations in the Mortagne pluton, Brittany redrawn after Guineberteau et al. (1987). The floor is deep ( > 5 km) for most of the pluton. Clearly, the steeply dipping walls are controlled by adjacent faults and magma intruded into a pull-apart structure induced by the shear on its walls.
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1
Fig. 4. Map of the Guitiriz granite, Spain. The pluton is situated between two shear zones and shows three main |acies, the limits of which are indicated by broken lines. The depth of its floor, interpreted from gravity data, is contoured at I-kin intervals. Grey arrows indicate the magmatic lineations (from Aranguren et al., 1992), the length of which is inversely proportional to their plunge. The crustal block, in between the faults, has been displaced northward as shown by the open arrow by strong E-W-trending compression. Two shallow root zones, parallel to o-1+trend east-west as to be expected in a brittle fracturing crust. The distribution of the lineations and the root zones clearly show that both types of infi~rmation are needed to determine the mode of emplacement of a pluton.
mation continues after the emplacement of granites (Hanmer and Vigneresse, 1980; Paterson and Fowler, 1993).
5. Plutons emplaced during a compressional phase Granites are also emplaced in regions undergoing compression, although popular opinion correlates granitic emplacement with extension. Examples of such situations have been reported in Australia (Doctors Flat, Bega), in the United States (Boulder and Tobacco Root plutons, Piute Mountain), in Ireland (Ox Mountains), and in France as well (Flamanville, Huelgoat) (Vigneresse, 1988; Burg and Wilson, 1988; Schmidt et ~d., 1990; B r u n e t al., 1990; Morand, 1992; McCaffrey, 1992; Karlstrom et al., 1993). In spite of the evidence of thrusts or large-scale folding, the plutons are always emplaced in locally extensional fractures or within the axial fold planes. Plutons are generally small, less than 200 km 3 in
volume and have only one root. The Flamanville granite (Brunet al., 1990) presents a small outcrop (7.5 × 4.5 km) and a limited thickness (3 kin). The asymmetrical shape of the outcrop (Fig. 5) also reflects the eastwardly displaced root, the emplacement of which is controlled by the plunge of the Cambrian arkoses. Cleavage and foliation envelop the pluton, showing a strain-free triple point northeast of the pluton. Stretch lineations are restricted to the northern and southern surroundings of the western part of the pluton, thus indicating the swelling of the magma which also created folds in the western Devonian series. The Huelgoat pluton, in western Brittany, also has a small volume, with one root, right at the center (Vigneresse, 1988). Because of the differences between the distal and the proximal field of deformation, granites intrusive during regional compression do have the adequate morphology (one root) whereas their emplacement is controlled by local extension. For instance, the Australian plutons all present a N-S-striking foliation related to E - W -
J.L. Vigneresse / Tectonophysics 249 (1995) 173-186
179
l
S3
_ S4
t
_ S5
"~
S2 s)
/
/
-OO,o - . . . ~ - . . ~
0
1
/ /
ba
i
2
..I
I
•
I
~ . . , . - - -- ~ "" ""
X b3
//
S'
/ /J -
/
1 km
Fig. 5. Map of the Flamanville granitic intrusion (redrawn after Bran et al., 1990). The limits of the outcrops, in large dots for the granite, and dashes for the Paleozoic series (xb3=Brioverian; b a = l o w e r Cambrian; s2 = L o w e r Ordovician; s3-s4-s5=Middle-Upper Ordovician, Silurian; d = Devonian axial plane). The depth contours are shown at l-km intervals. Lineations measured in the surrounding rocks are indicated by arrows.
oriented shortening (Burg and Wilson, 1988; Morand, 1992) but they all lie within locally extensional areas, as shown by their bulk orientation.
6. Piutons emplaced during extensional deformation Granites may also be intrusive during an extensional phase, or at the onset of extension due to the collapse of a large orogenic structure such as the Saint Sylvestre complex in the French Massif Cen-
tral (Audrain et al., 1989b), Pont l'Abb6 and Gu6rande (Fig. 1) in the southern French Armorican Massif (Vigneresse, 1983), or the Mykonos pluton in Greece (Faure and Bonneau, 1988). These plutons are very thin, with subhorizontal foliation planes (Mollier and Bouchez, 1982). Their volume, about 1000-1500 km 3, is comparable to that of plutons emplaced during shear. The average thickness may be as little as 2.6 km (Fig. 6) for Saint Sylvestre (Audrain et al., 1989b; Vasseur et al., 1990). The floor remains generally flat, but locally deep roots, up to 5 - 6 km, are observed in vertical cross sections
J.L. Vigneresse / Tectonophysics 249 (1995) 173-186
180
(Fig. 7). In the field, these small features, less than 5% of the whole unit, show vertical lineations. They are also interpreted as magma feeder zones. Mag-
i,
I/
i
matic activity persisted in these areas, as shown by fine-grained intrusives (Audrain et al., 1989b) or variously evolved magmas in outcrop, but deforma-
..tl
i
,
i
tzTO
I CJ,
)
;-:
150
:AO
I20
110
-
.",i
: 510
-
, 520
,550
54 0
Fig. 6. Map of the floor depth (in km) of the Saint Sylvestre granitic complex (French Massif Central) calculated from gravity data (from Audrain et al., 1989b). Note the overall shallow depth of the floor and the great number of small areas with a pronounced deepening of the floor which are interpreted as root zones. FN (Nantiat fault) and FAO (Ar~nes-Ouzilly fault) are ductile faults which were still active during the emplacement of the pluton.
J.L. Vigneresse / Tecwnophysics 249 (1995) 173-186
181
NW
SE
J
J
IO km
W
E
Fig. 7. (a) Cross section of the Pontivy pluton (see also Fig. 1), two distinct roots (dotted), one for each lobe of the complex. (b) Cross section of the Saint Sylvestrepluton (Fig. 6), a very thin main facies, in which vertical features (dotted) are intrusive.The foliation(adapted from Mollier and Bouchez, 1982) is also shown. The steep lines correspond to a ductile fault (FN in Fig. 6) within the pluton. In both cross sections, vertical and horizontal scales are identical.
tion of the host magma indicates temperatures above 300-400°C. Most of these small roots are not randomly oriented as demonstrated by their overall pattern. In the Saint Sylvestre massif, the late intrusives as well as the deeper zones are controlled by N020 °and Nl20°-oriented structures (Audrain et al., 1989b; Cuney et al., 1990). The stress field at that time involved a vertical shortening and a m a x i m u m horizontal stretching oriented towards the northwestsoutheast, showing that the root zones are in locally extensional areas. Ductile faults may locally affect the boundary of the granitic body (Mollier and Bouchez, 1982; Audrain et al., 1989b) offsetting its
floor, thus indicating that emplacement was near the brittle/ductile zone.
