The effect of volcanic constructs on rift fault patterns - GeoScienceWorld

The effect of volcanic constructs on rift fault patterns Benjamin van Wyk de Vries Department of Earth Sciences, Open University, Milton Keynes, United Kingdom Olivier Merle De´partement des Sciences de la Terre, Clermont-Ferrand, France ABSTRACT Volcanoes in rift situations often show distinctive “hour-glass” fault patterns, with increased fault throw as the volcano is approached. We present analogue models that show that this is caused by an interaction of the regional stress field with that set up by the volcano mass. For faults to be reorientated there must be a ductile layer below the volcano. This can be hot crust at mid-ocean ridges, continental rifts, or weak sedimentary strata. Increased volcano size or mass and lower brittle/ductile ratios lead to increased fault curvature, whereas an increased regional extension rate decreases the effect. Volcanoes on one side of a rift may capture it, forming the axis of a new rift, with curved or en echelon zones on either side. By concentrating extension, magma is more easily erupted, so volcanoes may erupt more magma of less-evolved composition. A positive feedback between increased extension and magma eruption rate will lead to rift narrowing, which can favor the formation of oceanic crust. INTRODUCTION Faults often show a distinctive curved geometry near volcanoes in rifts. Fieale volcano of the Asal Rift (De Chabalier, 1993; De Chabalier and Avouac, 1994) is one striking example. It has a well-developed “hour-glass” fault pattern, where the parallel rift faults bend in toward the volcano center (Fig. 1). Axial and Brown Bear seamounts at the Juan de Fuca Ridge show a different type of geometry (Fig. 1), in which the whole rift bends into the seamounts (Johnson and Embley, 1990). In both examples, fault displacements increase as the volcano is approached. At Fieale this produces spectacular fault cliffs, 100 m high, whereas at Axial, there are the 500-m-deep Helium and Brown Bear basins. Spreading volcanoes deform under their own weight, creating characteristic radial summit rifts (van Bemmelen, 1949; Merle and Borgia, 1996; van Wyk de Vries and Borgia, 1996). In natural examples there is often a deviation from the expected radial fault symmetry. At Maderas in Nicaragua, there is a biaxial symmetry, orientated normal to regional extension (Fig. 1). Maderas stands on flat lacustrine mudstones and is a fast-spreading cone (van Wyk de Vries and Borgia, 1996). Here the regional stress appears to affect the volcano fault pattern. The fault patterns in each of these examples suggest that there is an interplay between regional and volcano-induced stress. At Fieale, regional stress appears to be altered by volcano mass; at Maderas, the volcano spreading pattern is altered by regional stress. We suggest that there is a continuum of process between one extreme, where regional stress dominates faulting, to another, where volcano spreading dominates. To test this idea we have designed scaled analogue models that mimic the relationships be-

tween volcanic cone, rifting, and regional stress. The models are designed to isolate the effect of volcano loading and regional extension on fault patterns. Thus no magmatic system is modeled, and these experiments differ from those of Tibaldi (1995), who investigated the effect of regional faulting on cone morphology. The substrate consists of ductile material beneath a brittle layer, which is representative of vertical strength variations in oceanic and continental rift crust and sedimentary basins (Fig. 2). For volcanoes, the brittle/ductile thickness ratio is important in controlling the rate of spreading. The ratio of the brittle layer thickness (T) to ductile layer thickness (D) has been used as a pi number (P3) by Merle and Borgia (1996), so here we refer to a T/D substrate and ratio of P3 (Fig. 2). A volcano loading a T/D substrate causes the ductile layer to spread outward. Merle and Borgia (1996) show that by increasing D, or reducing T, spreading increases. Thus the degree of deformation is dependent on P3. In these experiments, we chose to vary P3 to assess the influence of volcano-induced deformation. Regional extension was created by ex-

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Figure 1. Examples of influence of volcanoes on faulting. A: Fieale volcano, drawn from De Chabalier and Avouac (1994), showing inward-curving faults. B: Axial and Brown Bear seamounts, drawn from Tivey and Johnson (1990), showing Juan de Fuca Ridge curving into volcanoes. BBB—Brown Bear basin, HB—Helium basin. C: Maderas volcano, drawn from van Wyk de Vries and Borgia (1996). Thick lines indicate major faults. This volcano spreads and rifts predominantly normal to regional extension (indicated by arrows). Shaded areas indicate upper part of each volcano. Contours are in metres.

Geology; July 1996; v. 24; no. 7; p. 643– 646; 4 figures; 2 tables.

