Download Lithosphere delamination in continental collisional orogens: A

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Journal of Geophysical Research: Solid Earth
RESEARCH ARTICLE
10.1002/2016JB013106
Key Points:
• Negative buoyancy from thickened
lithosphere drives delamination
• Rheology, density, and convergence
rate control lithosphere delamination
modes
• Initiation and migration styles of proplate versus retro-plate delamination
Lithosphere delamination in continental collisional
orogens: A systematic numerical study
Zhong-Hai Li1, Mian Liu1,2, and Taras Gerya3
1
Key Laboratory of Computational Geodynamics, College of Earth Sciences, University of Chinese Academy of Sciences,
Beijing, China, 2Department of Geological Sciences, University of Missouri, Columbia, Missouri, USA, 3Institute of
Geophysics, Department of Earth Sciences, ETH Zurich, Zurich, Switzerland
Abstract Lithosphere delamination is believed to have played a major role in mountain building; however,
Supporting Information:
• Supporting Information S1
Correspondence to:
Z.-H. Li,
[email protected]
Citation:
Li, Z.-H., M. Liu, and T. Gerya (2016),
Lithosphere delamination in continental
collisional orogens: A systematic
numerical study, J. Geophys. Res. Solid
Earth, 121, doi:10.1002/2016JB013106.
Received 21 APR 2016
Accepted 14 JUN 2016
Accepted article online 19 JUN 2016
the mechanism and dynamics of delamination remain poorly understood. Using a 2-D high-resolution
thermomechanical model, we systematically investigated the conditions for the initiation of lithosphere
delamination during orogenesis of continental collision and explored the key factors that control the various
modes of delamination. Our results indicate that the negative buoyancy from lithosphere thickening during
orogenesis could cause delamination, when the reference density of the lithospheric mantle is not lower than
that of the asthenosphere. In these cases, compositional rejuvenation of depleted continental lithosphere by
magmatic/metasomatic plume- and/or subduction-induced processes may play crucial roles for subsequent
lithosphere delamination. If the reference density of the lithospheric mantle is less than that of the
asthenosphere, additional promoting factors, such as lower crust eclogitization, are required for delamination.
Our numerical simulations predict three basic modes of lithosphere delamination: pro-plate delamination,
retro-plate delamination, and a transitional double-plates (both the pro-plate and retro-plate) delamination.
Pro-plate delamination is favored by low convergence rates, high lithospheric density, and relatively strong
retro-plate, whereas retro-plate delamination requires a weak retro-plate. The Northern Apennines and Central
Northern Tibetan Plateau are possible geological analogs for the pro-plate and retro-plate delamination modes,
respectively. Our model also shows significant impact of delamination on the topographic evolution of orogens.
Large-scale lithosphere delamination in continental collision zones would lead to wide and flat plateaus,
whereas relatively narrow and steep mountain belts are predicted in orogens without major delamination.
1. Introduction
Continental lithosphere delamination refers to the detachment and sinking (foundering) of lithospheric mantle, with or without a certain portion of the lower crust, from the tectonic plate. The concept of lithosphere
delamination was introduced by Bird [1978, 1979], who proposed that the dense lithospheric mantle might
peel off from the crust and sink into the underlying asthenosphere, if with an elongated conduit connecting
the hot asthenosphere with the base of continental crust. A similar mechanism is the convective removal or
thinning of continental lithosphere by the Rayleigh-Taylor gravitational instabilities developed in a thickened
lithospheric mantle [e.g., Houseman et al., 1981; England and Houseman, 1989; Houseman and Molnar, 1997;
Liu et al., 2004; Gorczyk et al., 2012; Gorczyk and Vogt, 2013]. In both cases the mantle lithosphere is removed
and sinks into the asthenosphere because of its negative buoyancy.
Delamination is used to explain the thinned or absent mantle lithosphere under orogens where the same
processes that thickened the crust should have also thickened the mantle lithosphere. The examples include
many orogens in the Alpine-Himalayan collisional belt such as the Tibetan Plateau [Bird, 1978; Jimenez-Munt
et al., 2008], the Anatolia [Keskin, 2003; Gogus and Pysklywec, 2008a], the Apennines [Giacomuzzi et al., 2011;
Chiarabba et al., 2014], and the Alboran Sea [Seber et al., 1996; Timoulalia et al., 2014]. Other orogenic belts with
proposed delamination include the Cordillera in western America (both North and South), including the
Altiplano-Puna Plateau [Kay and Kay, 1993; Beck and Zandt, 2002], the Sierra Nevada [Ducea and Saleeby,
1998; Zandt et al., 2004], the Colorado Plateau [Bird, 1979; Levander et al., 2011], and the Canadian Cordillera
[Bao et al., 2015]. Delamination has also been proposed for both ancient cratons such as the North China craton
[Gao et al., 2009; Zhu et al., 2012] and young orogens such as the New Guinea [Cloos et al., 2005].
©2016. American Geophysical Union.
All Rights Reserved.
LI ET AL.
In collisional orogens, lithosphere delamination may occur in either the overriding plate or the subducting
plate, or both, with variable spatial patterns. In the Himalayan-Tibetan system, delamination, or convective
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thinning, is proposed [England, 1993; Molnar et al., 1993] to have occurred under Central Northern Tibetan
Plateau, where the lithospheric mantle is thought to be abnormally thin or totally missing [Owens and
Zandt, 1997; Tilmann et al., 2003]. In contrast, delamination could occur in the subducting continental plate,
with the mantle lithosphere peeling off from the overlying crust. For example, the Northern Apennines is a
fold-and-thrust belt that formed in response to the postcollisional retreating of the subducted Adria microcontinental lithosphere that is delaminated from the overlying crust [Faccenna et al., 2001; Faccenda et al.,
2009; Chiarabba and Chiodini, 2013; Chiarabba et al., 2014].
Both analog and numerical models have been used to study the dynamics of delamination [e.g., Morency and
Doin, 2004; Valera et al., 2008, 2011; Gogus et al., 2011; Bajolet et al., 2012; Francois et al., 2014]. In previous
numerical studies, lithosphere delamination was usually induced by prescribed weak zones in the initial model
configuration [e.g., Bird, 1979; Schott and Schmeling, 1998; Gogus and Pysklywec, 2008a, 2008b; Bajolet et al.,
2012; Wang and Currie, 2015]. These models focus on the consequences of lithosphere delamination on the
topography as well as the mechanical and thermal evolution of the overlying continent. In addition, some models simulate delamination induced by sinking and retreating of high-density subducting lithosphere decoupled
from its overlying low-density crust [e.g., Beaumont et al., 2006; Faccenda et al., 2009; Gray and Pysklywec, 2012;
Ueda et al., 2012]. Alternatively, other models simulate various delamination processes induced by eclogitization of the thickened crust under modern [e.g., Krystopowicz and Currie, 2013] or Archean mantle temperature
conditions [e.g., Johnson et al., 2014; Sizova et al., 2015; Fischer and Gerya, 2016].
In this study, we constructed two-dimensional (2-D) high-resolution thermomechanical models to systematically investigate the initiation and evolution of continental lithosphere delamination during collisional orogenesis. We found that the modes of delamination depend on the rheology of the collisional plates, the
density contrast between lithospheric mantle and the asthenosphere, the eclogitization of the thickened
crust, and the convergence rates.