7. Thickness and number of roots The morphology at depth of granites is controlled by the regional stress field, but the plutons remain very thin (Fig. 8). Those plutons intrusive during a shear or a compressional phase are approximately 5 _ 2 km thick, in contrast with the very thin plutons (approximately 2 - 3 km) intrusive during an extensional phase. Such a difference is obvious from gravity data, although this is not unique and essen-
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--1
Extension A
A'
1
A,
A
Shear B
B'
c
~~ ~
,
B
Overlapping shear
B,
C~ - " ~ ~ ®
C'
c-Fig. 8. Schematic morphology of granitic plutons according to the regional field of deformation with dotted areas showing the root zones. The figures are highly simplified. The examples given are Saint Sylvestre (Fig. 5), Pontivy (Fig. 1) and Mortagne (Fig. 3). A map (on the left) and a cross section (on the right) with no vertical exaggeration represent the deformation processes described in the text: A = extension; B = wrench faulting; C = open fracturing in between overlapping shear zones. tially ambiguous. In the present case, the shape of the body which creates the gravity low is determined by its outcrop. Densities are measured from surface samples to incorporate facies changes into the inversion process, so each point of computation has its own density. Under those conditions, the range of acceptable solutions is narrow (Vigneresse, 1990). Input data errors (measurements, correction, density) are lower than 6%, because of the accuracy of measurements. This provides about 15% of the total estimate of the errors, after construction of the model, estimating that density may vary with depth or that an unsuspected body lies under the pluton. In no way does it explain the difference in thickness calculated in most cases (Brittany, Germany, Spain) compared to that of the plutons from the French Massif Central (Saint-Sylvestre, La Marche). Difference in the number of roots (Fig. 8) is also justified. The floor roughness depends partly on its
depth, and moreover on the surface distribution of measurements. Closer-spaced stations yield more detailed lateral resolution at depth. For measurements made in Spain or France, the coverage is about 1 p o i n t / k m 2 (760 p t / 1 0 0 0 km 2 for Cabeza de Araya, 248 p t / 3 4 km 2 for Flamanville), which compares to those in the French Massif Central (2241 p t / 2 0 2 5 km 2 in Saint-Sylvestre, 1983 p t / 1 8 0 0 km 2 in La Marche). The resolution at depth is quite similar and cannot explain the discrepancy between one root for the whole pluton as often observed and the many deep structures in those plutons emplaced at the onset of the extension in the French Massif Central.
8. G e o m e t r y o f the root zones
Whatever the phase of deformation, the granitic roots are within the local (near field) extensional
J.L. Vigneresse / Tectonophysics 249 (1995) 173-186
areas of the regional (far) field of deformation. This situation is easily understandable as soon as magma penetrates the upper brittle crust. The strength required to break the brittle crust is about four times greater under compression than under extension. Consequently, the easiest place for magma to intrude the crust in a mechanical sense is within regions with extensional stress. This agrees with observations, either in Brittany, in Galicia, or in plutons emplaced during transtension (Guineberteau et al., 1987; Hutton et al., 1990; Tikoff and Teyssier, 1992). In addition, the intrusion of magma in extensional areas of a deforming crust partly solves the dilemma of the so-called "space problem" which has puzzled the geological community for years (Bowen, 1948). If deformation controls the emplacement of granitic magma, then it will be tempting to relate its origin to deformation. Melting as a consequence of shear has been suggested in several cases (Strong and Hanmer, 1981). Melting results from the interaction of deformation and heat. It requires an important rheological contrast, for instance between a rigid crust and a soft mantle (Fleitout and Froidevaux, 1980). However, the process is inadequate to produce granitic plutons. One reason is the low volume of magma generated (approximately 1500 km3; Brun and Cobbold, 1980). The other reason is the lack of correlation between the root zone and the zone of major deformation (Vigneresse, 1983). The hypothesis of shear melting leading to plutons is therefore rejected. The root zone is not randomly oriented relative to the stress field. In all cases, the number of the roots and their distribution within the regional stress field emphasize the active role played by the deformation during the upwelling of magma and its emplacement (Pitcher, 1979). Geometrically, two extreme situations are observed. One, the less frequent, corresponds to roots aligned with the major stress component (O-l). This geometry is in accordance with the Andersonian theory of brittle fracture (Delaney et al., 1986). This is, for instance, the case in the Guitiriz massif, Spain. There (Fig. 3), the E-W-trending roots are parallel to 001- The entire block in which the granite is emplaced is shifted to the north. This is also the case with the Flamanville granite, which is known to have been emplaced at a very shallow crustal level. The geometry reflects an intrusion into
183
a brittle crust. In the other and far more common cases (see, for instance, Fig. 1), the roots trend at a high angle with o-~ and are aligned with extension. This geometry indicates a plastically deforming crust. Three models can explain this geometry: In the first model, the geometry of the root zones reflects a variation with depth of the classical distribution of fractures (Delaney et ai., 1986). Two hypotheses can be advanced. In the first, the root zone has the expected trend (near to o-~) at depth, but is too small ( < 500 m in radius) and too deep ( > 4 km) to be detected by gravity. In the second hypothesis, the actual shape of the root zone evolved from that at the onset of magma intrusion. An initial fracture was created by magma overpressure with the expected trend near to o'~, but the hot magma flow inflated the fracture and heated it up, thus interfering with the regional field of deformation. The conduit is reoriented because of the reduced crustal strength due to the heating, even if regional stresses do not vary (Reches and Fink, 1988). According to that hypothesis, the conduit is plastically deformed to its final elliptical shape due to regional shear. Under such conditions, the strain increases 'with depth, reflecting the deformation of the shape of the conduit. This hypothesis might explain the increase of the ellipticity ratio (Fig. 2) with depth deduced from the shape of the pluton (Vigneresse and Brun, 1983). An alternative to this model considers the geometry as a result of fractures opening under fluid pressure (Sibson et al., 1988). In Anderson's classical theory of rupture, shear failure occurs along planes that contain the intermediate stress component (0°2) and are oriented at 25-30 ° to the maximum principal stress component (001). In a fluid-saturated crust, all stress values are reduced by the amount of the fluid pressure. If the latter is greater than the smallest component (003), then fractures open along planes at a high angle to 001 (Sibson et al., 1988). This model explains mesothermal gold-quartz deposits through repetitive fault valve behavior along reactivated faults. During several cycles of seismic activity, sudden fluid pressure fluctuations are due to the release of fluids which infiltrate open fractures along faults. Also observed in partly molten gneisses (Hand and Dirks, 1992), it may be transposed to magma migration in a plastically deforming crust. The third model considers the geometry as two
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J.L. Vigneresse / Tectonophysics 249 (1995) 173 186
extreme cases o f intrusion relative to the deformation phase. In a brittle material, root zones initiate and remain near to cr~ as expected. Conversely, when magma intrudes a plastic crust, a fracture opens near to o.j and shear reorients it towards the direction of extension at a high angle to o.t- The root geometry would reflect the temperature controlled deformation of the crust. From the three proposed models, the first is the least probable, because it requires external conditions. The second is more plausible, but is valid only if enough fluid pressure exists, which cannot be predicted. The third combined with the second appears to be more realistic. It fits the observations on granites intruded at different levels of the crust (Brittany, Massif Central, Spain...). It would provide criteria to explain the geometry of granite emplacement as a function of regional deformation. When granites intrude a crust in which high-temperature schistosity develops with a well-defined lineation (biotite, sillimanite), that is in a plastic crust, the root is at a high angle to o-1. Conversely, when granites intrude a colder, or brittle crust, its root is near to o.~ as expected (Delaney et al., 1986). Deformation controls the geometry of granitic emplacements. It is tempting to ask if granites can be intruded without deformation. All cases presented are crustal-derived granites, ranging from calcalkaline granites to leucogranites. Ring-dyke complexes or hyper alkaline granites, which may contain a large mantle-derived component, have not been surveyed. Though they are described as forceful intrusions, those granites are also associated with large-scale extension. I suggest therefore that all granites are emplaced while the crust is deforming. In that sense, they are all synkinematic (Karlstrom, 1989).