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Figure 2. Diagram of experimental design and parameter notation, including schematic strength profile. See text for abbreviations.

tending the substrate, with experiments selected to model the geometric relationship shown at Fieale and Axial, using a cone symmetrically situated on a rift and on one side, respectively. Geometrical relationships were tested by varying the brittle layer, and hence P3. Temporal relationships between volcano-induced and regional stresses were explored by constructing the cone before or during extension. MODEL PROCEDURE AND SCALING Model parameters have to be scaled between nature and model in order to balance the change of forces, length, and time. The scaling argument for analogue volcano experiments is described in Merle and Borgia (1996). Following these authors, we used silicone putty to model ductile rock (D) with a viscosity of about 1018 Pa z s, and dry quartz sand to model the cone and brittle superficial rocks (T) with a cohesion of about 107 Pa. According to the model ratio, the model is 100 000 times weaker than the natural prototype. In addition, time is about 4.6 3 108 times shorter, and length is 60 000 times smaller (Table 1). This means that a 5-cmhigh cone in the model equates to a 3000m-high volcano and a natural extension rate of 1 cm yr21 is 0.82 cm hr21 in the model. The cone was chosen to be in the middle of the size range of volcanic constructs, smaller than an oceanic shield, but larger than an arc volcano such as Maderas. The models are constructed in a halved Perspex box with a thin plastic base, resting on a plastic table top. The box was 50 3 50 cm before extension in the brittle experiments. In experiments with a ductile base, the two sides were separated by 5 cm to allow a greater area of stretching in the silicone. Each half was attached to a computercontrolled motorized screwjack (Fig. 2). The model cone was built by dropping sand through a funnel to produce a near-perfect cone with 308 slopes. The cone was always 5 cm high. The ductile layer (silicone) was 0.5 cm thick in all cases.

Figure 3. Model results drawn from overhead photographs. Fault stipple indicates downthrow direction and stipple length is proportional to throw. A: Control experiment, showing fault sets X, Y, and Z. B: Experiment with P3 of 7. C: Experiment with P3 of `. D: Experiment with and P3 of 3. Inset shows “leaf graben” pattern of spreading volcano (Merle and Borgia, 1996). E: Experiment with cone on side of prospective rift. F: Cone emplaced after rift has developed (emplaced onto graben shown in A). 644

EXPERIMENTS A control experiment was run with no volcano (Fig. 3A). In this, the rift faults developed at roughly 908 to the direction of separation. A central graben (fault set X) was flanked by two other grabens, formed of fault sets Y and Z, creating two horsts on either side of the axial graben. In each of the experiments with a cone, the faulting pattern at the edge of the model (away from the cone) was similar to that without a cone. Symmetrically Sited Cone In an initial experiment with a P3 of 7 (3.5 cm sand and 0.5 cm silicone), the inner GEOLOGY, July 1996

faults (set X) ran straight toward the cone with increasing throw, creating a trough below it (Fig. 3B). The other faults (sets Y and Z) curved in, either joining the inner set, or dying out completely. The inward-dipping fault set Y joined the inner set X. The outward-dipping fault set Y disappeared, leading to a loss of the horst feature. Effect of Variable P3 We ran experiments identical to that described above, but altered the P3 by changing the sand layer thickness. The T/D arrangements used were P3 5 `, 11, 7, and 3. P3 5 7 and P3 5 11 probably are near the situation at Fieale and Axial. P3 5 ` represents a thick brittle crust, and P3 5 3 represents a crust on which a volcano would spread (Merle and Borgia, 1996), corresponding to Maderas. We measured the maximum angle of deflection of faults in each experiment. With decreasing P3 the fault deflection angle increases (Table 1). P3 5 `. The main faults grew from the edges of the separating plates and passed through the volcano with no deflection (Fig. 3C). An apparent bulge of fault traces in plan view was simply due to fault outcrop following topography. P3 5 3. Faults at up to 608 to the rift developed, indicating a major component of outward radial movement due to spreading (Fig. 3D). The overall pattern gave the appearance of a modified “leaf graben” set (Fig. 3D), as described by Merle and Borgia (1996), although no spreading-related compressional structures developed at the base of the cone. Cone on the Side of a Rift (P3 5 7) The rift developed in the usual place at the walls of the model, but the faults curved toward the offset cone, with decreasing throw. The cone developed a rift-parallel graben, with straight faults (Fig. 3E). The cone graben was dominated by only two faults, whereas the outer rift was similar to that in the symmetric experiments. The change of rift symmetry occurred in two ways: either the inner graben curved smoothly toward the cone, dying out at its base, while the outer faults disappeared, or an en echelon pattern developed, stepping toward the cone. It is not clear why two patterns develop, though we note that in each experiment one side curved, whereas the other was en echelon. Faults extending from the rift have progressively less throw as they approach the cone. The greatest throw over the volcano is at the center and decreases toward the edges. Volcano Postrift (P3 5 7) Extension was allowed to start before constructing a cone. At first a parallel rift GEOLOGY, July 1996

