Download Metamorphic core complexes

Subject: Oberseminar
25.05.2000
Title of presentation:
Metamorphic core complexes
Written by: Daniel Taugs
Care:
L. Ratschbacher
U. Kroner
K. Stanek
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Contents:
Summary
1.
Introduction
2.
Extension models
3.
Structural geometry of a metamorphic core complex
3.1 Upper plate
3.2 Lower plate
3.3 Detachment fault and multiple detachments
4. Construction
4.1 Metamorphism and deformation profile
4.2 Mylonite and ‘chloritic breccia’
5. Complex model of the development of a metamorphic core complex
6. Role of plutonism/ magmatism
7. Thermal consequences of extension
7.1 Evolution of the simple shear extension model
7.2 Uplift and heat flow
8. Basin and Range – Province
9. Literature
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Summary
Metamorphic core complexes represent regions where extensional shear zones on a
detachment mylonizied lower plate rocks move up from depth to the surface. The arching
of the footwall rocks is a consequence of isostatic, thermal uplift in connection with
continuing extension, decompression, and erosion. Metamorphic core complexes are
divided into three sections: upper plate, lower plate and detachment fault. The warped up
of the lower plate produced excisements and incisements splays (multiple detachments).
Through this process the upper and lower plate is more structured. The exhumation of the
lower plate is accompanied by a retrograde, uncompleted metamorphism, for example the
mylonites become overprinted by brecciation during uplift and fluid circulation. A
carapace of mylonites and ‘chloritic breccia’ are typically for the surfaces of metamorphic
core complexes. The formation of metamorphic core complexes may also be a result of
plutonism/ magmatism. The intrusions are associated with differential uplift and thermal
expansion. Depending on the distribution of magma chambers different kinds of
culmination are created. Short ductile deformation in the brittle crust takes place by
thermal pulse. Under this condition mylonites are formed. The dip angle of detachment
faults and the rate of extension influence the rift structures, especially the uplift and heat
flow. For small dip angles and extension, the amount of uplift and heat flow results in low
grades.
1. Introduction
Metamorphic core complexes were first recognised in the Cenozoic Basin and Range Province of the North American Cordillera, but core complexes are found in orogens
world wide; the Norwegian Caledonids, the Tauern Window of the Eastern Alps and the
Zentralmassiv are examples from Europe.
Metamorphic core complexes describe the outcrop of distinct bodies of metamorphic rocks
in the core or hinterland of an orogen. These gneiss domes form as a result of extreme
crustal extension, when the middle and lower crust is dragged beneath an extensional
detachment and is exposed at the surface. The metamorphic rocks in the footwall are
uplifted through different levels of metamorphic conditions and deformation types. A
profile through the crust and mantle segment which contains a metamorphic core complex
is given in Fig 2.1. The exhumation of the middle and lower crust results from isostatic
rebound due to tectonic denudation, additionally it may also be result of local isostatic
adjustments due to magmatic intrusions.
A metamorphic core complex is divided into an upper plate (hanging wall) and lower plate
(footwall), separated by a major detachment fault. The detachment fault cuts the entire
lithosphere, thus cutting through brittle and ductile deformation levels.
What distinguishes metamorphic core complexes from other regions of metamorphic rock
is a characteristic zonal arrangement of structures.
2. Extension models
Continental extension and it’s structural expression as metamorphic core complexes result
from the balance of horizontal movements in the crust and mantle. Extension commonly
results from gravitational adjustment of previously thickened crust. In the Basin and Range
– Province the subduction of hot oceanic lithosphere is additionally an important factor for
extensional tectonics. Continental extension is accomplished along normal faults in the
brittle crustal level and viscoplastic flow in the ductile level. There are two different types
of extension models, a pure and simple shear one. Pure shear extension and simple shear
extension produce different structural geometries and topography.