regional field of deformation. Their orientation corresponds, occasionally, to that expected during brittle fracture (near to o.~), but in most cases it is oriented at a high angle with the maximum stress component (o-i). Such a situation reflects magma intrusive in a plastically deforming crust. In all cases the number and disposition of the roots emphasize the role played by the regional deformation during the emplacement of magma. The location of the root zones in the case of granites associated with large wrench faults suggests that it is very unlikely that magma volume, as large as that of a granitic pluton, could only resuh from shear melting and emplacement within the fault zone. Deformation helps to transport magma, but does not generate it. It is also proposed that deformation plays an important role in determining the location of plutons.
9. Conclusions
References
Observations obtained from structural and gravity studies enable root zones to be identified beneath granitic plutons. They indicate that granites eraplaced during shear deformation or during compression have only one or a few roots, whilst granites emplaced during extensional deformation are thin with many root zones in which late melts are intrusive. In all cases, the root zones are located within the locally extensional stress field relative to the
Amice, M. and Bouchez, J.L., 1989. Susceptibilit6 magn6tique et zonation du batholithe granitique de Cabeza de Araya (Extremadura, Espagne). C.R. Acad. Sci. Paris, Ser. 2, 308: 1171-1178. Aranguren, A. and Tubia, J.M., 1989. Analisis estructural de los macizos graniticos de Puebla de Parga y Friol. Stud. Geol. Salmanticensia, 4: 17-26. Aranguren, A,, Tubia, J.M., Vigncrcssc, J.L. and Bouchez, J.L., 1992. Settlement of the granitic massif of Guitiriz, Spain. EOS, Trans. AGU, 73: 275.
Acknowledgements This paper is based on many gravity and structural surveys conducted over granitic areas with the help of students of the Universities of Bilbao, Montpellier, Miinchen, Nancy, Nantes, Rennes, Salamanca and Toulouse. They provided the data from which these ideas have been developed. Many constructive discussions with structural geologists have improved the reasoning. Special thanks to J.C. Bhattacharji, J.L. Bouchez, J.D. Clemens, B. Marsh, K.J.W. McCaffrey, A. Nicolas, W.M. Schwerdtner and J. Tullis who corrected the manuscript. Funding for the gravity surveys has been received through grants from the CNRS, INSU, DBT program, CREGU and from Cogema and Cisa, both uraniummining companies.
J.L. Vigneresse / Tecmnophysic~ 249 (1995) 173-186 Archanjo, C.J., Olivier P. and Bouchez, J.L., 1992. Plutons granitiques du Serido (Nordeste du Br~sil): Ecoulement magmatique parallble 5 la chaine relev6 par leur anisotropie magnEtique. Bull. Soc. GEol. Fr., 163: 509-520. Audrain, J., Amice, M., Vigneresse, J.L. and Bouchez, J.L., 1989a. GravimEtrie et gdomEtrie tri-dimensionnelle du pluton gl'anitique de Cabeza de Araya (ExtrEmadure, Espagne). C.R. Acad. Sci. Paris, Set. 2, 309: 1757-1764. Audrain, J., Vigneresse, J.L., Cuney, M. and Friedrich, M., 1989b. Modble gravimdtrique et raise en place du complexe granitique hyperalumineux de Saint Sylvestre (Massif Central fi'an~ais). C.R. Acad. Sci. Paris, Set. 2, 309: 1907-1914. Bouchez, J.L., Gleizes, G., Djouadi, T. and Rochette, P., 1990, Microstructure and magnetic susceptibility applied to emplacement kinematics of granites: the example of the Foix pluton (French Pyrenees). Yectonophysics, 184:157-171. Bowen, N.L., 1948. The granite problem and the method of multiple prejudices. In: J. Giluly et al. (Editors), Origin of Granite. Geol. Soc. Am. Mere., 28: 79-90. Brun, J.P. and Cobbold, P.R., 1980. Strain heating and thermal softening in continental shear zones. J. Struct. Geol., 2: 149158. Bran, J.P., Gapais, D., Cogn~, J.P., Ledru, P. and Vigneresse, J.L., 1990. The Flamanville granite (NW France): An unequivocal example of an expanding pluton. Geol. J., 25: 271-286. Burg, J.P. and Wilson, J.L.W., 1988. A kinematic analysis of the southernmost part of the Bega Batholith. Aust. J. Earth Sci., 35: 1-13. Courrioux, G., 1983. Exemple de raise en place d'un leucogranite pendant le fonctionnement d'une zone de cisaillement, le granite de Puentedeume (Galice Espagne). Bull. Soc. GEol. Fr., 25: 301-307. Cuney, M., Friedrich, M., Blumenfeld, P., Bourguignon, A., Boiron, M.C., Vigneresse, J.L. and Poty, B., 1990. Metallogenesis in the French part of the Variscan orogen, Part l: U preconcentration in the pre-Variscan formations. A comparison with Sn, W and Au. Tectonophysics, 177: 39-51. Delaney, P.T., Pollard, D.D., Ziony, J.l. and McKee, E.H., 1986. Field relations between dikes and joints: emplacement processes and paleostress analysis. J. Geophys. Res., 91: 49204938. Faure, M. and Bonneau, M., 1988. Donnres nouvelles sur I'extension ndog?:ne de I'Egde, la deformation ductile du granite miocene de Mykonos (Cyclades, Grace). C.R. Acad. Sci. Paris, Ser. 2, 307: 1553-1559. Fleitout, L. and Froidevaux, C., 1980. Thermal and mechanical evolution of shear zones. J. Struct. Geol., 2: 159-164. Guillet, P., Bouchez, J.L. and Wagner, J.J., 1983. Anisotropy of magnetic susceptibility and magmatic structures in the GuErande granite massif (France). Tectonics, 2: 419-429. Guillet, P., Bouchez, J.L. and Vigneresse, J.L., 1985. Le complexe granitique de Plouaret. Mise en Evidences structurale et gravimEtrique de diapirs emboitrs. Bull. Soc. GEol. Fr., 8: 503-513. Guineberteau, B., Bouchez, J.L. and Vigneresse, J.L., 1987. The Mortagne granite pluton (France) emplaced by pull apart along