Figure 4. Stress orientation around base of cone for T/D substratum. A: Orientation of stress axes in rift without volcano (sr). B: Fault and stress pattern around nonspreading volcano (P3 > 3), showing volcano-induced stress (s3v 5 concentric) and regional stress (s3r 5 rift normal). C: Fault and stress pattern around spreading volcano (P3 < 3), where s1v away from cone is radial. See text for abbreviations.

developed (Fig. 3A). Once the cone was constructed, the faults in the central part of the cone (set X) began to develop rapidly, while those on either side (set Y and Z) ceased movement. Set Z faults were cut by new inwardly curving faults, which splayed away from the deactivating outer fault set Z (Fig. 3F). The fault pattern that remained was nearly that of the pre-emplacement state; however, the movement along the faults was significantly altered. Slow-growing volcanoes would have similar fault patterns, dominantly rift parallel, but with extension progressively concentrated at the cone. MECHANISM OF FAULT REORIENTATION In these experiments, the load of the cone and regional extension are the only parameters controlling the fault patterns. The

stress field that causes faulting depends on these and the physical properties of the substratum and cone. Thus the distribution of stress is dependent on the brittle and viscous response of the sand and silicone. For this discussion we denote volcano-induced stress as sv, regional stress as sr, and composite volcano and regional stress as sc. For regional extension with no cone, s3r lies in the direction of extension, s2r is parallel to it, and s1r is vertical. Faults form normal to s3r and parallel to s2r (Fig. 4A). Without regional extension a cone on a T/D substratum causes outward flow of the ductile layer. The rate of flow is determined by P3 and the cone load. We first take the case in which the cone does not spread (P3 . 3). Around the cone, s1v is vertical, s2v is radial, and s3v is concentric (Fig. 4B). If the two stress systems are combined 645

(sc), the pattern evolves to bilateral symmetry (Fig. 4B). s3c tends to become concentric near the cone. On the rift axis, s3v and s3r are parallel and combine to create a greater composite s3c with no change of direction. In the axial region of the cone, therefore, extension should be greatest, and this is seen in the models. If the circumference of the cone is followed round from the rift axis, then s3v increasingly becomes more parallel to s2r. This has two effects: First, it causes s3c to rotate from rift normal toward cone concentric. Second, it decreases the magnitude of s3c. The faulting direction expected to form in the composite stress pattern is shown in Figure 4B. This shows clearly the fault pattern seen in the experiments. In the case of a cone that can spread under its own weight, the stress axes within the cone are the same, but outside, s1v becomes radial, s2v is concentric, and s3v is vertical (Fig. 4C). If failure occurs, tangential thrusts form below the cone, and radial normal faults form within the cone (Merle and Borgia 1996). In this case, on the rift axis, s2v and s3r are parallel and create a larger s3c with no orientation change. Around the cone, s2v rotates toward s2r, but s1v becomes parallel to s3r. This reduces the magnitude of s3c, leading to inhibited fault growth, or in the extreme leads to spreading-related concentric thrust faulting. The experiments show that the fault pattern is dependent on P3: the smaller it is, the higher the fault angle (Table 2). The above analysis suggests that increased cone size (higher sv) will also increase angles, and that increased extension rate (sr) will do the opposite. DISCUSSION The fault patterns in the natural examples (Fig. 1) are very similar to those in the models (Fig. 3). The experiment with P3 5 7 (Fig. 3C) has a fault pattern and fault curvature very similar to those of Fieale. At Axial–Brown Bear, fault curves are about 258, again roughly comparable to the P3 5 7 offset model (Fig. 3E). Because the model cone was three times larger than Fieale and twice as large as Axial–Brown Bear, the real volcanoes will have substrata with P3 , 7. 646

