Dike induced Faults. Thomas Heinig. TU Bergakademie Freiberg, B. v. Cotta-Straße 2

Transcription

1 Dike induced Faults Thomas Heinig TU Bergakademie Freiberg, B. v. Cotta-Straße 2 Abstract. The preferred region of dike induced faults can be found in rifts. They develop because of the extensional forces there. Dikes are growing upward because of the overpressure in a magmatic reservoir and occur subvertical and subparallel. They chance into inclined normal faults when they propagate in higher crustal levels. The Dikes link with downward expanding fractures. Introduction The problem of fault behavior is always a reason for discussion. In this report the focus lies especially on the relationship between fault and magmatic activities that lead to their appearance. To understand the mechanics of fault evolution it is important to analyze the fault characteristics and the systematic of brittle failure (Tentler; 2005). There are several models how faults grow in space and time, for example their preferred growing direction. Some say normal faults evolve upward from seismogenic levels (Langley and Martel, 2000; Peacock and Parfitt, 2002; Walsh et al., 2003). Others prefer a downward propagation from the surface. But unfortunately it is not that easy to get an idea of the fault panes, because visualizations of the shape in 3D are hard to find. Only small fractions of the whole fault plane can be investigated with the help of mine data-sets, some outcrops on the surface and in seismic cross-sections (Tentler; 2005). The main work to understand the fault behavior and brittle failure characteristics in the upper crust is done in laboratories by simulating the conditions in the depth.

2 2 Thomas Heinig Faults There are three main types of faults. Which own occurs depends on the stress field put on a specific rock. The normal fault is the type that usually characterizes areas of extension. It is characterized by a relatively downward movement from the hanging wall to the footwall. The main stress vector ϭ 1 stands vertical upon the horizon and rectangular to the extension direction ϭ 3, which is in this case negative. Ϭ 2 lies in the strike direction of the fault plane (Fig.1). These fault planes are often listric planes which roots in the detachment. Their dipping angle decreases in the depth. The gap build during the extension causes structures like roll over anticlines, syn- or antithetic faults. Normal faults occur in mid ocean rifts seen in Island and East African Rift Zone, back-arc spreading regions, orogen collapse structures or mantle domes and along the rimes of the continents. Fig.1. Schematic models of the three fault types, the corresponding stress vectors and the relative movement of the blocks. (A) normal fault; (B) reverse fault; (C) strike-slip fault Reverse faults are caused by compression of two plates or masses in the horizontal direction. The hanging wall moves up relative to the footwall (Fig.1). It s called a thrust fault when the dip is less than 45. With a low dip and a large displacement the thrust fault is termed as overthrust fault.

3 Dike induced Faults 3 The stress vector ϭ 1 is horizontal and lies in the compression direction. Ϭ3 is the vector of least stress and is vertical. Ϭ 2 lies in the strike direction of the fault plane. Reverse (Thrust) Faults appear foreland fold- and thrust belts and all types of collisions. Strike-slip faults are caused by shear stress. The stress vector ϭ 1 is horizontal and stands nearly 45 on the fault plane (Fig.1). It marks the direction of shortening. Ϭ 2 is vertical and ϭ 3 has also a horizontal direction with an angle of 45 to the vertical fault plane. Strike-slip fault occur at mid ocean ridges, as tear faults by folding processes, as transform faults along plate borders and as transcurrent faults in continental plates. Dike development Gudmundsson (1998, 2000) describes that around central volcanoes the regional dyke swarms have an extension of tens of kilometer. That means that same of the dikes must be partly fed by shallower chambers or by large magma reservoirs at the contact between mantle and crust or in the deeper crust. Fig.2. Schematic 3D representation of a dyke swarm in Iceland without the surrounding host rocks. The connection between the magma chamber and the melting zone is drawn as an example. The strikes of the intrusions are assumed to be controlled by the variation of the stress distribution along the swarm. Modified from Paquet et al. (2007). This leads to two main sources of the magma which fills the dikes. The first opportunity is the magma chamber under a volcano and the second is an ever greater reservoir at the mantle crust border. In areas with a thin