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a shear zone: structural and gravimetric arguments and regional implication. Geol. Soc. Am. Bull., 99: 763-770. Hand, M. and Dirks, P.H.G.M., 1992. The influence of deformation of axial planar leucosomes and the segregation of small melt bodies within the migmatitic Napperby gneiss, Central Australia. J. Struct. Geol., 14: 591-604. Hanmer, S. and Vigneresse, J.L., 1980. Mise en place de diapirs syntectoniques dans la chaine hercynienne, exemple des massifs leucogranitiques de Locronan et de Pontivy. Bull. Soc. GEol. Fr., 22: 193-202. Hollister, L.S. and Crawford, M.L., 1986. Melt-enhanced deformation: a major tectonic process. Geology, 14: 558-561. Hutton, D.H.W., Dempster, T.J., Brown, P.E. and Decker, S.D., 1990. A new mechanism of granite emplacement: intrusion in active extensional shear zones. Nature, 343: 452-455. Karlstrom, K.E., 1989. Towards a syntectonic paradigm for granites. EOS Trans. Am. Geophys. Union, 70: 762-770. Karlstrom, K.E., Miller, C.F., Kingsbury, J.A. and Wooden, J.L., 1993. Pluton emplacement along an active ductile thrust zone, Piute Mountains, southeastern California: Interaction between deformational and solidification processes. Geol. Soc. Am. Bull., 105: 213-230. McCaffrey, K.J.W., 1992. Igneous emplacement in a transpressive shear zone: Ox Mountains igneous complex. J. Geol Soc. London, 149: 221-235. Mollier, B. and Bouchez, J.L., 1982. Structuration magmatique du complexe granitique de Brame-Saint-Sylvestre-Saint-Goussaud (Limousin, Massif Central Fran~ais). C.R. Acad. Sci. Paris, Ser. 2, 294: 1329-1333. Morand, V.J., 1992. Pluton emplacement in a strike-slip fault zone: the Doctors Flat pluton Victoria Australia. J. Struct. Geol., 14: 205-213. Paterson, S.C. and Fowler, T.K., 1993. Re-examining pluton emplacement mechanisms. J. Struct. Geol., 15:191-206. Paterson, S.C., Vernon, R.H. and Tobisch, O.T., 1989. A review of criteria for the identification of magmatic and tectonic foliations in granitoids. J. Struct. Geol., I I: 349-363. Pitcher, W.S., 1979. The nature ascent end emplacement of granitic magmas. J. Geol. Soc. London, 136: 627-662. Reches, Z. and Fink, J., 1988. The mechanism of intrusion of the Inyo dike, Long Valley, California. J. Geophys. Res., 93: 4321-4334. Schmidt, C.J., Smedes, J.W. and O'Neill, J.M., 1990. Syncompressional emplacement of the Boulder and Tobacco Root batholiths (Montana, USA) by pull-apart along the fault zones. Geol. J., 25: 305-318. Sibson, R.H., Robert, F. and Poulsen, K.H., 1988. High angle reverse faults, fluid-pressure cycling and mesothermal goldquartz deposits. Geology, 16:551-555. Strong, D.F. and Hanmer, S.K., 1981. The leucogranites of southern Brittany, origin by faulting, frictional heating, fluid flux and fractional melting. Can. Mineral., 19: 163-176. Tikoff, B. and Teyssier, C., 1992. Crustal scale, en Echelon "P-shear" tensional bridges: A possible solution to the batholithic room problem. Geology, 20: 927-930. Vasseur, G., Dupis, A., Gallart, J. and Robin, G., 1990 Donnres
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gEophysiques sur la structure du massif leucogranitique du Limousin. Bull. Soc. G6ol. Fr., 8(6): 3-11. Vigneresse, J.L., 1983. Enracinement des granites armoricains estimE d'aprbs la gravimEtrie. Bull. Soc. GEol. Mineral. Bretagne, 15C: 1-15. Vigneresse, J.L., 1988. Formes et volumes des plutons granitiques. Bull. Soc. GEol. Ft., 8: 897-906.
Vigneresse, J.L., 1990. Use and misuse of geophysical data to determine the shape at depth of granitic intrusions. Geol. J., 25: 249-260. Vigneresse, J.L. and Brun, J.P., 1983. Les leucogranites armoricains marqueurs de la deformation rEgionale: apport de la gravimEtrie. Bull. Soc. GEol. Fr., 25: 357-366.
ELSEVIER
Control of granite emplacement by regional deformation Jean Louis Vigneresse
1
CREGU, 54501 Vandoeuvre / Nancy Cedex. France
Received 14 February 1994; accepted 11 January 1995
Abstract A combination of structural measurements and gravity data is used to investigate the mode of granite emplacement in various tectonic settings. The deepest zones revealed by gravity data, provided those zones also show vertical lineations in outcrop, are interpreted as magma feeder channels. These structures, or root zones, reflect the flow pattern of the upwelling magma while being emplaced. They can be interpreted within the regional stress field active during that time. Granites intruded during transcurrent shearing have only a few feeder channels. These roots are not connected to the main shear zone. All of them lie within locally extensional areas of the regional stress field. This situation is also observed in granites emplaced during a compressional phase of deformation. When emplaced during a regionally extensional phase, the granitic plutons are very thin, with many small deeper zones in which variously evolved granite types may be intrusive. These regions, vertically differentiated, are also interpreted as feeder channels. The different morphology at depth, as well as the difference in the number of root zones suggest that deformation controls the emplacement of granitic plutons. The root zones are oriented at a high angle to the direction of maximum compression when magma is emplaced in a plastic crust. If the pluton is emplaced higher in a brittle crust, then the roots are aligned with the maximum compression, according to the Andersonian theory of fracture. The orientation of the root zone along the direction of extension in a plastic crust may be explained by shearing which acts as a valve mechanism for segregating magma. In all the examples, the geometry of the root zone relative to the major fault plane implies that the shear zone does not induce magma generation and that it does not serve as a guide for magma emplacement.