2. Numerical Methodology
2.1. Governing Equations
The lithospheric deformation and delamination are simulated numerically by solving the governing equations of conservation of mass, momentum, and energy and the constitutive equations with the 2-D finite
difference and marker-in-cell methods in an irregularly spaced staggered grid in Eulerian coordinates
[Gerya and Yuen, 2003, 2007; Gerya, 2010; Li, 2014].
1. Mass conservation (Continuity equation)
The conservation of mass is approximated by the incompressible continuity equation (extended Boussinesq
approximation):
∂v x ∂v y
þ
¼0
∂x
∂y
(1)
where vx and vy are velocities in the x and y directions, respectively.
2. Momentum conservation (Stokes equations)
′
∂σ ′xx ∂σ xy ∂P
þ
¼
∂x
∂x
∂y
∂σ ′yx ∂σ ′yy ∂P
gρðC; P; T Þ
þ
¼
∂y
∂x
∂y
(2)
where x and y denote the horizontal and vertical coordinates; P is the dynamic pressure; g is the gravitational
acceleration; σ ′xx , σ ′xy , and σ ′yy are the deviatoric stress tensor components, with constitutive relations as follows:
:
:
:
; σ ′yy ¼ 2η
σ ′xx ¼ 2ηeff ε xx ; σ ′xy ¼ 2ηeff ε xy
eff ε yy
∂v y :
∂v x :
1 ∂v x ∂v y
(3)
:
ε xx ¼
; ε yy ¼
; ε xy ¼
þ
2 ∂y
∂x
∂y
∂x
where ηeff is the effective viscosity that depends on the composition, pressure, temperature, and strain rate;
: :
:
ε xx , ε yy , and ε xy are the deviatoric strain rate components.
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Table 1. Viscous Flow Laws Used in the Numerical Experiments
ID Symbol
RA
RB
RC
RD
Flow Law
1
E (kJ mol
Wet quartzite
Plagioclase An75
Dry olivine
Wet olivine
154
238
532
470
)
a
1
V (J MPa
1
mol
)
8
8
8
8
a
10.1002/2016JB013106
n
2.3
3.2
3.5
4.0
n 1
AD (MPa
s
4
3.2 × 10
4
3.3 × 10
4
2.5 × 10
3
2.0 × 10
)
η0 (Pa s)
17
1.97 × 10
22
4.80 × 10
16
3.98 × 10
20
5.01 × 10
(6n)
η0 is the reference viscosity, which is calculated by η0 = (1/AD) × 10 . References are from Kirby [1983], Kirby and
Kronenberg [1987], Ranalli and Murphy [1987], Ji and Zhao [1993], and Ranalli [1995].
For a specific rock type, i.e., composition (C), its density ρ depends explicitly on pressure (P) and temperature (T):
ρP;T ¼ ρ0 ½1 αðT T 0 Þ½1 þ βðP P0 Þ
(4)
where the reference density ρ0 is the density under the conditions of P0 = 0.1 MPa and T0 = 298 K (i.e., pressure
and temperature at Earth’s surface); α and β are the thermal expansion coefficient and the compressibility
coefficient, respectively.
3. Energy conservation (Energy equation)
∂qy
∂q
¼ x
þ Hr þ Ha þ HS
∂x
∂y
∂T
∂T
qx ¼ k ðT; P; C Þ ; qy ¼ k ðT; P; C Þ
∂x
∂y
dP
Ha ¼ Tα ;
dt
:
:
:
HS ¼ σ ′xx ε xx þ σ ′yy ε yy þ 2σ ′xy ε xy
ρC p
DT
Dt
(5)
where T represents temperature; Cp is the effective isobaric heat capacity; DT/Dt is the substantive (material)
time derivative of temperature; qx and qy are the thermal heat flux components; k is the thermal conductivity,
which depends on temperature (T), pressure (P), and composition (C); Hr, Ha, and HS denote the radioactive
heat production, adiabatic heat production, and shear heating, respectively.
2.2. Viscoplastic Rheology
The viscosity of rocks deforming by dislocation creep is defined as
1
E þ PV
: 1n
(6)
ηductile ¼ ð ε II Þ n ðAD Þn exp
nRT
: : 1=2
:
is the second invariant of the strain rate tensor; AD (preexponential factor), E
where ε II ¼ 0:5 ε ij ε ij
(activation energy), V (activation volume), and n (creep exponent) are experimentally determined flow law
parameters (Table 1). The parameter R is the gas constant.
We combine the viscous rheology with an extended Drucker-Prager yield criterion [e.g., Ranalli, 1995], which
is equivalent to Mohr-Coulomb criterion in 2-D plane strain approximation, to simulate the viscoplastic behavior of the rocks:
σ yield
ηplastic ¼ :
2 ε II
(7)
σ yield ¼ C 0 þ Psin φdry λ
where σ yield is the yield stress; P is the dynamic pressure; C0 is the residual rock strength at P = 0; φdry is the
internal frictional angle of dry rocks; λ is the pore fluid/melt coefficient that controls the brittle strength of
P
fluid/melt containing porous or fractured media, which can be interpreted as λ ¼ 1 fluid=melt
. Because the
P
fluid/melt parameter Pfluid/melt is not directly calculated in the model, we use a range of “sin(φdry)λ” values
to represent the effective friction coefficient, based on previous systematic investigations [e.g., Gerya and
Meilick, 2011; Vogt et al., 2012; Gerya et al., 2015].
The minimum value of the viscous or plastic viscosity defines the effective composite viscosity in the model
[Ranalli, 1995]:
ηeff ¼ min ηductile ; ηplastic
(8)
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Figure 1
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a
Table 2. Material Properties Used in the Numerical Experiments
3
b
Section
ρ0 (kg m
Material
All plates
Pro-plate and end-plate
Retro-plate
f
Sediment (1, 2)
Weak-zone mantle (9)
Asthenosphere (10)
Upper crust (3)
Lower crust (4)
Lithospheric mantle (7)
Upper crust (5)
Lower crust (6)
Lithospheric mantle (8)
References
a
1
1
2700
3300
3300
2700
3000
3300
2700
3000
3300
1, 2
)
c
1
k (W m
K1
K3
K3
K1
K2
K3
K1
K2
K3
3
1
K
)
3
Hr (μW m
)
e
Plastic C0 (MPa)
Plastic sin(ϕ dry)λ
RA
RD
RC
RA
RB
RC
RA
RB/RA
RC/RD
4
10
10
10
10
10
10
10
10
10
5
0.15
0.006
0.6
0.15
0.15
0.6
0.15
0.15
0.6/0.006
5
1
0.022
0.022
1
1
0.022
1
1
0.022
1
5
d
Viscous Flow Law
1
5
e
1
The isobaric heat capacity Cp = 1000 J kg K , thermal expansion coefficient α = 3 × 10 K , and the compressibility coefficient β = 1 × 10 MPa are
used for all rock types.
b
Numbers of materials correspond to Figure 1c.
c
K1 = [0.64 + 807/(TK + 77)] exp(0.00004PMPa); K2 = [1.18 + 474/(TK + 77)] exp(0.00004PMPa); K3 = [0.73 + 1293/(TK + 77)] exp(0.00004PMPa).
d
Parameters of viscous flow laws are shown in Table 1.
e
Strain weakening effect is applied for plastic rheology, in which both cohesion (C0) and effective friction coefficient (sin(ϕ dry)λ) decrease with larger strain rate.
f
References 1–5 are Turcotte and Schubert [1982], Bittner and Schmeling [1995], Clauser and Huenges [1995], Ranalli [1995], and Vogt et al. [2012], respectively.