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Pure shear extension results in a symmetric structural architecture. The model is
schematically illustrated in Fig. 2.1a. The symmetry is characterised by a regular thinning
and an equal amount of uplift of the rift shoulders. During the extension the asthenosphere
in the footwall is warped up as a result of isostatic uplift. The shear zone or detachment
(decollement) represents the decoupling zone between ductile and brittle extension, in
other words it represents the brittle – ductile transition (e.g., Miller et al., 1983).
Simple shear extension (Fig. 2.1b) results
in asymmetric uplift, structural geometry,
topography and heat flow distribution.
The extension occurs along a shallowly
dipping detachment fault, which is a
lithosphere – scale dislocation [Wernicke,
1985]. Along the detachment, lower
crustal segments may be observed.
The dip of the shear zone, its width and
the rate of movement influence the
exhumation of the footwall.
A description of these facts and their
influences is given on page 10/11.
a)
b)
Fig . 2.1 schematic geologic models (a) pure shear
extension and (b) simple shear extension (Buck et
al., 1988)
Geophysical and geological fieldstudies and observations in the sedimentary records of
passive margins characterise the origin of metamorphic core complexes in association with
simple shear extension. It has been found that the simple shear model explains the
geological and geophysical data, in particular, the pure shear model cannot predict the
asymmetries in structures, thinning and continental uplift.
3. Structural geometry of a metamorphic core complex
Metamorphic core complexes are divided into an upper plate, a lower plate and a
detachment fault. The detachment fault separates the single plate segments and is described
by a shallowly dipping normal fault geometry through the entire lithosphere.
3.1 Upper plate
The upper plate is characterised by brittle stretching as reflected by normal fault
geometries. Normal faults are shown approximately planar from the surface to the zone
where they root in the subhorizontal detachment. They nucleate near the base of the brittle
upper crust in depth up to 10 km with a dip between 30° and 60°. The faults flatten with
depth and become horizontal (listric character ).
Normal faults result normal-fault earthquakes. Such earthquakes on the continents nucleate
in the depth range of 6 km (e.g. Greece/ Turkey) to 15 km (e.g. Basin and Range Province). As a result of an enormous increase of strain rate during large earthquakes
normal fault ruptures continue into the ‘ductile’ lower crust with gentler dips. Under these
conditions the lower crust becomes brittle for a short time.
The dip of the normal faults is influenced by rotation. The thinning of the upper plate
results in rotation of faults and blocks towards a horizontal orientation. There are two main
reasons for rotation. First, movements on planar faults represent simple shear yet the
overall result must be pure shear. Pure shear is enforced by vertical confinement, and is
achieved by a combination of rotation and simple shear (Fig. 3.1.1).
The second reason is the existence of the planar non-rotational detachment fault zone in the
footwall of the upper plate over the width of the entire lithosphere. The planar non4
rotational fault zone produces an isostatic disequilibrium and a long-wavelength gravity
anomaly. In addition such a fault zone leads to a Moho depression. In other words, the
upper plate faults are truncated at their base by very low angle faults of large areal extent,
which appear to be normal slip detachment faults.
With continuous extension the blocks and faults rotate. The amount of rotation is described
by the extension factor β. The extension factor is defined as quotient between post
extension length l1 and the pre – extension non –
rotational length l0 (Fig . 3.1.1). For example a
normal fault, which started with a dip of 60° and
rotated into a dip of 30° implies a relative
extension, β, of 1.7. Indeed, a value for β of 1.7 is
the maximum amount of extension which can be
achieved along a single set of faults. If stretching
exceeds β = 1.7, a new generation of normal faults
is formed. The second generation (active system)
cuts the rotated first fault generation. This
Fig. 3.1.1 connection between the modes of
processes are associated with seismic activity
shear and rotation , ß = l1/lo (Jackson, 1987)
on a single generation of normal faults.
In summary, there are two modes of extensional faulting in the brittle upper crust in
metamorphic core complexes, the seismically active steep normal faults (30 and 60°) and
the low angle (<20°) normal faults (detachment), generally seismically inactive. The
rotation is associated with a continuing thinning of the upper plate, which results in the
exhumation of the middle and lower crust. Rotation of blocks along steep normal faults
results in multiple generation of high angle normal faults.