1. Introduction C u r r e n t observations o f granites and s u r r o u n d i n g rocks describe the structure and shape of granitic bodies as observed in outcrop. Petrographic variations, structures or m i n e r a l i z a t i o n s are described, which m a i n l y reflect the very late history of the granitic pluton. E x a m i n a t i o n o f the s u r r o u n d i n g rocks
Also at ENS G6ologie, Nancy, France.
shows the t h e r m o - m e c h a n i c a l c o n s e q u e n c e s o f m a g m a emplacement. On a larger scale, the schistosity displays the influence o f m a g m a intrusion on the regional pattern o f deformation. Field information is restricted to the final stage of e m p l a c e m e n t and processes taking place at depth must be extrapolated. C o m b i n i n g structural and gravity data indicates how the m a g m a was intruded. Structural studies on granites are related to deformation caused by the m a g m a as it intrudes the surrounding crust. T h e y are c o m m o n l y restricted to surface observations, or in a few cases to structures eroded d o w n to 1 or 2 kin,
0040-1951/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0040- 1951(95)00004-6
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still limited compared to the inferred thickness (approximately 5 kin) of granitic plutons (Vigneresse, 1988). Information on deeper features is derived from gravity data. The resolution of the gravity survey must be geared to a detailed investigation of the magma feeder channels. Both sets of data provide information only on the final stages of magma emplacement. They do not record the successive conditions of intruding magma, though the host rocks provide information about the regional stress field. This paper examines the morphology at depth as well as the position, shape and number of the root zones due to the stress field active at the time of magma emplacement. Most of the technical information on individual plutons has already been published and these discussions relate to the specific conditions of emplacement (Guillet et al., 1985; Guineberteau et al., 1987; Vigneresse, 1988; Audrain et al., 1989a,b; B r u n e t al., 1990; Aranguren et al., 1992). They concern mainly biotite or biotite-cordierite granites, the most evolved being two-mica granites. The conclusions are therefore restricted to magmas derived from the middle or lower crust and do not apply to alkaline granites, ring-dyke complexes, nor to mantle-derived magmas.
maps, granitic facies are contoured according to the total iron content of the magma which is deduced from the intensity of the magnetic susceptibility (Bouchez et al., 1990). It shows the chronology of each phase of emplacement. To detect deeper structures, geophysical data are needed (Vigneresse, 1990). Gravity surveys provide evidence for the shape at depth of the granitic bodies. Readings are taken at the same density (approximately 1 point/km 2) as that of the geological observations, in order that both sets of data can be correlated. Subsequent data treatment differentiates the anomaly generated by the granitic body from larger anomalies which could obscure its effects. Geological variations such as igneous facies changes must also be taken into account as they cause changes in density contrasts (Vigneresse, 1990). The precision of depth and shape ineasurements depends on the density of stations, as well as the intrinsic precision of the measurements and the accuracy of the density readings at depth. The estimated precision is about 15%, because of the loss of confidence in estimating the density values at depth. The floor is modeled with better precision, about 5%, since it reflects the homogeneity and the density of the readings.
2. Structure and shape of granites
3. Root zones in granites
Structural studies describe the present geometry of granitic bodies, from which conclusions are drawn on their emplacement into the upper crust, thus influencing the regional field of deformation. The stress field is recorded in the schistosity of the surrounding rocks (Hanmer and Vigneresse, 1980; Paterson et al., 1989) and in the relationship between internal granitic structures and regional deformation. Within the pluton structures are mapped by conventional means (Guillet et al., 1985; Guineberteau et al., 1987) or by measuring the anisotropy of the magnetic susceptibility (Guillet et al., 1983; Bouchez et al., 1990). These techniques are used to map the foliation and the lineation recorded by the first minerals crystallizing while the magma was still viscous. They determine the pattern of the magma flow, during the final stages of emplacement. On such
The combination of both structural and geophysical data aid the modeling of the mode of pluton emplacement (Guillet et al., 1985; Guineberteau et al., 1987; Audrain et al., 1989a,b; Brunct al., 1990). When erosion only affects the uppermost part of a pluton, the magmatic structures reflect the interaction between pluton emplacement and its surrounding rocks (Hanmer and Vigneresse, 1980). For a more deeply eroded pluton, zones where magmatic lineations are vertical, often correlate with the deepest zones computed from gravity data. Conversely, zones where lineations are gently dipping, often denote areas of shallow to medium depth. The deeper zones, if they show vertical lineations, are interpreted as magma feeder zones. They are apparently active during the time the pluton is being emplaccd and they are often reflected in outcrop by chemically
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m o r e e v o l v e d m a g m a or late intrusive facies. For instance in the C a b e z a de A r a y a c o m p l e x , Spain, a m o r e leucocratic m a g m a crops out o v e r the root zones, w h e r e a s the r e m a i n i n g part o f the pluton is mainly biotitic ( A m i c e and B o u c h e z , 1989; Audrain et al., 1989a). The location and g e o m e t r y o f the root zones are not i n f l u e n c e d by the nature o f the specific m a g m a associated with that area since changes in density induced by facies variations are taken into account during the inversion process (Vigneresse, 1990). T h e s e zones can be d e t e r m i n e d in cross sec-
tions drawn through a pluton, because the slope o f the floor o f the granitic pluton strongly increases when approaching the root zone. R o o t zones are located b e t w e e n 5 and 14 km depth and h a v e a cross-sectional area o f 1 to 10 km 2, c o m p a r e d to several hundreds o f square kilometers for the surface area o f a massif. The g e o m e t r y and orientation o f the roots p r o v i d e direct information on the t h e o l o g i c a l properties o f the crust during intrusion. T h e y reflect the lateral spreading o f the m a g m a and its e v o l u t i o n within the regional stress field.
°c,
0-1
Po ~
\.O"F_,~-~./.,. /
Gu
L_g Pab
0 :
5 0 km i
Fig. 1. Depth (in kin) of the floor of the granitic bodies in Brittany deduced from gravity data (modified from Vigneresse, 1983). The root zones trend (dashed line) at high angles with the major stress component (o-j) responsible for the main dextral fault zone. Granitic plutons quoted in the text: Lo = Locronan: Po = Pontivy-Rostrenen; Gu = Gu6henno; Lg = La Gacilly; Qu = Questembert; Pab = Pont I'Abb6; PIo = Ploeumeur; Gue = Gu6rande. The Mortagne (Mo) massif, quoted in the text and in Fig. 2, is 20 km to the southeast of this map. The cross section across the Pontivy pluton is shown in Fig. 7.
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J.L. Vigneresse / Tectonophysics 249 (1995) 173 186
!