3. Initial Model Configuration and Boundary Conditions
We used a large-scale model domain (4000 × 400 km) to study the dynamics of continental collision and lithosphere delamination. Using a nonuniform rectangular numerical grid, we represent the collision zone with a
1 × 1 km high resolution while using a 5 × 1 km for the rest of the model domain. A dense grid with more than
15 million of active Lagrangian markers is used to trace and mark internal lithological boundaries, material
properties, and temperature. Although the model length is 4000 km, the deformation localizes in a relatively
narrow (ca. 1000 km) region of interest. Effects of the Earth’s curvature are therefore neglected in our simplified Cartesian model.
In the initial model, the continental plate includes a pro-plate, a retro-plate, and an end-plate (Figure 1b). An
initial weak zone is placed between the pro-plate and retro-plate, which is applied to locate the initial convergence, not for initiating delamination. The initial continental lithosphere includes a 20 km thick upper
crust, a 15 km thick lower crust, a 65 km thick lithospheric mantle, and the subjacent asthenosphere.
Properties of variable rock types (Figure 1c) are summarized in Tables 1 and 2. The initial thermal structure
of the lithosphere (white lines in Figure 1b) is laterally uniform with a linear gradient defined by 0°C at the
surface and 1350°C at the bottom of the lithospheric mantle [e.g., Turcotte and Schubert, 2002]. The initial
thermal gradient in the asthenosphere is 0.5°C/km.
The top surface of the lithosphere is calculated dynamically as an essentially internal free surface by using a
buffer layer of “sticky air” [Gerya and Yuen, 2003; Schmeling et al., 2008; Crameri et al., 2012]. It is an initially
10 km thick layer with a low viscosity (1018 Pa s) above the upper crust. Crameri et al. [2012] found that the
sticky air approach is adequate as long as the term (ηst/ηch)/(hst/L)3 is small, where ηst and hst are the viscosity
Figure 1. Initial model configuration. (a) Topography. (b) Composition and temperature. Enlargement (1700 × 400 km) of
the numerical box (4000 × 400 km) is shown. White lines are isotherms in °C. Colors indicate the rock types, specified by the
(c) colorgrid: 1,2, sediment; 3,4, continental upper and lower crust of the pro-plate and end-plate, respectively; 5,6,
continental upper and lower crust of the retro-plate, respectively; 7, lithospheric mantle of the pro-plate and end-plate; 8,
lithospheric mantle of the retro-plate; 9, initial weak zone mantle; 10, asthenosphere. (d)–(f) Three different configurations
of initial density profile, with changing reference density contrast between the lithospheric mantle (ρLith
0 ) and asthenosphere (ρAsth
0 ) as shown in the figures. The density property of the initial model is homogeneous in the horizontal x
direction. (g)–(j) Four different configurations of the initial rheological properties, with changing effective viscosity of the
retro-plate lower crust and/or lithospheric mantle. The inset figures demonstrate the vertical cross sections, i.e., black solid
lines, in the retro-plate. Pro-plate, retro-plate, and end-plate with the same rheological layering, i.e., homogeneous layering
model (Figure 1g). Weak lower crust of the retro-plate (Figure 1h). Weak lithospheric mantle of the retro-plate (Figure 1i).
Both weak lower crust and weak lithospheric mantle of the retro-plate (Figure 1j). The flow laws used in the numerical
models can be found in the text and Tables 1 and 2. It is worth noting that the rheological configurations of pro-plate and
end-plate are kept unchanged. The strain rate dependence of rheological properties (equation 1) may result in differences
of the pro-plate effective viscosity in different models (cf. Figure 1j and Figures 1g–1i).
LI ET AL.
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and thickness of the sticky air layer, and ηch and L are the characteristic viscosity and length scale of the
model, respectively. According to this criterion, the quality of the internal free-surface condition in our models is rather moderate. Further decrease in the sticky air viscosity is precluded by numerical limitations.
Indeed, the large viscosity contrast caused by this low-viscosity boundary layer minimizes shear stresses at
the top of the solid portion of the model [Burg and Gerya, 2005]. The interface between this weak layer
and the underlying crust is treated as an internal erosion/sedimentation surface [e.g., Burg and Gerya,
2005]. Large-scale erosion and sedimentation rates in our models are assumed to be negligible, but surface
slopes are limited by prescribing the maximum stable slope angle “θmax”. In the numerical models, tan(θmax)
= 0.1 is applied, which is maintained at each time step by computing an instantaneous mass redistribution in
the regions of unstable surface slopes. Our parameter studies indicate that variability of the prescribed maximum slope angle within a realistic range does not affect significantly the large-scale model evolution (Figure
S1 in the supporting information).
One of the most important driving forces for lithosphere delamination is the positive density contrast
between the lithospheric mantle and the asthenosphere. However, the reference density, i.e., at the same
pressure and temperature, of the melt-depleted continental lithospheric mantle is generally lower than that
of more fertile asthenosphere [Djomani et al., 2001; Schutt and Lesher, 2006]. The lithospheric density may,
however, be increased by eclogitized magmatic intrusions (dykes) from (ancient) plumes [Sobolev et al.,
2011], which could result in compositionally dense lithospheric mantle (e.g., pyroxenite), compared to the
underlain peridotite mantle [Jull and Kelemen, 2001; Lee et al., 2006, 2011]. In this aspect, compositional rejuvenation of depleted continental lithosphere by magmatic/metasomatic plume- and/or subduction-induced
processes may result in positive density contrast between the lithospheric mantle and asthenosphere. In
addition, thermal contraction can also contribute to high density of the lithosphere, because it is cooler than
the asthenosphere [Houseman and Molnar, 1997]. In this study, we construct three sets of models with,
respectively, neutral (Figure 1d), negative (Figure 1e), and positive (Figure 1f) reference density contrast
between the lithospheric mantle and asthenosphere, to explore how the compositional density contrast
affects the continental collision and delamination.
Besides the density contrast, rheological properties also play significant roles in lithosphere delamination
[e.g., Krystopowicz and Currie, 2013]. However, there has been much debate concerning the long-term (i.e.,
>1 Ma) strength of continental lithosphere. In one model, dubbed “jelly sandwich,” the strength resides in
the crust and the uppermost mantle; whereas in another model, dubbed “creme brulee,” the mantle is weak
and the strength is limited to the crust [Kohlstedt et al., 1995; Turcotte and Schubert, 2002; Burov and Watts,
2006]. Therefore, we have designed four different rheological profiles of the retro-plate lithosphere
(Figures 1g–1j). In the first model, the same rheological profile is applied for the pro-plate, retro-plate, and
end-plate (Figure 1g), in which the flow laws of “wet quartzite” is used for the upper continental crust,
“Plagioclase An75” for the lower continental crust, and “dry Olivine” for the lithospheric mantle and the asthenosphere (Tables 1 and 2), as generally used for a strong continental lithosphere. In the second model, the
effective viscosity of the retro-plate lower crust is decreased by using the rheology of wet quartzite, same as
in the upper crust (Figure 1h). This retro-plate rheology has a jelly sandwich style with strong upper crust and
lithospheric mantle but weak lower crust. In the first two models, where dry olivine rheology is used for all
sections of the lithospheric mantle (Figures 1g and 1h), the fluid/melt weakening effects are neglected.