Maderas, the spreading volcano, has a P3 of 0.02 (van Wyk de Vries and Borgia, 1996), which means that bilateral symmetry is less well developed than in the P3 5 3 experiment (Fig. 3D), and that fault deflection is higher, approaching 908 (Fig. 1, Table 2). We conclude that the experiments have successfully simulated the natural process of fault reorientation and have demonstrated that the volcano mass is the controlling factor. Because no fault deflection was found in brittle experiments, we also conclude that the presence of a ductile substratum is a necessary condition, and that with decreasing P3 the fault curvature increases. We have deliberately not considered the effect of magma. Some dike patterns are similar to faulting patterns in the experiments (Johnson, 1968; Muller and Pollard, 1977; Nakamura, 1977). Other studies show that magma bodies may by accompanied by significant deformation (Merle and Vendeville, 1995). Our experiments show that there need not be an active magmatic component for increased extension and fault deflections to occur. Where a large volcano has been present, such orientations may be caused by the cone load, rather than magma overpressure. The presence of magmatic heat below a cone will tend to increase ductility, reduce brittle thickness, and hence reduce P3, leading to increased fault curvature. The concentration of extension at a volcano is likely to lead to increased eruption volumes because of increased ease of dike injection. Greater output decreases residence times, in general leading to lessevolved magma. Thus, where there is a ductile substrate, as a volcano grows and its load increases, magma compositions should become less evolved. In a rift with a line of volcanoes, a feedback situation might develop in which added mass leads to increased extension, which in turn leads to increased eruption. This would concentrate volcanism and narrow the rift. In the extreme this could lead to the rift forming an oceanic spreading center. CONCLUSIONS Volcanoes load the crust, increasing and reorientating stresses. In a rift situation,

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faults will curve toward a volcano, forming “hour-glass” patterns, such as at Fieale. If a volcano is to one side of a rift, or fault, it may capture it, as at Axial–Brown Bear seamounts. The orientation of structures at spreading volcanoes can be given bilateral symmetry by tectonic stress, as seen at Maderas. In rifts, extension will be concentrated at the volcano, which will lead to a narrowing and deepening of the rift. As a consequence, the extending volcano is likely to erupt greater volumes, and produce less-evolved magma. A feedback system can be set up in which rift volcanoes concentrate extension, produce more erupted mass, and can facilitate ocean crust formation by narrowing the rift. ACKNOWLEDGMENTS A Leverhulme Trust Grant supports van Wyk de Vries. The modeling was carried out at the Geosciences Rennes, Universite´ de Rennes, ably set up by Centre Nationale de Recherche Scientifique technician J. J. Kermarrec. We thank P. W. Francis, F. Garland, and N. W. Rodgers for constructive criticism of the manuscript. REFERENCES CITED De Chabalier, J.-B., 1993, Topographie et diformation tridimensionelle du Rift d’Asal (Djibouti): de la disparition d’un volcan ´a la naissance d’un oce´an [Ph.D. thesis]: Paris, Universite´ Paris 7 (Orsay), 264 p. De Chabalier, J.-B., and Avouac, J.-P., 1994, Kinematics of the Asal Rift (Djibouti) determined from the deformation of Fieale volcano: Science, v. 265, p. 1677–1681. Johnson, P. H., and Embley, R. W., 1990, Axial Seamount: An active ridge axis volcano on the central Juan de Fuca Ridge: Journal of Geophysical Research, v. 95, p. 12689 –12696. Johnson, R. B., 1968, Geology of the igneous rocks of the Spanish Peaks region, Colorado: U.S. Geological Survey Professional Paper 594-G, 47 p. Merle, O., and Borgia, A., 1996, Scaled experiments of volcanic spreading: Journal of Geophysical Research (in press). Merle, O., and Vendeville, B., 1995, Experimental modelling of thin-skinned shortening around magmatic intrusions: Bulletin of Volcanology, v. 57, p. 33– 43. Muller, O. H., and Pollard, D. D., 1977, The stress state near Spanish Peaks, Colorado, determined from a dike pattern: Pure and Applied Geophysics, v. 115, p. 69– 86. Nakamura, K., 1977, Volcanoes as possible indicators of tectonic stress orientation—Principal and proposal: Journal of Volcanology and Geothermal Research, v. 2, p. 1–16. Tibaldi, A., 1995, Morphology of pyroclastic cones and tectonics: Journal of Geophysical Research, v. 100, p. 24521–24535. Tivey, M. A., and Johnson, H. P., 1990, The magnetic structure of Axial Seamount, Juan de Fuca ridge: Journal of Geophysical Research, v. 95, p. 12735–12750. van Bemmelen, R. W., 1949, The geology of Indonesia: General geology of Indonesia and adjacent archipelagos, Volume 1A: The Hague, Netherlands, Government Printing Office, 732 p. van Wyk de Vries, B., and Borgia, A., 1996, The role of basement in McGuire, W. C., et al., eds., Volcano instability on the Earth and other planets: Geological Society of London Special Publication 110, p. 95–110. Manuscript received December 26, 1995 Revised manuscript received April 4, 1996 Manuscript accepted April 16, 1996

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