Mo Qu Lg Gu
4. Granites emplaced during shear deformation Some granitic intrusions may be spatially associated with major strike-slip shear zones as in Brittany, western France (Vigneresse, 1983) or in Galicia, northwestern Spain (Courrioux, 1983; Aranguren and Tubia, 1989). In such cases, the shear plane is subvertical and the lineations associated with the shear are horizontal with the same strike as the fault zone. Such plutons have a moderate volume, less than 1500 km ~, and crop out at surface over a few hundred square kilometers (Vigneresse, 1988) (Fig. I). At outcrop level, their overall shape is elliptical, reflecting shear deformation. Their ellipticity, defined as the ratio of the minor to the major axis, is less than 0.8 in Brittany, with a small component of shortening ( 5 - 1 0 % ) superimposed on a dextral shear (Vigneresse and Brun, 1983). In Galicia, the shear is sinistral. Greater stretching yields a lower (0.4) ellipticity ratio lk~r the plutons. At the present surface level, the ellipticity ratio varies according to the shear intensity on a regional scale. Within granitic bodies, it decreases with depth (Fig. 2), down to 0.3 at 6 km in Brittany, but its gradient remains constant with depth on a regional scale. In those plutons emplaced during wrench-faulting, only one single root is observed, two in larger plutons. The floor plunges gently to 5 + 2 km, after which a sharp increase in the dip of the floor indicates the root zone which extends down to 6 - 1 2 km. The root zone is very small in size compared to the pluton itself. Its cross-sectional area represents only 1 to 5% of that of the whole pluton. Several geometrical patterns of the root relative to the shear zone are observed, depending if granites have either been emplaced within overlapping shear zones, or are adjacent to the shear zone. When magma intrudes anastomosing shear planes, the volume of granite is generally small and deformation may be enhanced (Hollister and Crawford, 1986). When the shear zone is wider or discontinuous, with en 6chelon relay faults, or under tensional stress, magma fills the space created by the opening of pull-apart-like structures (Guineberteau et al., 1987; Hutton et al., 1990; Tikoff and Teyssier, 1992). In this case the granitic body has steep walls controlled by the shear zone. The morphology has been detailed for the Mortagne massif in Brittany, France
/ /,s 2v o
/
8-
Z(km)
Po Lo
II II /,I /I
I
=t
o
O5
I
to B/A
Fig. 2. Diagram of the ellipticity ratio versus depth of granitic plutons in Brittany. The ellipticity ratio is defined as the ratio of minor axis B to major axis A. The pluton trends with an angle /3 relative to the shear direction. Depth contours have been obtained from gravity data. Captions referring to granitic plutons are simi lar to those for Fig. I. Generally plutons (Locronan to Mortagne) show a W-E-oriented decrease in their ellipticity ratio (B/A), but the gradient with depth is constant with an average value of 0.055 km l showing an increase of deformation with depth.
(Guineberteau et al., 1987). There, magma intruded while overlapping faults were active (Fig. 3). This situation is limited to transtensive environments which provide simultaneous shear and space due to the extensional component of the deformation. In the remaining cases, which are the majority, the root zone is always located off the major deformation plane, and no connection exists between them. The distance of the root from the maior shear plane does not correlate with any other parameter, such as the volume of erupted magma or shear intensity. Two situations exist, depending on the orientation of the root zone relative to the major stress component (o-1). In the first, which is more fl'equent, the roots are at a high angle with %. For instance in Brittany (Fig. 1), the roots strike northeast relative to a dextral shear zone oriented N I l 0 °. According to the regional stress field, the roots are at a high angle to the major stress component, and are aligned with the direction of tensional stress. In the second case, the roots are aligned with o-I, as to be expected in the case of brittle fracture (Delaney et al., 1986). This is particularly obvious in the Guitiriz massif, Galicia,
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Spain (Aranguren and Tubia, 1989; Aranguren et al., 1992). There (Fig. 4), two disconnected diverging shear zones lead to the northward expulsion of a piece of crust into which the granitic pluton of Guitiriz intruded. According to the shear direction along both faults, the major stress component is E - W oriented. In spite of the pronounced N-S-oriented structural pattern (Aranguren and Tubia, 1989) within the Guitiriz massif, gravity data reveal two root zones trending N100 ° (Aranguren et al., 1992). The granitic root zones are always situated in locally extensional areas relative to the regional field of detbrmation. This is observed in single plutons (Brittany, Galicia) or even at a larger scale. For instance, the intrusion pattern of Proterozoic granites in northeastern Brazil clearly relates to E-W-oriented dextral shear zones (Archanjo et al., 1992). Though some of these granites show an intense
deformation and reorientation due to shearing, all plutons are oriented along N045°-trending structures, within the extensional direction as to be expected in a plastic crust. This does not conform to the Andersonian theory of fracture which would predict N135°-oriented tension gashes. On a larger scale, plutons were intruded within local extensional areas of the stress field. In most plutons emplaced during shearing, conjugate shear zones affect the granitic pluton along its borders. Gravity data do not indicate a connection between the root zones and the deformation planes, as shown, for instance, in Brittany or in Galicia (Figs. 1 and 3). This precludes a direct relationship between the zone of intense deformation and the root zones. However, those fracture planes show well-developed mylonites with low-temperature C-S structures and broken quartz attesting that intense defor-
granite outcrop and faults- - %.._ -. . ~ ~ - ...
Lineations ..... > ®
0-45* 45-90 o
Depth
contours
,x
I L ~
10
km
.
in km
,
~~
Fig. 3. Map of the granitic floor and corresponding lineations in the Mortagne pluton, Brittany redrawn after Guineberteau et al. (1987). The floor is deep ( > 5 km) for most of the pluton. Clearly, the steeply dipping walls are controlled by adjacent faults and magma intruded into a pull-apart structure induced by the shear on its walls.
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1
Fig. 4. Map of the Guitiriz granite, Spain. The pluton is situated between two shear zones and shows three main |acies, the limits of which are indicated by broken lines. The depth of its floor, interpreted from gravity data, is contoured at I-kin intervals. Grey arrows indicate the magmatic lineations (from Aranguren et al., 1992), the length of which is inversely proportional to their plunge. The crustal block, in between the faults, has been displaced northward as shown by the open arrow by strong E-W-trending compression. Two shallow root zones, parallel to o-1+trend east-west as to be expected in a brittle fracturing crust. The distribution of the lineations and the root zones clearly show that both types of infi~rmation are needed to determine the mode of emplacement of a pluton.
mation continues after the emplacement of granites (Hanmer and Vigneresse, 1980; Paterson and Fowler, 1993).
5. Plutons emplaced during a compressional phase Granites are also emplaced in regions undergoing compression, although popular opinion correlates granitic emplacement with extension. Examples of such situations have been reported in Australia (Doctors Flat, Bega), in the United States (Boulder and Tobacco Root plutons, Piute Mountain), in Ireland (Ox Mountains), and in France as well (Flamanville, Huelgoat) (Vigneresse, 1988; Burg and Wilson, 1988; Schmidt et ~d., 1990; B r u n e t al., 1990; Morand, 1992; McCaffrey, 1992; Karlstrom et al., 1993). In spite of the evidence of thrusts or large-scale folding, the plutons are always emplaced in locally extensional fractures or within the axial fold planes. Plutons are generally small, less than 200 km 3 in
volume and have only one root. The Flamanville granite (Brunet al., 1990) presents a small outcrop (7.5 × 4.5 km) and a limited thickness (3 kin). The asymmetrical shape of the outcrop (Fig. 5) also reflects the eastwardly displaced root, the emplacement of which is controlled by the plunge of the Cambrian arkoses. Cleavage and foliation envelop the pluton, showing a strain-free triple point northeast of the pluton. Stretch lineations are restricted to the northern and southern surroundings of the western part of the pluton, thus indicating the swelling of the magma which also created folds in the western Devonian series. The Huelgoat pluton, in western Brittany, also has a small volume, with one root, right at the center (Vigneresse, 1988). Because of the differences between the distal and the proximal field of deformation, granites intrusive during regional compression do have the adequate morphology (one root) whereas their emplacement is controlled by local extension. For instance, the Australian plutons all present a N-S-striking foliation related to E - W -
J.L. Vigneresse / Tectonophysics 249 (1995) 173-186
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/ /J -
/
1 km
Fig. 5. Map of the Flamanville granitic intrusion (redrawn after Bran et al., 1990). The limits of the outcrops, in large dots for the granite, and dashes for the Paleozoic series (xb3=Brioverian; b a = l o w e r Cambrian; s2 = L o w e r Ordovician; s3-s4-s5=Middle-Upper Ordovician, Silurian; d = Devonian axial plane). The depth contours are shown at l-km intervals. Lineations measured in the surrounding rocks are indicated by arrows.
oriented shortening (Burg and Wilson, 1988; Morand, 1992) but they all lie within locally extensional areas, as shown by their bulk orientation.