Thereby, high plastic effective friction coefficient (sin(φdry)λ = 0.6) is used for the dry and strong lithospheric
mantle (equation (7)). However, in the third model, we assume that the lithospheric mantle of the retro-plate
experienced potential fluid/melt weakening during the previous oceanic subduction or terrane accretion,
which may lead to a creme brulee rheological profile. Consequently, the wet olivine rheology is used for
the retro-plate lithospheric mantle [Ranalli, 1995]. In addition, low plastic effective friction coefficient (sin
(φdry)λ = 0.006) is used for the weak lithospheric mantle (Figure 1i), based on previous systematic investigations of the fluid/melt effect during subduction and/or plume-lithosphere interactions [e.g., Gerya and
Meilick, 2011; Vogt et al., 2012; Gerya et al., 2015]. The fourth model combines the weak lower continental
crust (Figure 1h) and weak lithospheric mantle (Figure 1i) of the retro-plate, which, thereby, is the endmember scenario of the weakest retro-plate lithosphere (Figure 1j).
The velocity boundary conditions are free slip for the left, right, and top boundaries. The lower boundary is
permeable, along which a mass-conservative permeable boundary condition is imposed [e.g., Burg and
Gerya, 2005]. This infinity-like external free slip condition along the lower boundary implies free slip condition
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to be satisfied at about 100 km below the base of the model (external lower boundary as indicated in Figure 1b).
As for the usual free slip condition, external free slip allows global conservation of mass in the computational
domain and is implemented by using the following limitation for velocity components at the lower boundary:
∂vx/∂y = 0 and ∂vy/∂y = vy/Δyexternal, where Δyexternal is the vertical distance from the lower boundary to the
external boundary where free slip (∂vx/∂y = 0, vy = 0) is satisfied. It is worth noting that the external region, i.e.,
dashed rectangle below the permeable lower boundary and above the external boundary, is not modeled
directly, but its presence is implied by the permeable boundary condition. The pro-plate is pushed rightward
at a constant convergence rate (Vx) imposed to a small internal domain in the left part of the pro-plate that
remains fixed with respect to the Eulerian coordinate (Figure 1b). No convergent condition is applied to the
right side of the mode, i.e., to the end-plate.
The thermal boundary conditions have a fixed value (0°C) for the upper boundary and zero horizontal heat
flux across the vertical boundaries. For the lower thermal boundary, an infinity-like external constant
temperature condition is imposed (e.g., Figure 1b), which allows both temperatures and vertical heat fluxes
to vary along the permeable lower boundary [e.g., Burg and Gerya, 2005], implying that the constant temperature condition is satisfied at about 1000 km below the bottom of the model (Figure 1b). This condition is
implemented by requiring ∂T/∂y = (Texternal T)/Δyexternal, where Texternal is the temperature at the external
boundary and Δyexternal is the vertical distance from the lower boundary to the external boundary. This
condition allows the temperature and heat flux at the bottom boundary to be adjusted dynamically during
the simulation. It is worth noting that the external boundary for temperature is fully independent from the
external velocity boundary.
4. Model Results
With three different density profiles (Figures 1d–1f) and four different rheological configurations (Figures 1g–1j),
we constructed 12 numerical experiments, which demonstrate variable modes of continental collision and
lithosphere delamination.
4.1. Neutral Reference Density of the Lithospheric Mantle and Asthenosphere (ρLith
¼ ρAsth
)
0
0
4.1.1. A Model of Homogeneous Layering
To illustrate the effects of rheological structure of the colliding continents on delamination, we started with a
reference model in which the rheological properties for the pro-plate, retro-plate, and end-plate are the same
(Figure 1g).
In this model, the pro-plate is pushed from the left side at 5 cm/yr, causing it to subduct continuously into the
asthenosphere. In the process its upper-middle crust, being relatively weak, is scraped off (Figure 2a). The
detached crustal materials are accumulated in the collision zone and accreted to the overriding retro-plate.
During this process, local erosion and sedimentation take place, which is mainly controlled by the prescribed
maximum surface slope angle. When the slope is steep, the materials in the higher side are eroded and then
deposited in the neighboring lower side as sediments. Consequently, a belt with series of thrust faults are
formed, in which sediments become incorporated in the upper crust as dipping wedges (Figures 2a–2c).
The upper crust of the retro-plate is also detached from the lower crust and thickened by the collision
(Figures 2b and 2c). In this case, the surface location of the suture between pro-plate and retro-plate is different from that at the depth of lithospheric mantle (Figure 2c). During the convergence, the collision zone
uplifts as the upper crust thickens there, reaching as high as 8 km from the original elevation of 1 km
(Figures 2c and 1a). The effective viscosity of the middle crust of the thickened collisional orogen is particularly
low (Figure 2f) because of crustal thickening and temperature increase during collision.
In the sublithospheric depth, a certain portion of the subducted lower crust may detach from the slab,
intruding into the mantle wedge (Figures 2b and 2c) and forming a compositionally buoyant (Figures 2d
and 2e) and rheologically weak (Figure 2f) thermal-chemical crustal plume (diapir), the dynamics of which
is systematically investigated in Currie et al. [2007] and Li and Gerya [2009]. The growth and migration of
the sublithospheric crustal plume, as well as the induced small-scale mantle flow (Figure 2f), leads to bottom
erosion of the overriding lithosphere, which further results in a limited, small-scale delamination of the retroplate lithosphere (Figure 2c). However, no major lithosphere delamination is predicted in this model of
homogeneous rheological layering.
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Figure 2. Results of the model with laterally homogeneous layering (Figure 1g) and neutral reference density contrast
between the lithospheric mantle and the asthenosphere (Figure 1d). (a)–(c) Composition and temperature evolution.
Colors of rock types are as in Figure 1c. Time (Ma) of shortening is given in each panel. The black triangle in Figure 2c
indicates the crustal suture zone between the pro-plate and retro-plate. The (d) density field, (e) density anomaly field, and
(f) effective viscosity field with velocity vectors are shown for the last time step of 20.0 Ma (Figure 2c).
4.1.2. Model With a Weak Lower Crust of the Retro-plate
In the second model, we test the effects of a weak lower crust in the retro-plate on potential development of
delamination. In this case, the rheological property of the lower crust is changed from that of a relatively
strong Plagioclase An75 to that of a wet quartzite (Figure 1h and Table 1). The model results are generally
similar to those of the reference model (cf. Figures 2c and 3a, both at 20.0 Ma). The weak lower crust in the
retro-plate alone is insufficient to induce major delamination. The plate convergence is still dominated by
subduction of the pro-plate.
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Figure 3. Results of the model with weak lower crust of the retro-plate (Figure 1h) and neutral reference density contrast
between the lithospheric mantle and the asthenosphere (Figure 1d). (a) Composition and temperature. Colors of rock types
are as in Figure 1c. The black triangle in Figure 3a indicates the crustal suture zone between the pro-plate and retro-plate.