3.2 Lower plate
The lower plate is subject to intense ductile deformation in intracrustal zones of noncoaxial
laminar flow (Lister and Davis, 1983). These zones are underlain by nonmylonitic
metamorphic (high grade) rocks and granitoid intrusions.
3.3 Detachment fault and multiple detachments
Low angle normal faults or detachment faults are characterised by gentle dip and
penetration of the whole crust (Wernicke, 1981). “An alternative is that detachment faults
are merely upper crustal manifestations of shallow – dipping, normal – slip shear zones
which widen with depth,...” (Reynolds, 1982, 1985). The following description is based on
the Wernicke model.
The possible origin for such structures in the lower crust may be either: (i) through rotation
– where second-generation upper-crustal faults continue to transfer motion on to old firstgeneration faults in the lower crust; or (ii) as weak layers”(Jackson, 1987). But weak layers
controlled by temperature are likely to be roughly horizontal and are unlikely to cut
downwards through the entire lower crust.
Multiple generations of detachment faults splay from the same movement zone at depth as
a result of uplift of the middle and lower crust. During the extension, especially during the
formation of the culmination, the first generation detachment fault becomes progressively
curved, until finally a new splay develops. The new fault cuts upwards into the lower
portions of the upper plate. In Fig. 3.3.2.1, four generations of detachments are shown
above the same basement culmination. The development of a culmination means that the
active master detachment fault becomes progressively more bent. As a result, a new
detachment fault propagates from the culmination, allowing for easier slip. The youngest
generation shows the smallest deviation from planarity.
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Fig. 3.3.1 development of splays as a result of
basement culmination (Lister/ Davis, 1989)
Fig. 3.3.2 multiple detachments leads to a new
active normal fault system with listric character
(Lister/ Davis, 1989)
Fig. 3.3.3 development of incisement – excisement structures as the lower plate bows upwards
(Lister/ Davis, 1989)
When a splay bites into the upper plate (climb upwards), excisement is formed. During this
process the first generation of detachment faults becomes kinematically inactive and a new
active system with younger listric nonrotational, high – angle normal faults is formed
associated with a new detachment surface. Fig. 3.3.2.2a shows a major splay that has
branched from the active detachment in the footwall. In (b) this splay has become the new
active detachment surface. Younger, listric, normal faults have formed in connection with
a new active system.
When a splay bites into the lower plate, incisement is formed. The new detachment surface
sliced the upper portions of the lower plate. In this way, relatively deep levels during the
exhumation process can be exposed. In Fig. 3.3.2.3, incisement and excisement are
illustrated as a result of bowing of the lower plate.
Summarising, there appear to be two different types in which a detachment fault can
operate, exiscement and incisement.
4. Construction
4.1 Metamorphism and deformation profile
Major detachment zones cut through different
metamorphic conditions and deformation types
(Fig. 4.1.1a and b). With increasing depth
major shear zones evolve from cataclastic rocks
into regions where mylonities are dominant
(Sibson 1977, 1983).
In most of the metamorphic core complexes the
exposed succession begins in the footwall with
Fig . 4.1.1 metamorphism/ deformation profile
(Lister/ Davis, 1989)
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deformation by non – coaxial laminar flow and amphibolite (800°C/ 8 kbar) or high
greenschist fazies (500°C/ 6 kbar) conditions. Typical microstructures are micafish and
asymmetric quartz aggregates in LS – tectonites (Lister and Davis, 1983).
The brittle – ductile transition marks the transition from brittle to ductile deformation and
is characterised by structures typically for both deformation levels. The transition occurs at
lowest greenschist facies conditions. In the brittle crust, faults or shear zones are associated
with brecciation, cataclasis and seismic slip.
4.2 Mylonite and ‘chloritic breccia’
In the normal slip shear zones, as the lower plate is dragged out from beneath the upper
plate, mylonites form as a result of movement in depth > 10 km. This depth is
characterised by ductile deformation (see also ‘Role of plutonism/ magmatism’). The sense
of shear for the mylonites is the same as for detachment fault. In general, the movement
vectors for brittle and ductile events are parallel.