6. Piutons emplaced during extensional deformation Granites may also be intrusive during an extensional phase, or at the onset of extension due to the collapse of a large orogenic structure such as the Saint Sylvestre complex in the French Massif Cen-
tral (Audrain et al., 1989b), Pont l'Abb6 and Gu6rande (Fig. 1) in the southern French Armorican Massif (Vigneresse, 1983), or the Mykonos pluton in Greece (Faure and Bonneau, 1988). These plutons are very thin, with subhorizontal foliation planes (Mollier and Bouchez, 1982). Their volume, about 1000-1500 km 3, is comparable to that of plutons emplaced during shear. The average thickness may be as little as 2.6 km (Fig. 6) for Saint Sylvestre (Audrain et al., 1989b; Vasseur et al., 1990). The floor remains generally flat, but locally deep roots, up to 5 - 6 km, are observed in vertical cross sections
J.L. Vigneresse / Tectonophysics 249 (1995) 173-186
180
(Fig. 7). In the field, these small features, less than 5% of the whole unit, show vertical lineations. They are also interpreted as magma feeder zones. Mag-
i,
I/
i
matic activity persisted in these areas, as shown by fine-grained intrusives (Audrain et al., 1989b) or variously evolved magmas in outcrop, but deforma-
..tl
i
,
i
tzTO
I CJ,
)
;-:
150
:AO
I20
110
-
.",i
: 510
-
, 520
,550
54 0
Fig. 6. Map of the floor depth (in km) of the Saint Sylvestre granitic complex (French Massif Central) calculated from gravity data (from Audrain et al., 1989b). Note the overall shallow depth of the floor and the great number of small areas with a pronounced deepening of the floor which are interpreted as root zones. FN (Nantiat fault) and FAO (Ar~nes-Ouzilly fault) are ductile faults which were still active during the emplacement of the pluton.
J.L. Vigneresse / Tecwnophysics 249 (1995) 173-186
181
NW
SE
J
J
IO km
W
E
Fig. 7. (a) Cross section of the Pontivy pluton (see also Fig. 1), two distinct roots (dotted), one for each lobe of the complex. (b) Cross section of the Saint Sylvestrepluton (Fig. 6), a very thin main facies, in which vertical features (dotted) are intrusive.The foliation(adapted from Mollier and Bouchez, 1982) is also shown. The steep lines correspond to a ductile fault (FN in Fig. 6) within the pluton. In both cross sections, vertical and horizontal scales are identical.
tion of the host magma indicates temperatures above 300-400°C. Most of these small roots are not randomly oriented as demonstrated by their overall pattern. In the Saint Sylvestre massif, the late intrusives as well as the deeper zones are controlled by N020 °and Nl20°-oriented structures (Audrain et al., 1989b; Cuney et al., 1990). The stress field at that time involved a vertical shortening and a m a x i m u m horizontal stretching oriented towards the northwestsoutheast, showing that the root zones are in locally extensional areas. Ductile faults may locally affect the boundary of the granitic body (Mollier and Bouchez, 1982; Audrain et al., 1989b) offsetting its
floor, thus indicating that emplacement was near the brittle/ductile zone.
7. Thickness and number of roots The morphology at depth of granites is controlled by the regional stress field, but the plutons remain very thin (Fig. 8). Those plutons intrusive during a shear or a compressional phase are approximately 5 _ 2 km thick, in contrast with the very thin plutons (approximately 2 - 3 km) intrusive during an extensional phase. Such a difference is obvious from gravity data, although this is not unique and essen-
182
J.L. Vigneresse / Tectonophysics 249 (1995) 173-186
--1
Extension A
A'
1
A,
A
Shear B
B'
c
~~ ~
,
B
Overlapping shear
B,
C~ - " ~ ~ ®
C'
c-Fig. 8. Schematic morphology of granitic plutons according to the regional field of deformation with dotted areas showing the root zones. The figures are highly simplified. The examples given are Saint Sylvestre (Fig. 5), Pontivy (Fig. 1) and Mortagne (Fig. 3). A map (on the left) and a cross section (on the right) with no vertical exaggeration represent the deformation processes described in the text: A = extension; B = wrench faulting; C = open fracturing in between overlapping shear zones. tially ambiguous. In the present case, the shape of the body which creates the gravity low is determined by its outcrop. Densities are measured from surface samples to incorporate facies changes into the inversion process, so each point of computation has its own density. Under those conditions, the range of acceptable solutions is narrow (Vigneresse, 1990). Input data errors (measurements, correction, density) are lower than 6%, because of the accuracy of measurements. This provides about 15% of the total estimate of the errors, after construction of the model, estimating that density may vary with depth or that an unsuspected body lies under the pluton. In no way does it explain the difference in thickness calculated in most cases (Brittany, Germany, Spain) compared to that of the plutons from the French Massif Central (Saint-Sylvestre, La Marche). Difference in the number of roots (Fig. 8) is also justified. The floor roughness depends partly on its
depth, and moreover on the surface distribution of measurements. Closer-spaced stations yield more detailed lateral resolution at depth. For measurements made in Spain or France, the coverage is about 1 p o i n t / k m 2 (760 p t / 1 0 0 0 km 2 for Cabeza de Araya, 248 p t / 3 4 km 2 for Flamanville), which compares to those in the French Massif Central (2241 p t / 2 0 2 5 km 2 in Saint-Sylvestre, 1983 p t / 1 8 0 0 km 2 in La Marche). The resolution at depth is quite similar and cannot explain the discrepancy between one root for the whole pluton as often observed and the many deep structures in those plutons emplaced at the onset of the extension in the French Massif Central.