(b) Density field. (c) Density anomaly field. (d) Effective viscosity field with velocity vectors. Time (Ma) of shortening is
20.0 Ma, as shown in the panels.
4.1.3. Model With a Weak Lithospheric Mantle of the Retro-plate
In the third model, we show the effects of a weak lithospheric mantle in the retro-plate on delamination. Here
we replace the rheological properties of the mantle lithosphere of the retro-plate with those of a weak wet olivine (Table 1) and decreased the effective friction coefficient sin(φdry)λ for a lower plastic yield stress (Figure 1i).
The results are significantly different from the previous two cases, with plate convergence mainly accommodated by thickening of the whole retro-plate lithosphere (Figure 4a), which produces a broad plateau over
the entire retro-plate. Continuous convergence results in major lithosphere thickening. Some of the mantle
lithospheric material is dragged into the asthenosphere by the subducting plate (Figure 4a), whereas lithospheric dripping developed at the far end of the weak retro-plate, close to the end-plate (Figures 4b and 4c),
which eventually leads to a large-scale delamination of the lithospheric mantle (Figure 4d).
The development of delamination in this case may be better understood by examining the density anomaly
and effective viscosity fields (Figures 4c′ and 4c″). Positive density anomaly is resulted from the thickening
of the cold lithospheric mantle of the retro-plate (Figure 4c′), due to temperature dependence of the effective density (equation (4)). In addition, the effective viscosity field indicates the weakness of the lithospheric
mantle of retro-plate (Figure 4c″). Both conditions contribute to a major lithosphere delamination (Figure 4d).
The density anomaly below the suture zone (Figure 4c′) does not lead to major delamination (Figure 4d),
perhaps because of the relatively high effective viscosity there (Figure 4c″).
4.1.4. Model With a Weak Lower Crust and a Weak Lithospheric Mantle of the Retro-plate
In the fourth model, the retro-plate has both a weak lower crust and a weak lithospheric mantle (Figure 1j).
In this case, lithosphere delamination occurs in the retro-plate but faster than in the previous model. Small
lithospheric blobs start to delaminate in the collision zone a few mega annum after collision started
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Figure 4. Results of the model with weak lithospheric mantle of the retro-plate (Figure 1i) and neutral reference density
contrast between the lithospheric mantle and the asthenosphere (Figure 1d). (a)–(d) Composition and temperature
evolution. Colors of rock types are as in Figure 1c. Time (Ma) of shortening is given in each panel. The (c′) density anomaly
field and (c″) effective viscosity field with velocity vectors are shown for the time step of 31.0 Ma, with delamination
initiation.
(Figure 5a). The broad crustal thickening of the retro-plate produces broad high topography over the entire
retro-plate. Lithospheric thickening eventually leads to major lithosphere delamination in the far end of the
retro-plate (Figure 5b). Further convergence results in another round of thickening and delamination
(Figures 5c and 5d). Finally, the lithospheric mantle of the retro-plate is almost completely delaminated, forming a wide and relatively flat plateau with thickened continental crust of 60–70 km and a high topography
(Figure 5e). The end-plate is underthrusted beneath the plateau in this case (Figure 5e).
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Figure 5. Results of the model with weak lower crust and weak lithospheric mantle of the retro-plate (Figure 1j), as well as
neutral reference density contrast between the lithospheric mantle and the asthenosphere (Figure 1d). (a)–(e) Composition
and temperature evolution. Colors of rock types are as in Figure 1c. Time (Ma) of shortening is given in each panel.
Besides the positive density anomaly and low effective viscosity of the thickened lithospheric mantle, delamination is facilitated by the weak lower crust of the retro-plate (Figure 6), which allows the delaminating
lithospheric mantle to be decoupled from the crust. This is one of the most favorable conditions for the delamination of lithospheric mantle as originally proposed by Bird [1979].
It is worth noting that the plate deformation and delamination in our models are mainly induced
by lithosphere thickening from plate convergence, not by the underlying asthenospheric mantle
flow. Moreover, lithosphere delamination in our model drives significant asthenospheric mantle flow
(Figure 6), which, in turn, may further contribute to the delamination. The main factors for large-scale
delamination are density increase in the thickened lithosphere and rheological weakness of the lithosphere (Figure 6).
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Figure 6. (a–e) Effective viscosity field with velocity vectors are shown for the same model as in Figure 5. (c′) and (d′) Density
anomaly fields for two time steps 17.6 Ma (Figure 6c) and 18.1 Ma (Figure 6d), respectively. Time (Ma) of shortening is given in
each panel.
4.2. Negative Reference Density Contrast Between Lithospheric Mantle and Asthenosphere
(ρLith
ρAsth
= 30 kg/m3)
0
0
In the model with negative reference density contrast between the lithospheric mantle and the asthenosphere (Figure 1e), no major delamination is observed in the experiments with variable rheological structures
(Figure 7).
When the retro-plate is strong, the convergence is mainly accommodated by underthrusting of the pro-plate,
with its upper-middle crust scraped off (Figures 7a and 7b). In contrast, a relatively weak retro-plate results in
pure-shear thickening of the whole retro-plate lithosphere, with significant high topography (Figures 7c and
7d). However, delamination is still prevented by the negative reference density contrast between the lithospheric mantle and the asthenosphere.
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3
Asth
Figure 7. Numerical models with negative reference density contrast (ρLith
= 30 kg/m ) between the lithospheric
0 ρ0
mantle and the asthenosphere as shown in Figure 1e. (a)–(d) Models with rheological profiles as in Figures 1g–1j, respectively, where LC represents lower crust and LM for lithospheric mantle. Colors of rock types are as in Figure 1c. Time (Ma) of
shortening is given in each panel.
4.3. Positive Reference Density Contrast Between Lithospheric Mantle and Asthenosphere (ρLith
ρAsth
0
0
3
= 30 kg/m )
4.3.1. Model of Homogeneous Layering
In this model, the pro-plate subducts continuously into the asthenosphere, with its upper-middle crust
scraped off and accreted to the overriding plate (Figure 8a). No obvious lithosphere delamination is predicted. The results are generally similar to the reference model with neutral reference density of the lithospheric mantle and the asthenosphere (Figure 2), except for the absence of sublithospheric crustal plume
in this model (Figure 8a).
4.3.2. Model With a Weak Lower Crust of the Retro-plate
In this model, the plate convergence is still dominated by subduction of the pro-plate, with its upper-middle
crust scraped off and accumulated in the collision zone (Figure 8b). A piece of lithospheric mantle is detached
from the bottom of retro-plate due to the erosion of sublithospheric crustal plume; however, the weak lower
crust alone is insufficient to induce major delamination.
4.3.3. Model With a Weak Lithospheric Mantle of the Retro-plate
In this case, lithosphere initially thickens in the collision zone, and small blocks of the thickened lithosphere
delaminate (Figure 9a). Further convergence thickens the lithosphere across the retro-plate, especially in the
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Figure 8. Results of the model (a) with laterally homogeneous layering as in Figure 1g or (b) with a weak retro-plate lower
crust as in Figure 1h. Positive reference density contrast between the lithospheric mantle and the asthenosphere is applied,
3
Asth
ρLith
= 30 kg/m , as in Figure 1f. Colors of rock types are as in Figure 1c. Time (Ma) of shortening for either model is
0 ρ0
21.0 Ma as shown in the panels. The black triangles indicate the crustal suture zone between the pro-plate and retro-plate.
far-end region close to the end-plate, resulting in drippings at the bottom of the lithospheric mantle
(Figures 9b and 9c). The broad crustal thickening of the retro-plate produces broad high topography over
the entire retro-plate. Lithosphere thickening eventually leads to a large-scale delamination of the lithospheric mantle, starting from the far end of the retro-plate and migrating toward the suture zone
(Figures 9d and 9e), eventually removing the entire lithospheric mantle of the retro-plate and forming a wide
and relatively flat plateau with thickened continental crust of 60–70 km (Figure 9f). The large-scale delamination caused rapid uplift of the topography (cf. Figures 9c–9e). At the end of lithosphere delamination, the elevation of the plateau can reach 6–7 km.