During the uplift, widespread hydrothermal alteration takes place, forming ‘chloritic
breccia’. In fact, the deformation and formation process depends on the pressured pore
fluids. The fluids influence the embrittelment criteria (from ductile to brittle behaviour)
and describe a retrograde uncompleted metamorphism. When the rocks pass the brittle –
ductile transition, the ductile fabrics and microstructures are overprinting by brittle
deformation structures, and in many cases ultracataclasites are formed.
In Fig 4.2.1 the generalised geometry of a metamorphic core complex is shown, in
particular the location of mylonites and ‘chloritic breccia’. The interior zone (middle and
lower crust) is surrounded by a carapace of mylonite. Rocks above this zone are
nonmylonitic. The basal contact of the mylonite is gradational, such the fabric intensity
decreases downward into the lower nonmylonitic rocks of the interior zone. The foliation is
arched into a gently dome dependenton the formation of a culmination.
Rotational normal faults cut the hanging wall above the detachment, but do not cut the
detachment fault itself. “In fact, locally, faulting has isolated individual blocks of the
hanging wall, so that they appear as islands of unmetamorphosed strata floating in a
‘sea’ of mylonite”(Pluijm, Marshak, 1997).
Fig. 4.2.1 generalised geometry of a
metamorphic core complex
(Pluijm, Marshak, 1997)
5. Complex model of the development of a metamorphic core complex
The evolution of a ‘typical’ Basin and Range metamorphic core complex or detachment
terrain is illustrated in Fig. 5.1 (last page), but this model contains only the general
geometric structures, the multiple detachments and the development of new active systems
are not documented.
In Fig. 5.1a the typical structural feature of a metamorphic core complex is shown, the
shallow – dipping detachment fault (10 – 30°) dividing lithosphere into an upper plate and
a lower plate. During continued extension the upper plate becomes more complex, high –
angle normal faults are formed which flatten with depth (Fig. 5.1b). The thinning of the
upper plate results in rotation of blocks along the normal faults dependent on extensional
process (Fig. 5.1c). In the last stage, the middle and lower crust is dragged out from
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beneath the detachment fault as a result of unloading and isostatic rebound (Fig. 5.1d). The
complex model gives also an overview about the time scale and the dimension of
exhumation of metamorphic core complexes. The time period from the starting point of
extension to the exhumation is approximately 10 – 14 Ma and the denudation is about 20
km (approximately 6 kbar) in the Basin and Range - Province.
6. Role of plutonism/ magmatism
The rapid unloading in connection with thinning of the upper plate results in extreme
decompression of the mantle and underlying asthenosphere. The formation mafic magma is
based on this decompression. When the mafic magma is trapped in the continental crust,
partial melting of the adjacent crust is possible.
The lower plate of a metamorphic core complex can be intruded by synkinematic
granitoids as one of the main results of decompression. The granitoids may ductilely
overprinted dependent on uplift process. The granitoids, especially as sills or sill swarms,
are found in the central part of gneiss domes and are emplaced into shallow crustal levels.
Gravity lows and aeromagnetic data (Klein, 1982) may confirm the presence of intrusions
at depth. The formation of metamorphic core complexes may be behind the theory of
isostatic rebound (e.g. Wernicke, 1988) also a result of plutonic activity during episodes of
continental extension. Such intrusions may be the major cause for differential uplift of the
footwall during tectonic denudation of metamorphic core complexes. The uplift results
from the emplacements of sill – like magma chambers. During this process the lower plate
becomes more warped. When the intrusions are localised in one region the uplift is
concentrated and results in one culmination. When the magma impulses are regionally, the
uplift is also distributed and leads to a chain of culmination (Fig. 6.1b-c).