4 4 Thomas Heinig crust like the central rift zone in Iceland this may causes dike propagation in higher crustal levels. Dike swarms are usually 50 km long and 5 to 10 km wide (Gudmundsson, 2000). The dike intensity increases with depth and each swarm contains hundreds of dikes. They are mostly subvertical and subparallel (Fig.2). The dikes have a narrow strike distribution because they were direct controlled by the regional extensional stress field (Gudmundsson, 2002). The strikes of the intrusions are even more influenced by the stress distribution, the farer away they are from the magma camber. As long as the magma pressure is high enough the dikes move on as blade like magma filled fractures in a plane perpendicular to ϭ 3 (Rubin, 1992). Near the eruptive centre where the magmatic support is high, the stress field becomes almost isotropic leading to a radial distribution of dykes (Fig.2, Fig3). In Iceland dikes were commonly observed in depth between 500 and 1500 m. One regional dike has may be a thickness from a few centimeters to 60 m. In Tertiary rocks regional dike swarms produce an average crustal dilation of 10%. For example, the crustal displacement generated by faulting has an average number below 2% (Forslund and Gudmundsson, 1991, 1992). Fig.3. (A) Stress field around a magma chamber having a circular vertical crosssection subject to a remote tensile stress of 5 MPa (p e = 0). The ticks represent the direction of the maximum principal compressive stress, ϭ 1, along which dikes injected from the chamber would propagate under these extensional stress conditions. (B) Stress field around a magma chamber having a circular vertical crosssection subject to internal magmatic excess pressure p e = 5 MPa. The ticks represent the direction of the maximum principal compressive stress, ϭ 1, along which dikes injected from the chamber would propagate. Modified from Gudmundsson (2002). The initialization of a dike is connected with an overpressure in a magma filled chamber (Fig.3 B). It ruptures when: p 1 + p e = ϭ 3 + T 0 (1)

5 Dike induced Faults 5 In this case p 1 is the lithostatic stress at the depth in which the chamber is situated; p e = P t p 1, the excess magmatic pressure, is the difference between the total magma pressure, P t, in the chamber at the time of its rupture and the lithostatic stress; ϭ 3 is the minimum principal stress; and T 0 is the in situ tensile strength, in the roof of the chamber. Dike injection appears when the conditions of Eq.1 are reached, no matter at which point of the chamber wall and independent from its shape or depth below the surface (Gudmundsson, 2002). Furthermore it is unimportant whether the condition of Eq. 1 is reached if p e increases because of a fluid addition to the chamber, or when ϭ 3 decreases as a result of an increase from the tensile stress concentration (Gudmundsson, 1998). For most of its lifetime a magma chamber is a long- lived fluid-filled structure which is probably in lithostatic balance with the rocks and forces around. For that, the usually situation along the chamber wall is: p 1 = ϭ 3 (= ϭ 1 ). Only during short events when p e > 0 through ϭ 3 reduction or an absolute increase in fluid pressure, the condition in Eq. 1can by fulfilled and the result in a new dike. On the other hand the total fluid pressure Pt can be written as: P t = ϭ 3 + T 0 (2) A dike initialization take place when the conditions in Eqs.1 and 2 meeting each other. The propagation of this dike depends on some parameters. Most of the dikes get arrested soon after or even during their initialization. When the host rock of the chamber is elastic and the magma flow is vertical it is may be possible to propagate away from the source. To continue this propagation a driving pressure P 0 must be available (Gudmundsson, 1990): P 0 = - (ρ r ρ m ) gz + p e (3) where z is the vertical co-ordinate, ρ m the density of the magma, ρ r the density of the host rock, p e is the excess pressure and g the acceleration because of the gravity. P 0 could also be called net pressure or overpressure. Eq.3 applies when the principal stresses are equal. This is only the case when the host rock is in an isotropic state of stress. To make the equation work even under anisotropic conditions, the term ϭ d = ϭ 1 - ϭ 3 has to be added. For this two-dimentional problem ϭ d is the difference between the maximum and minimum principal stresses: P 0 = - (ρ r ρ m ) gz + p e + ϭ d (4)