4.3.4. Model With a Weak Lower Crust and a Weak Lithospheric Mantle of the Retro-plate
Not surprisingly, adding a weak lower crust to the previous case further enhances delamination. Small lithospheric blobs start to delaminate a few mega annum after the continental collision started (Figures 10a and
10b). Further convergence thickens the whole lithosphere of the retro-plate, especially the far-end region
close to the end-plate (Figures 10c and 10d), which is similar to the previous model (Figures 9c and 9d). In
about 15 Ma the entire mantle lithosphere of the retro-plate has delaminated (Figures 10e and 10f). A wide
and flat plateau is formed, with altitude of about 5–6 km (Figure 10f).
5. Effects of Other Factors
Other factors may affect the delamination dynamics in collisional orogens. Here we discuss the effects of
possible eclogitization of the lower crust and the convergence rates.
5.1. Lower Crust Eclogitization
Eclogitization, a phase transformation of mafic crustal materials to denser eclogite, has been suggested as a
major cause of delamination [e.g., Leech, 2001; Krystopowicz and Currie, 2013]. Lower crust eclogitization can
increase density by 10% [e.g., Austrheim, 1987; Jolivet et al., 2005] or 300–600 kg/m3 [e.g., Doin and Henry,
2001]. To illustrate its first-order impact, we simplified eclogitization in the model by increasing the lower
crustal density by 300 kg/m3 once the pressure is greater than 1.5 GPa (about 50 km in depth).
Asth
= 30 kg/m3)
First, we applied eclogitization to the models with negative reference density contrast (ρLith
0 ρ0
between the lithospheric mantle and the asthenosphere, which predict no major delamination without eclogitization (Figure 7). Having lower crust eclogitization significantly changes the model evolution. When the retroplate lithospheric mantle is strong, steep subduction of pro-plate lithosphere occurs (Figures 11a and 11b);
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Figure 9. Results of the model with weak lithospheric mantle of the retro-plate as in Figure 1i. Positive reference density
3
Asth
contrast between the lithospheric mantle and the asthenosphere is applied, ρLith
= 30 kg/m , as in Figure 1f. (a)–(f)
0 ρ0
Composition and temperature evolution. Colors of rock types are as in Figure 1c. Time (Ma) of shortening is given in each panel.
whereas flat subduction (underthrusting) is predicted in the models without eclogitization (Figures 7a and 7b).
With a weak retro-plate lithospheric mantle and strong lower crust, retro-plate thickening dominates but no
delamination occurs (Figure 11c), similar to the comparable cases but without eclogitization (Figure 7c).
When both the lower crust and mantle lithosphere in the retro-plate are weak, eclogitization is shown to lead
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Figure 10. Results of the model with both weak lower crust and weak lithospheric mantle of the retro-plate as in Figure 1j.
3
Asth
Positive reference density contrast between the lithospheric mantle and the asthenosphere is applied, ρLith
= 30 kg/m ,
0 ρ0
as in Figure 1f. (a)–(f) Composition and temperature evolution. Colors of rock types are as in Figure 1c. Time (Ma) of shortening
is given in each panel.
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Asth
Figure 11. Effects of lower crust eclogitization for the models with negative reference density contrast ( ρLith
0 ρ0
3
3
= 30 kg/m ). The density of the lower crust is increased by 300 kg/m when the pressure is greater than 1.5 GPa, i.e.,
eclogitization occurs. (a–d) Models with rheological profiles as in Figures 1g–1j, respectively, where LC represents lower
crust and LM for lithospheric mantle. Colors of rock types are as in Figure 1c. Time (Ma) of shortening is given in each panel.
to major delamination in the retro-plate near the collision zone (Figure 11d). This could not occur without eclogitization (Figure 7d), because of the lower reference density of the mantle lithosphere.
¼ ρAsth
Eclogitization is also applied to the reference models with neutral reference density contrast (ρLith
0
0 )
between the lithospheric mantle and the asthenosphere. Delamination occurs mainly in the pro-plate when
the retro-plate is not very weak (Figures 12a–12c). In these cases, eclogitization promotes the sinking of the
pro-plate. Delamination, on the other hand, would occur mainly in the retro-plate when it is weakest
(Figure 12d). The retro-plate delamination is similar to the comparable model without eclogitization in
Figure 5. In both models, major delamination initiates in the far end of the retro-plate. However, the predicted
delamination initiates earlier and moves faster with eclogitization.
Asth
When eclogitization is applied to the models with positive density contrast (ρLith
= 30 kg/m3), pro-plate
0 ρ0
delamination is similar to the previous models (Figure 12) but migrates faster, when the retro-plate is not very
weak (Figures 13a–13c). In contrast, delamination occurs in retro-plate and migrates from the suture zone to
the far end when the retro-plate is weakest (Figure 13d). The migration of retro-plate delamination differs
significantly from the comparable model without eclogitization (Figure 10), in which opposite migration
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3
Asth
Figure 12. Effects of lower crust eclogitization for the models with neutral reference density contrast (ρLith
= 0 kg/m ).
0 ρ0
3
The density of lower crust is increased by 300 kg/m when the pressure is greater than 1.5 GPa, i.e., eclogitization occurs. (a–d)
Models with rheological profiles as in Figures 1g–1j, respectively, where LC represents lower crust and LM for lithospheric
mantle. Colors of rock types are as in Figure 1c. Time (Ma) of shortening is given in each panel.
direction of retro-plate delamination is observed. These results indicate that lower crust eclogitization could
play a major role in initiating delamination in the collision zone, where lithosphere thickening starts.
5.2. Convergence Rate
The convergence rate plays a significant role in the dynamics of continental collision and mountain building
[e.g., Faccenda et al., 2008; Li et al., 2011, 2013, 2015]; it may also influence delamination. In the models
presented so far, a constant convergence rate of Vx = 5 cm/yr is applied. We conducted additional models
with lower (Vx = 2 cm/yr) and higher (Vx = 8 cm/yr) convergence rates (Figures S2–S7), in order to show how
variations of Vx affect the model results.
Asth
When the reference density of the mantle lithosphere is lower than that of the asthenosphere (ρLith
0 ρ0
3
= 30 kg/m ), no delamination is predicted regardless of the convergence rates (Figures S2 and S3). The main
difference is that lower convergence rate of Vx = 2 cm/yr leads to steep subduction in the models with relatively strong retro-plate (Figure S2), whereas flat subduction, or underthrusting, is predicted with higher
(Vx = 5 cm/yr, Figure 7, and Vx = 8 cm/yr, Figure S3) convergence rates.
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Asth
Figure 13. Effects of lower crust eclogitization for the models with positive reference density contrast (ρLith
= 30 kg/m ).
0 ρ0
3
The density of the lower crust is increased by 300 kg/m when the pressure is greater than 1.5 GPa, i.e., eclogitization occurs.
