Download Dike induced Faults

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.
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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 ϭ1 stands vertical upon the horizon and rectangular to the extension direction ϭ3, which is in this case
negative. Ϭ2 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.
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The stress vector ϭ1 is horizontal and lies in the compression direction. Ϭ3
is the vector of least stress and is vertical. Ϭ2 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 ϭ1 is horizontal and stands nearly 45° on the fault plane (Fig.1). It marks the direction
of shortening. Ϭ2 is vertical and ϭ3 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
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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 ϭ3(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 pe = 5 MPa. The ticks
represent the direction of the maximum principal compressive stress, ϭ1, 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:
p1 + pe = ϭ3 + T0
(1)
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In this case p1 is the lithostatic stress at the depth in which the chamber is
situated; pe = Pt – p1, the excess magmatic pressure, is the difference between the total magma pressure, Pt, in the chamber at the time of its rupture and the lithostatic stress; ϭ3 is the minimum principal stress; and T0 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 pe increases because of a
fluid addition to the chamber, or when ϭ3 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: p1 = ϭ3 (=
ϭ1). Only during short events when pe > 0 through ϭ3 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:
Pt = ϭ3 + T0
(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 P0 must be available (Gudmundsson,
1990):
P0 = - (ρr – ρm) gz + pe
(3)
where z is the vertical co-ordinate, ρm the density of the magma, ρr the
density of the host rock, pe is the excess pressure and g the acceleration because of the gravity. P0 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 ϭd = ϭ1 - ϭ3 has to be
added. For this two-dimentional problem ϭd is the difference between the
maximum and minimum principal stresses:
P0 = - (ρr – ρm) gz + pe + ϭd
(4)
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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 ϭ3 and strong magmatic pressure (Fig. 4,
Mode I). This pressure pumps magma upwards through the fractures.
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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; ϭ3 is horizontal
and rift normal, ϭ2 is horizontal and rift-parallel, ϭ1 is vertical. Modified from
Tentler 2005
Away from the overpressured magma reservoir in higher crustal levels, the
stress conditions change during the propagation. ϭ3 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
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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 ϭ3 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
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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.
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