8. G e o m e t r y o f the root zones
Whatever the phase of deformation, the granitic roots are within the local (near field) extensional
J.L. Vigneresse / Tectonophysics 249 (1995) 173-186
areas of the regional (far) field of deformation. This situation is easily understandable as soon as magma penetrates the upper brittle crust. The strength required to break the brittle crust is about four times greater under compression than under extension. Consequently, the easiest place for magma to intrude the crust in a mechanical sense is within regions with extensional stress. This agrees with observations, either in Brittany, in Galicia, or in plutons emplaced during transtension (Guineberteau et al., 1987; Hutton et al., 1990; Tikoff and Teyssier, 1992). In addition, the intrusion of magma in extensional areas of a deforming crust partly solves the dilemma of the so-called "space problem" which has puzzled the geological community for years (Bowen, 1948). If deformation controls the emplacement of granitic magma, then it will be tempting to relate its origin to deformation. Melting as a consequence of shear has been suggested in several cases (Strong and Hanmer, 1981). Melting results from the interaction of deformation and heat. It requires an important rheological contrast, for instance between a rigid crust and a soft mantle (Fleitout and Froidevaux, 1980). However, the process is inadequate to produce granitic plutons. One reason is the low volume of magma generated (approximately 1500 km3; Brun and Cobbold, 1980). The other reason is the lack of correlation between the root zone and the zone of major deformation (Vigneresse, 1983). The hypothesis of shear melting leading to plutons is therefore rejected. The root zone is not randomly oriented relative to the stress field. In all cases, the number of the roots and their distribution within the regional stress field emphasize the active role played by the deformation during the upwelling of magma and its emplacement (Pitcher, 1979). Geometrically, two extreme situations are observed. One, the less frequent, corresponds to roots aligned with the major stress component (O-l). This geometry is in accordance with the Andersonian theory of brittle fracture (Delaney et al., 1986). This is, for instance, the case in the Guitiriz massif, Spain. There (Fig. 3), the E-W-trending roots are parallel to 001- The entire block in which the granite is emplaced is shifted to the north. This is also the case with the Flamanville granite, which is known to have been emplaced at a very shallow crustal level. The geometry reflects an intrusion into
183
a brittle crust. In the other and far more common cases (see, for instance, Fig. 1), the roots trend at a high angle with o-~ and are aligned with extension. This geometry indicates a plastically deforming crust. Three models can explain this geometry: In the first model, the geometry of the root zones reflects a variation with depth of the classical distribution of fractures (Delaney et ai., 1986). Two hypotheses can be advanced. In the first, the root zone has the expected trend (near to o-~) at depth, but is too small ( < 500 m in radius) and too deep ( > 4 km) to be detected by gravity. In the second hypothesis, the actual shape of the root zone evolved from that at the onset of magma intrusion. An initial fracture was created by magma overpressure with the expected trend near to o'~, but the hot magma flow inflated the fracture and heated it up, thus interfering with the regional field of deformation. The conduit is reoriented because of the reduced crustal strength due to the heating, even if regional stresses do not vary (Reches and Fink, 1988). According to that hypothesis, the conduit is plastically deformed to its final elliptical shape due to regional shear. Under such conditions, the strain increases 'with depth, reflecting the deformation of the shape of the conduit. This hypothesis might explain the increase of the ellipticity ratio (Fig. 2) with depth deduced from the shape of the pluton (Vigneresse and Brun, 1983). An alternative to this model considers the geometry as a result of fractures opening under fluid pressure (Sibson et al., 1988). In Anderson's classical theory of rupture, shear failure occurs along planes that contain the intermediate stress component (0°2) and are oriented at 25-30 ° to the maximum principal stress component (001). In a fluid-saturated crust, all stress values are reduced by the amount of the fluid pressure. If the latter is greater than the smallest component (003), then fractures open along planes at a high angle to 001 (Sibson et al., 1988). This model explains mesothermal gold-quartz deposits through repetitive fault valve behavior along reactivated faults. During several cycles of seismic activity, sudden fluid pressure fluctuations are due to the release of fluids which infiltrate open fractures along faults. Also observed in partly molten gneisses (Hand and Dirks, 1992), it may be transposed to magma migration in a plastically deforming crust. The third model considers the geometry as two
184
J.L. Vigneresse / Tectonophysics 249 (1995) 173 186
extreme cases o f intrusion relative to the deformation phase. In a brittle material, root zones initiate and remain near to cr~ as expected. Conversely, when magma intrudes a plastic crust, a fracture opens near to o.j and shear reorients it towards the direction of extension at a high angle to o.t- The root geometry would reflect the temperature controlled deformation of the crust. From the three proposed models, the first is the least probable, because it requires external conditions. The second is more plausible, but is valid only if enough fluid pressure exists, which cannot be predicted. The third combined with the second appears to be more realistic. It fits the observations on granites intruded at different levels of the crust (Brittany, Massif Central, Spain...). It would provide criteria to explain the geometry of granite emplacement as a function of regional deformation. When granites intrude a crust in which high-temperature schistosity develops with a well-defined lineation (biotite, sillimanite), that is in a plastic crust, the root is at a high angle to o-1. Conversely, when granites intrude a colder, or brittle crust, its root is near to o.~ as expected (Delaney et al., 1986). Deformation controls the geometry of granitic emplacements. It is tempting to ask if granites can be intruded without deformation. All cases presented are crustal-derived granites, ranging from calcalkaline granites to leucogranites. Ring-dyke complexes or hyper alkaline granites, which may contain a large mantle-derived component, have not been surveyed. Though they are described as forceful intrusions, those granites are also associated with large-scale extension. I suggest therefore that all granites are emplaced while the crust is deforming. In that sense, they are all synkinematic (Karlstrom, 1989).
regional field of deformation. Their orientation corresponds, occasionally, to that expected during brittle fracture (near to o.~), but in most cases it is oriented at a high angle with the maximum stress component (o-i). Such a situation reflects magma intrusive in a plastically deforming crust. In all cases the number and disposition of the roots emphasize the role played by the regional deformation during the emplacement of magma. The location of the root zones in the case of granites associated with large wrench faults suggests that it is very unlikely that magma volume, as large as that of a granitic pluton, could only resuh from shear melting and emplacement within the fault zone. Deformation helps to transport magma, but does not generate it. It is also proposed that deformation plays an important role in determining the location of plutons.
9. Conclusions
References
Observations obtained from structural and gravity studies enable root zones to be identified beneath granitic plutons. They indicate that granites eraplaced during shear deformation or during compression have only one or a few roots, whilst granites emplaced during extensional deformation are thin with many root zones in which late melts are intrusive. In all cases, the root zones are located within the locally extensional stress field relative to the
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Acknowledgements This paper is based on many gravity and structural surveys conducted over granitic areas with the help of students of the Universities of Bilbao, Montpellier, Miinchen, Nancy, Nantes, Rennes, Salamanca and Toulouse. They provided the data from which these ideas have been developed. Many constructive discussions with structural geologists have improved the reasoning. Special thanks to J.C. Bhattacharji, J.L. Bouchez, J.D. Clemens, B. Marsh, K.J.W. McCaffrey, A. Nicolas, W.M. Schwerdtner and J. Tullis who corrected the manuscript. Funding for the gravity surveys has been received through grants from the CNRS, INSU, DBT program, CREGU and from Cogema and Cisa, both uraniummining companies.
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