(a–d) Models with rheological profiles as in Figures 1g–1j, respectively, where LC represents lower crust and LM for lithospheric
mantle. Colors of rock types are as in Figure 1c. Time (Ma) of shortening is given in each panel.
When the reference density is the same for the lithospheric mantle and the asthenosphere (ρLith
¼ ρAsth
),
0
0
lower convergence rate (Vx = 2 cm/yr) results in pro-plate delamination, when the retro-plate is not very weak
(Figures S4a–S4c). It indicates that the prescribed convergence rate is lower than the local sinking velocity
driven by the negative buoyancy of the subducting slab with the crustal materials detached. Double-plates
delamination is predicted when the retro-plate is weakest (Figure S4d), in which pro-plate delamination
occurs following a major retro-plate delamination near the suture zone. The faster convergence rate
(Vx = 8 cm/yr) does not change the general delamination mode in comparison with the reference models with
Vx = 5 cm/yr, although details of the collisional styles are different (cf. Figures S5 and 2–5).
Asth
= 30 kg/m3), a lower convergence rate of Vx = 2 cm/yr
In the models of positive density contrast (ρLith
0 ρ0
causes different styles of delamination from the reference models (Figure S6). If the retro-plate lithospheric
mantle is relatively strong, delamination occurs in the pro-plate (Figures S6a and S6b). When the retro-plate
has a weak lithospheric mantle but a strong lower crust, slow convergence leads to delamination in both the
pro-plate and retro-plate (Figure S6c). In contrast, delamination occurs dominantly in the retro-plate when
both its lower crust and lithospheric mantle are weak (Figure S6d). With faster convergence (Vx = 8 cm/yr),
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the pro-plate subduction occurs if the lithospheric mantle of the retro-plate is relatively strong (Figures S7a
and S7b). In contrast, delamination occurs dominantly in the retro-plate when its lithospheric mantle is weak
(Figures S7c and S7d). In these cases, the results are similar to those of the reference models with Vx = 5 cm/yr
(Figures 9 and 10).
6. Discussion
6.1. Delamination Mode Selection
Our numerical simulations show three basic modes of lithosphere delamination in collisional orogens: proplate delamination, retro-plate delamination, and the transitional mode of double-plates delamination (both
pro-plate and retro-plate). Based on the numerical results, we constructed a 3-D regime diagram
(Figures 14a–14c) to illustrate how the delamination mode depends on the convergence rate, retro-plate
rheological properties, and the density contrast between lithospheric mantle and asthenosphere. Another
regime diagram is constructed in order to show the effects of lower crust eclogitization on the delamination
mode selection (Figure 14d).
The regime diagram shows the absence of delamination in models when the reference density of the lithospheric mantle is lower than that of the asthenosphere (Figure 14a). The buoyant-converging lithospheres
prevent any types of delamination, except when the lower crust eclogitization is considered (Figure 14d,
top). The density increase due to eclogitization may result in lithosphere delamination in the rather
weak retro-plate.
With neutral or positive density contrast between the lithospheric mantle and the asthenosphere, delamination occurs in different modes under various conditions (Figures 14b and 14c). The prerequisite for delamination of the retro-plate is its weakness (Figure 14); otherwise, it would prevent major deformation and
delamination of the retro-plate. The promoting factors of retro-plate delamination include faster converAsth
gence and a higher value of ρLith
0 ρ0 .
Pro-plate delamination is favored by lower convergence rate, relatively strong retro-plate, and the lower crust
eclogitization (Figure 14). In these cases, plate convergence is accommodated mainly by sinking of the proplate lithospheric mantle with (part of) the lower crust. The local velocity of slab sinking is larger than
horizontal plate convergence rate (Vx), leading to the retreat of the sinking slab that detaches from the
upper crust.
Delamination of both the pro-plate and retro-plate could occur simultaneously, but only in the models with
low convergence rates and within a narrow range of parameters (Figures 14b and 14c). This mode of delamination can be regarded as a transition between the pro-plate and retro-plate delamination modes.
6.2. Comparison With Previous Models
The dynamics of lithosphere delamination has been studied in Morency and Doin [2004] based on a mantle
convection model with a stagnant lid. They found that lithosphere delamination initiates systematically when
the lowermost crust and the top lithospheric mantle are sufficiently soft. They further constructed a 2-D
regime diagram with the two axes of crust viscosity and mantle viscosity, which was separated into two
domains: one with lower viscosity values, where delamination initiates; another one with higher viscosity
values, where no delamination occurs. In addition, Schott and Schmeling [1998] proposed that delamination
and detachment of the mantle lithosphere are only possible for a thickened lithosphere, which has been
significantly weakened, e.g., by water. Our models also indicate that the weakness of the retro-plate is a prerequisite for its delamination during continental collision. With respect to the influence of rheological
strength on lithosphere delamination, our results are thus consistent with previous numerical experiments,
although our plate tectonics-like numerical setup is significantly different.
Our systematic numerical experiments predict three basic modes of delamination: pro-plate, retro-plate, and
double-plates delamination. Each of them has been reported in previous models, but separately. The proplate delamination has been simulated in models with a sinking and retreating subducting lithosphere
peeling off from its overlying crust [e.g., Faccenda et al., 2009; Ueda et al., 2012]. These numerical models
assume a strong retro-plate, so lithosphere delamination occurs only in the pro-plate. In addition, the prescribed convergence in Ueda et al. [2012] is turned off after a certain time of plate subduction. The resulted
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Figure 14. Delamination regime diagram. (a)–(c) Dependence of delamination modes on the density contrast between the
lithospheric mantle and the asthenosphere, and between the retro-plate rheology and the convergence rate. For the
rheological properties, “reference,” “weak LC,” “weak LM,” and “weak LC + LM” represent rheological profiles as in
Figures 1g–1j, respectively, where LC represents lower crust and LM for lithospheric mantle. The 3-D regime diagram is
visualized in 2-D for clarity. (d) Comparisons of delamination mode selection for the models with or without lower crust
eclogitization, in which constant convergence rate is applied.
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slow convergence by solely slab pull contributed to the retreating of the pro-plate. These results are generally
consistent with our predictions with low convergence rate and relatively strong retro-plate as favorable
conditions for pro-plate delamination (Figure 14).
Retro-plate delamination has been simulated by Krystopowicz and Currie [2013] (“K-C models” henceforth). In
their models, one weak plate is located between two strong plates. In the K-C models crustal eclogitization is
critical to lithospheric delamination. In addition, a weak lower crust is generally required for the full detachment of the sinking mantle lithosphere. In our models, a weak retro-plate is necessary for delamination of its
lithospheric mantle, and a weak lower crust promotes delamination. These results agree with the K-C models.
However, in our models the effects of lower crust eclogitization depend on the density contrast between the
lithospheric mantle and the asthenosphere, which play significant roles in the delamination processes. When
the reference density of the continental lithospheric mantle is the same or greater than that of the asthenosphere, lithosphere delamination can occur during the retro-plate shortening without eclogitization
(Figures 14b and 14c). Otherwise, lower crust eclogitization is needed for delamination (Figures 14a and 14d).