6 6 Thomas Heinig Own important point is to know that dikes propagate by expanding their tips. They create an extensional stress field over its upper tip line which leads to the occurrence of offset fractures and their connection when they were filled with magma (Tentler, 2005). Surface fractures Magmatic bodies in the depth can initiate graben on the surface. In a computed model from Pollard et al. (1983) which shows the stresses and the elastic displacements in a 2D, isotropic half space driven by a magmafilled blade-like dike, the dike emplacement leads to a significant in the local stress over the top of the dike. It also predicted the depth, inclination and height of subsurface dikes only from ground displacement profiles that were taken within the central rift zone of Iceland. They came to the conclusion that inclined faults were produced due to stresses induced by dike injections and fissures at the surface occur because of the following upward displacement triggered subsidence over a dike. Data from geodetic leveling in Iceland from 1966 to 1980 show, that dike intrusions can cause a displacement on the surface up to 1 m (Tryggvason, 1984). Fractures preferably develop along joints with an orientation after the regional stress field. As a result of the joining from many individual shorter segments continuous fractures were built. The fracture development depends on the stress concentration above an underlying fault and results from the faults upward and lateral growth (Tentler; 2005). Dike induced faults The combination of the three previous points leads to dike induced faults. The type of faults created by dikes, are normal faults. The fact that magma is beneath the crust in a rift zone like Iceland is the key to understand fault initiation. The mechanical properties of the lithosphere and local stress conditions alter due the variations in magma supply (Behn et al., 2002). Owing to higher stress periods, extension is created all over the rift. This leads to the initiation of planar vertical magma-filled fractures at the contact of brittle crust and liquid magma (Fig. 4a). These blade-like structures develop in regions with negative ϭ 3 and strong magmatic pressure (Fig. 4, Mode I). This pressure pumps magma upwards through the fractures.

7 Dike induced Faults 7 Fig.4. Model of a dike induced fault. (a) Failure is initiated by magma-filled fractures. (b) Failure propagates upwards as an inclined shear fault and induces dilational fractures on the surface. (c) Shear fault links with surface forming a continuous rupture. Inset: stress configurations for propagation of magma-filled dilational fracture, compressive failure and surface dilational fracture; ϭ 3 is horizontal and rift normal, ϭ 2 is horizontal and rift-parallel, ϭ 1 is vertical. Modified from Tentler 2005 Away from the overpressured magma reservoir in higher crustal levels, the stress conditions change during the propagation. ϭ 3 becomes positive (compressive) because vertical body force stress within the ascending magma is negotiated by the horizontal compressive stress (Fig. 4, Mode II). In normal case the magma stops its rising at that depth, but with the presence of constant regional extension, the shear velocity of the growing rupture allows further upward propagation (Le Gall et al., 2000) as a mode

8 8 Thomas Heinig II inclined normal fault (Fig. 4b). The local stress field is affected by variation in stresses because of magma supply and brittle faulting. A normal fault concentrates stresses in its tip zone and releases stress on both sides (Pollard and Aydin, 1984). The fault continues its growing and propagates upwards to shallower levels in the crust as a discrete plane of failure (Tentler, 2005) where it s spread into several fault surfaces with distributed slip. As a response to high stress accumulations extensional fractures occur above the upper tip line of the fault zone when the failure reaches the surface (Gudmundsson, 1992). They may propagate downward as mode I vertical dilation fractures because the stress produced by the fault slip bear down the strength of the surrounding rocks where ϭ 3 is negative (Fig. 4, inset 1). These self-induced dilational fractures would meet a link the upward-growing near the surface fault zones (Fig. 4c). The link between the vertical fracture and the inclined normal fault is relatively abrupt instead of listric as expected from brittle structures. The depth where this linkage happened is laterally constant if the faulted media is mechanically homogeneous. In the transition of mode I (magma dilational fracturing) and mode II (shear failure) a mixed-made is possible. Blade-like dike propagation is becoming subject to shear but still held open by a tensile normal stress (Tentler 2005). Fig.5 Diagrams to show how pattern of fault segmentation can result from the progressive propagation of failure: suggested growth of western faults of (a) Sveinagia and (b) Veggir graben. Modified from Tentler 2005