Igneous intrusions (sills or dykes or deeper plutons) cause also periodic thermal pulse (heat
input) in the surrounded country rock. If intrusions take place at shallow crustal levels, the
thermal pulse may induce ductile behaviour in otherwise brittle rocks. Through the
increase of temperature (> 300 – 450°C) ductile deformation takes place during a relatively
short period of time, but the temperature decreases rapidly away from the igneous body.
Dependent on the magmatism a short – lived ductile shear zone is created. The shear zone
shows approximately horizontal character (see Fig. 6.1a). Mylonite formation coincides
with the thermal pulse and short – lived ductile shear zone. Mylonite forms under
greenschist and/or amphibolite facies conditions. During the exhumation the mylonites
become exposed at the surface and described a ‘mylonitic front’. The ‘mylonitic front’
marks the abrupt first appearance of mylonite fabrics and has been interpreted as the depth
– dependent transition between brittle and ductile behaviour.
Fig. 6.1 (a) gently dipping shear zone beneath
high angle normal faults, (b) and (c)
differential uplift in connection of plutonism
(Lister and Baldwin, 1993)
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7. Thermal consequences of extension
Simple shear extension creates asymmetries in topography, uplift, subsidence and heat
flow. Especially the dip angle of detachment faults, the rate of extension (see extension
factor β) and the crustal thickness influence these parameters. The following description
gives a short overview about the temporal development of a simple shear extension model
and the connection between the dip angle and rate of movement on the one side, uplift and
heat flow on the other side.
7.1. Evolution of the simple shear extension model
Figure 7.1.1 illustrates the evolution of a simple shear extension model. The panels show
the development of topography, subsidence, thermal uplift and heat flow at 4 m.y. intervals
between 2 and 22 m.y. after the start of rifting. The dip of the shear zone is 15°. In this case
the upper plate is fixed and the lower plate is moving to the left by a rate of 1 cm/year. The
shear zone has zero width.
The topography (panel a) shows typical asymmetry, the left side is characterised by crustal
thinning, rotation and tilting (negative topography anomaly), the right side describes the
antithetic fault structures and documents the positive topography anomaly. The width of
the basin floor is equal to the subsidence (panel b). The subsidence is caused by the
extension and by the cooling of the lithosphere. The maximum of thermal uplift and heat
flow (panels c/d) is centred over “the intersection of the shear zone with the surface”( Buck
et al., 1988). The thermal uplift is broader than the region of heat flow. The differences are
based on temperature influences, whereas the thermal uplift is a function of the vertical
changes of temperature within the entire lithosphere, the heat flow is only characterised by
the temperature or thermal gradient at the surface. Thermal uplift and heat flow are
concentrated in this region where the crust has not been thinned by brittle reactions.
All parameters are influenced by the asthenosphere uplift in the footwall, especially the
thermal uplift mimics the asthensphere uprise. The peaks of graphic illustrations are all
centred in the region where the crustal overlay is smallest (see also Fig. 2.1).
Fig. 7.1.1 evolution of the simple shear extension model
(Buck et al., 1988)
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7.2 Uplift and heat flow
The dip angle of detachment fault and the rate of extension influence the uplift of the upper
and lower plate rift shoulders and the heat flow. In Figure 7.2.1a the hanging wall and the
footwall uplifts are documented for detachment dips of 15, 30 and 45° and a plate
separation rate of 1 cm/ year as a function of time (x – axis). The hanging wall uplift
(upper plate) is greater than the footwall (lower plate) uplift as a result of different heating,
because the upper plate rift shoulder is characterised by the heating from the advection of
hotter lower plate lithosphere and asthenosphere material and by material circulation along
the detachment fault. The lower plate rift shoulder is thermally influenced only by material
beneath the detachment fault. Further for small dip angles, the uplift results in small
amounts. Fig. 7.2.1b shows equal results for different plate separations by a detachment of
15°. In fact, the smaller the separation rate, the smaller is the hanging wall and footwall
uplift, because the greater are the influences of erosion.