The double-plates delamination predicted in our model has also been observed in the numerical models of
Gray and Pysklywec [2012]. Their models used a higher reference density contrast (50 kg/m3) between the
lithospheric mantle and the asthenosphere, as well as extremely high radioactive heating of a wide retroplate margin that may decrease its rheological strength to initiate the retro-plate delamination. Finally, a
mode with delamination on both the pro-plate and retro-plate is observed. Their results are consistent with
our models in terms of needing a relatively large density contrast and weak retro-plate (Figure 14).
Houseman and Molnar [1997] drew a useful distinction between the concept of delamination as proposed
by Bird [1979] and that of convective thinning. In delamination, the locus of downwelling migrates backward as the delaminating plate sinks downward. In convective thinning, the loci of downwelling are
stationary as the denser layer flows toward them and then vertically downward. In our experiments,
however, it is rather difficult to clearly distinguish the convective thinning mode from the delamination
mode. In several experiments (e.g., Figures 9 and 10), the detachment and sinking of lithospheric mantle
initially occur in a stationary locus (i.e., similar to the convective removal), but then the downwelling may
migrate to its sides (i.e, similar to the delamination mode). Therefore, we prefer using the term “delamination” in this study to describe a general (rather complex) process of lithosphere removal and discriminate
among different “delamination modes.”
6.3. Delamination-Induced Topographic Uplift
Topographic uplift is a direct and most significant response to continental collision. Our numerical simulations indicate that a relatively narrow mountain belt, i.e., around 500 km in width, is resulted during continental collision without delamination (Figures 2 and 3). The averaged elevation is around 4 km, with the peak of
up to 8 km (e.g., Figure 3). In contrast, delamination of the mantle lithosphere tends to form a broad plateau
(e.g., Figures 4 and 5, as well as other similar cases).
6.4. Geological Implications
The numerical models presented here are necessarily simplified, yet the results may provide some useful
insight into mountain building in many continental collision zones that may have involved delamination.
The pro-plate delamination mode implies that the mantle lithosphere of the subducting continental plate
could delaminate from the overlying crust and sinks (e.g., Figures 12a–12c and 13a–13c). A possible geological analog is the Northern Apennines (Figure 15a), which is a fold-and-thrust belt that formed in response to
the eastward retreating of the subducted Adria microcontinental lithosphere delaminating from the overlying continental crust [Faccenna et al., 2001, 2009; Chiarabba and Chiodini, 2013; Chiarabba et al., 2014].
Geological evidence indicates that the Northern Apennines underwent two phases of eastward migrating
deformation that caused early compressional structures being dissected by later extensional deformation
since the late Oligocene-Miocene [Elter et al., 1975; Faccenda et al., 2009]. Extension, due to pro-plate lithosphere delamination, produced progressive younging toward the east syntectonic extensional basins and
associated magmatic activity.
The Himalayan-Tibetan system may provide an example of retro-plate delamination (Figure 15b) [e.g.,
England, 1993; Owens and Zandt, 1997]. The collision between the Indian and Eurasian plates at about
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Figure 15. (a) Simplified tectonic profile of the Northern Apennines (modified from Barchi et al. [1998], Mele and Sandvol
[2003], and Carminati et al. [2004]). (b) Characteristic profile of the Tibetan plateau from SW to NE (modified from Owens
and Zandt [1997]).
5 cm/yr in the past 50 Ma has nearly doubled the thickness of the Tibetan crust, and should have done the
same for mantle lithosphere. Although some surface wave inversions indicate a thick lithosphere beneath
the entire Tibetan plateau [McKenzie and Priestley, 2008; Priestley and McKenzie, 2013], most geophysical
observations suggest relatively thin, or even the absence of, lithospheric mantle in much of the central
and northern Tibetan plateau [McNamara et al., 1995; Owens and Zandt, 1997; Tilmann et al., 2003]. This thin
lithosphere is attributed to delamination or convective thinning [Molnar et al., 1993]. Our model results
suggest that retro-plate delamination mode (e.g., Figures 4 and 5) is favored when the mantle lithosphere,
perhaps the crust as well, is rheologically weak. This is reasonable, because the Tibetan lithosphere had
experienced a series of (paleo)-Tethyan subduction and the collision of multiple continental terranes before
the final collision with the Indian Plate [Yin and Harrison, 2000]. Thus, the Tibetan lithosphere may be
weakened by significant hydration and partial melting.
For the Himalayan-Tibetan system, the 3-D process with variation in the third dimension and significant
lateral extrusion may influence the extent of deformation in the retro-plate [England et al., 1985;
Tapponnier et al., 2001; Royden et al., 2008; Li et al., 2013; Capitanio et al., 2015]. It may further affect the lithosphere delamination in a certain degree, which is currently unclear. As a first inference, the lateral extrusion of
retro-plate may postpone its delamination, because more convergence is needed to thicken the lithosphere
and build enough negative buoyancy. On the other hand, the lateral extrusion, along large-scale strike-slip
faults, may decrease the strain-dependent rheological strength of the faults and the retro-plate, which will
facilitate the delamination. The effects of 3-D process, e.g., lateral extrusion, on lithosphere delamination in
collisional orogens need further investigation with 3-D models. It is worth emphasizing that the goal of this
work is not to reproduce the tectonic evolution of specific orogens with possible delamination but to illustrate the basic physics behind the delamination and orogenesis.
7. Conclusions
We have used high-resolution thermomechanical numerical models to systematically investigate the
dynamics of lithosphere delamination during continental collision. The main conclusions we may draw from
this study include the following:
1. Lithosphere thickening from continental collision can sufficiently increase the negative buoyancy to drive
lithosphere delamination, when the reference density of the continental lithospheric mantle is the same
or greater than that of the asthenosphere. However, if the reference density of the lithospheric mantle is
LI ET AL.
DYNAMICS OF LITHOSPHERE DELAMINATION
23
Journal of Geophysical Research: Solid Earth
2.
3.
4.
5.
Acknowledgments
This work was supported by the 973
Project of China (2015CB856106), the
Major Research Program of NSFC
(41590860), the Strategic Priority
Research Program (B) of CAS
(XDB18020104), the NSFC project for
young scholars (41304071), and the
National “Qian-Ren” Program to Z. H. Li.
M. Liu acknowledges support from the
University of Chinese Academy of
Sciences. Numerical simulations were
run with the clusters of National
Supercomputer Center in Guangzhou
(Tianhe-II). The data for this paper
are available by contacting the
corresponding author ([email protected]). Thorough and constructive
reviews by two anonymous reviewers
and the Associate Editor improved
the paper.
LI ET AL.
10.1002/2016JB013106
less than that of the asthenosphere, additional promoting factors, e.g., lower crust eclogitization, are
required for delamination.
Lithosphere delamination can occur in pro-plate, retro-plate, or both during continental collision, depending on the density anomaly, the convergence rate, as well as the relative rheological strength of the proplate and retro-plate.
The pro-plate delamination is favored by lower convergence rate, relatively strong retro-plate, and the
lower crust eclogitization. The Northern Apennines may provide as a geological analog for the pro-plate
delamination mode.
Retro-plate delamination requires weak rheology of the retro-plate, and is favored by faster convergence
and larger density contrast between the retro-plate lithospheric mantle and the asthenosphere. This
process may have contributed to the interpreted thin lithosphere under Central Northern Tibetan Plateau.
Lithosphere delamination during continental collision generally results in a wide and flat mountain belt
or plateau. In collisional orogens without major delamination, relatively narrow and steep mountain belts
are predicted.
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