9 Dike induced Faults 9 The propagation of normal faults and linkage of their segments appears by means of dilational fractures. Because of successive rupture events that produce dilation and a heterogeneous distribution of slip irregularities in displacement profiles are generated. So a reconstruction of the history of segment linkage (Fig.5a) and the different stages in their propagation may is possible (Fig. 5b). The age can be measured directly based on the displacement if the segments grew at the same rate along the fault. The side with the maximum displacement would represent the area where the fault first reaches the surface. Repeated episodes of growth can be seen in throw profiles (Fig. 5b). At first a nucleated segments occur (Fig. 5b (I)) and propagate laterally. Until the segment tips link (Fig. 5b (II)) the throw accumulation is falling short. To satisfy the general displacement-length scaling, a stage of throw accumulation is following to compensate the throw deficit (Fig. 5b (III)). At this stage lateral displacement is retarded. The next segment linkage cycle is started when throw compensation in finished (Fig. 5b (IV) and it is followed by further throw accumulation (Fig. 5b (V). Conclusion Normal faults can be caused by dikes. Most dikes commonly cease to rise in shallower crustal levels, because of the high stress intensity. The upward termination of most magmatic intrusions is located in a region at 1-2 km depth. Fault slips accommodate most of the dikes subsidence. The model supports the theory of growing upward faults instead of growing downwards. The dominant component of crustal displacement along MORs seams to by magmatic emplacement. Tertiary rocks in Iceland have an extension because of dikes three times higher that due to faults. Which type, normal fault or dike, is preferred depends on the volume of available magma. Faults preferential were formed along slow ridges with lower magma volumes, but still be triggered by magma in greater depth. They develop larger planes, greater displacement and stronger seismic signatures. When the amount of magma is great, the intrusions reach shallower levels and are more numerous producing volcanic systems or faults that were limited to the uppermost crust. The faults produce lesser displacement and shallower seismic.

10 10 Thomas Heinig References Behn, M.D., Lin, j., Zuber, M.T., Mechanisms of normal fault development at mid-ocean ridges, J. Geophys. Res. 107 (B4), Langley, J.s., Martel, S., Propagation of normal faults to the surface in basalt Eos Trans. AGU 81 (48), F1121. Gudmundsson, A., Emplacement of dikes, sills and crustal magma chambers on intrusion and extension frequencies. Tectonophysics 176, Gudmundsson, A., Formation and growth of normal faults at the divergent plate boundary in Iceland. Terra Nova 4, Gudmundsson, A., Magma chambers modelled as cavities explain the formation of rift zone central volcanoes and their eruption and intrusion and intrusion statistics. J. Geophys. Res. 103, Gudmundsson, A., Dynamics of volcanic systems in Iceland: examples of tectonism and volcanism at juxtaposed hot spot and mid-ocean ridge system. Annu. Rev. Earth Planet. Sci. 28, Gudmundsson, A., Emplacement and arrest of sheets and dykes in central volcanoes. Journal of Volcanology and Geothermal Research 116 (2002) Le Gall, B.,. Tiercelin, J.-J., Richert, J.-P., Gente, P., Surchio, N.C., Stead, D., Le Turdu, C., A morphotectonic study of an extensional fault zone in a magma-rich rift: the Baringo trachyte fault system, Central Kenia rift. Tectonophysics 320, Paquet, F., Dauteuil, O., Hallot, E., Moreau, F., Tectonics and magma dynamics coupling in a dyke swarm of Iceland. J. Struct. Geol. 29 (2007) Peacock, D.C.P., Parfitt, E.A., Active relay ramps and normal fault propagation on Kilauea Volcano, Haiwaii. J. Struct. Geol. 24, Pollard, D.D., Aydin, A., Propagation and linkage of oceanic ridge segments. J. Geophys. Res. 89, Pollard, D.D., Delaney, P.T., Duffield, W.a., Endo, E.T., Okamura, A.T., Surface deformation in volcanic rift zones. Tectonopysics 94, Rubin, A.M., Dike-induced faulting and graben subsidence in volcanic rift zones. J. Geophys. Res. 97 (B2), Tentler, T., Propagation of brittle failure triggered by magma in Iceland. Tectonophysicis 406 (2005) Tryggvason, E., The widening of the Krafla fissure swarm during the volcanotectonic episode. Bull. Volcanol. 47, Walsh, J.J., Baily W.R., Childs, C., Nicol, A., Bonson, C.G., Formation of segmented normal faults: a 3-D perspective. J. Struct. Geol. 25,