The dip angle also controls the rate of heat flow. Peak heat flow as a function of time for
different detachment dips are shown in Fig. 7.2.1c (1 cm/year). Fig. 7.2.1d shows general
results for variations in the separation rates. For high dip angles and high velocity
extension the rate of vertical advection (depends on the vertical velocity of material) can be
higher than the rate of cooling. This leads to linear increase in the heat flow.
One of the most obvious results of simple shear extension is that uplift and the other
parameters are limited by the dip angle and the total amount of extension. A small rate of
extension or dip angle can never produce large uplift or heat flow. In other words, the more
shallow the dip angle and the smaller the rate of extension, the lower is the maximum
uplift.
a)
b)
c)
d)
Fig. 7.2.1 (a-d) dependence of uplift and heat flow on the dip angle of the detachment
fault and on the rate of extension (Buck et al., 1988)
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8. Basin and Range – Province
The Basin and Range - Province extension is documented by Cenozoic ages. A thin crust
(25 km thick), high heat flow and topography is best explained by the simple shear model.
The overall of extension is more than 300 – 400%, that is 250 km stretching. The Basin
and Range – Province is the youngest extensional structure in the North American
Cordillera. Major extension is documented for the last 16 Ma by a rate of movement of 15
mmy-1.
Between British Columbia and Mexico about 30 metamorphic core complexes are located
(Wernicke et al., 1987). In Fig. 8.1.1, the distribution of the metamorphic core complexes
in the north of American Cordillera is illustrated. The Basin and Range – Province is
located in the centre of illustration. The complexes are situated between the Mesozoic
batholiths in the west and the foreland thrust belt in the east. The orientation of structures
is equal to the orientation of the collision zone on the west side. The metamorphic core
complexes are a key structure for late Cretaceous to Holocene extension in the Cordillera.
The exhumation of gneiss domes was a process which occurred over a short time period,
e.g. the Whipple mountain core complex in SE California was exhumed from mid – crustal
levels in less than 2 Ma (between 20 to 18 Ma). The largest metamorphic core complex
with 500 km length is the “Shuswap Complex in the Omineca belt of British Columbia”
(Okulitch, 1984; Ranalli et al., 1989).The structure and history is different from those of
the Basin and Range - Province.
Fig. : 8.1.1 distribution of metamorphic core in
the North American Cordillera
(Windley, 1996)
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Figure to point 5.
Fig. 5.1 complex model of the formation of a metamorphic core complex (Wernicke 1988)
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9. Literature
Lister G. S., Davis G. A. (1989): The origin of metamorphic core complexes and
detachment faults formed during Tertiary continental extension in the northern Colorado
River region, U.S.A.. Journal of Structural Geology, Vol.11, 65 - 94.
Jackson, J. A. (1987): Active normal faulting and crustal extension. Continental
Extensional Tectonics. Geological Society Special Publication, No. 28, 3 - 17.
Buck W. R., Martinez F., Steckler M. S., Cochran J. R. April (1988): Thermal
consequences of lithospheric extension: pure and simple. Tectonics, Vol. 7, No. 2, 213 –
234.
Lister G. S., Baldwin S. L. July (1993): Plutonism and the origin of metamorphic core
complexes. Geology, v. 21, 607 - 610.
Wernicke B. (1985): Uniform – sense normal simple shear of the continental lithosphere.
Can. J. Earth Sci., No 22, 108 - 125.
Voorhoeve H. (1988): Th thermal evolution of lithosphere extending on a low – angle
detachment zone. Basin Research, No 1, 1 - 9.
Lister G. S., Etheridge M. A., Symoneds P. A. March (1986): Detachment faulting and the
evolution of passive continental margins. Geology, v. 14, 246 - 250.
Windley B. F. (1996): The Evolving Continents. Wiley, third edition, 98 – 104.
Van der Pluijem B. A., Marshak S. (1997): Earth Structure – an introduction to structural
geology and tectonics. WCB/ McCraw-Hill, 326 – 330.
Kroner U. (1994): Postkollisionale Extension am Nordrand der Böhmischen Masse – Die
Exhumierung des Sächsischen Granulitgebirges. Dissertation, 81 – 101.
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