Geophysical Research Letters

Supporting Information for

Slab horizontal subduction and slab tearing beneath East Asia

Pengfei Ma1, Shaofeng Liu1*, Michael Gurnis2, Bo Zhang1

1State Key Laboratory of Geological Processes and Mineral Resources and School of Geosciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China.

2Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA.

Corresponding author: Shaofeng Liu (shaofeng@cugb.edu.cn)

Contents of this file

Tables S1 to S5

Figures S1 to S8

Introduction

This Supporting Information contains background information of our geodynamic models and modelling parameters, cases, and results.

Incompressible global mantle convection was simulated by solving partial differential equations for the conservation of mass, momentum, and energy in a shell domain (Zhong et al., 2008). The spherical shell was divided into approximately 12.6x106 elements with radial mesh refinement so that the average vertical resolution in the upper mantle reached approximately 24 km.

The Rayleigh number, which determines the vigour of mantle convection, was defined as

equation (1)where is the coefficient of thermal expansion, is the density, is the acceleration due to gravity, is the temperature range between the Earth’s surface and the core-mantle boundary, is the thickness of the mantle, is the thermal diffusivity, and is the viscosity. The subscript “0” denotes the reference values. The viscosity of the mantle depends on the depth and temperature and is defined as

equation (2)

where is the depth-dependent pre-factor that is defined with respect to the reference viscosity , is the dimensional activation energy ( for the upper mantle and for the lower mantle), R is the universal gas constant, T is the mantle temperature, is the temperature offset, and is the ambient mantle temperature outside the thermal lithosphere, slabs and basal thermal boundary layer.

To investigate the effect of the phase transition across 660 km on the slab behaviour in the mantle transition zone, the Clapeyron slope of the phase change was considered as a parameter (Table S2). The phase function is

equation (3)

where r is the depth; and are the ambient depth and phase-change temperature, respectively; and is the width of the phase transition. All the physical constants are listed in Table S3.

The initial temperature field is reconstruction-based, including the thermal boundary layer and thermal structure of slabs. The thermal structure of the surface boundary layer was created according to the half-space cooling model, where the distribution of the temperature field depends on the depth and the age of the lithosphere. For the oceanic basins, the ages are adopted from the reconstructed seafloor agegrid at the start age. In the continent, the lithosphere is considered as three types of columns based on the tectonothermal age of the continents (Artemieva,2006), i.e. Archean, Proterozoic, and Phanerozoic; the corresponding ages within the three types are uniformly set 368 Ma, 235 Ma, and 151 Ma, respectively. The slab morphology was determined by the age of initial subduction at the start of the assimilation (either 200 or 55 Ma), assuming they dip 45° in the upper mantle and then sink vertically in the lower mantle. The sinking rates in upper and lower mantle for all the slabs were uniformly assumed as constant (3 cm/yr and 1.2 cm/yr, respectively).

We created a series of slabs that were assimilated into the flow models with the reconstructed tectonic history (Table S2). Case 1-1 was set as the reference case. Our new reconstruction was used for Cases 1-1 to 1-6. The boundary conditions outside our region of focus and before 55 Ma were extracted from Zahirovic et al.’s (2016) dataset. The slab morphology above 350-km depth was set based on the reconstructed paleogeography and the present observations from the Wadati-Benioff seismicity. The subduction history along the East Asia margin was specifically set. We assumed a short flat-slab subduction stage of Pacific subduction after the subduction of the Izanagi-Pacific ridge. The dip angle later increased to 45º at 26 Ma and then gradually decreased to 26º from 23 Ma to 13 Ma linearly during the Philippine Sea plate’s approach. We assumed that the Pacific slab fully detached from the Ryukyu Trench-Nankai Trough at 6 Ma, before the Philippine Sea slab steepened.

For Case 2-1, all the boundary conditions were extracted from the original reconstruction dataset. For Case 1-2, the boundary conditions were the same as those in Case 1-1, but the initial simulation age was set to 55 Ma (Table S2). Therefore, the Mesozoic Izanagi subduction was not considered in this case. In addition, variations in viscosity layering and the Clapeyron slope were varied among the models for Case 1-3 to Case 1-6, which had the same boundary conditions as Case 1-1 (Table S2).

As many as 3108 particles (tracers) were advected in the shell. Such tracers were used to represent the higher viscosities and differing buoyancies of continental lithosphere and crustal components and in turn suppressed instabilities at the base of the lithosphere. The evolution of slabs were followed by passive tracers. Every 5 to 10 Myr, the leading edge of the slabs at 400 to 450 km depth were marked to trace material entering the transition zone.

To evaluate the match between the Case 1-1 and tomographic models, we computed the area of intersecting polygons between predicted present-day thermal field (10% colder than ambient) and tomographic images (0.02% larger than average) below 350km depth in vertical profiles, following Seton et al. (2015). Case 1-1 achieves an intersection as high as 90 % with GAP-P4 model (Table S4) and 70% for the MIT-P08 model.

Table S1. Geological and geophysical constraints for the plate-tectonic history.

Geological event

Age

Methods and evidence

Widespread rifting in East Asia

Cretaceous

Subsidence in rifting basins

Opening of the proto South China Sea

~65 Ma

Onset of tectonic subsidence along

South China margin

Izanagi-Pacific ridge subduction

beneath East Asia

60 Ma~50 Ma

Plate reconstruction,

geodynamic modelling, and paleo-geothermal analysis

Izu-Bonin subduction initiation

52 Ma

Radiometric dating and

geochemical analysis

Hotspot activity near the central

basin spreading centre

50 Ma~35 Ma

Radiometric dating and

geochemical analysis

Cessation of seafloor spreading

at the Amami-Sankaku basin

49 Ma

Radiometric and paleontological

dating

Oblique consumption of

marginal oceanic basins around

the modern Philippines archipelago

50 Ma~

Geochemical analysis, zonation

and radiometric dating of ophiolites,

correlation of lithologies, chemical compositions, and age

Onset of seafloor spreading at the SCS basin

32 Ma

Magnetic lineation identification

Huagang Movement in the East China

Sea shelf basin

Mid-Oligocene~

Early Miocene

Interpretation of seismic reflection

data and well logs

Onset of seafloor spreading in the

Shikoku and Parece-Vela

Basins

28 Ma

Incorporated analysis of magnetic, gravity, and bathymetric data

Subduction inception at the Manila

Trench

22 Ma

Radiometric and paleontological dating

Collision of the Philippine Sea plate

with western Shikoku Island

~20 Ma

Structural analysis and paleo-geothermal evaluation

Clockwise rotation of the SW Japan

Arc

17 Ma~15 Ma

Radiometric dating and geomagnetic assessment

Subduction of Shikoku basin

crust beneath SW Japan

17 Ma~12 Ma

Radiometric dating of magmatism

Cessation of seafloor spreading

at the SCS basin

16 Ma

Identification of magnetic lineation

Cessation of seafloor spreading in the Shikoku and Parece-

Vela basins

15 Ma

Analysis of gravity, bathymetric,

and magnetic anomalies

Cessation of extension in the Japan Sea basin

15~12 Ma

Decrease in subsidence rate

First phase of rifting at the north-central

Okinawa Trough

Middle Miocene

Analysis of seismic

data

First collision of the Izu-Bonin arc

at the Izu peninsula

15~12 Ma

Palaeontology analysis of trough sediments

First collision of the Luzon arc at Ryukyu

Late Miocene

Plate reconstruction and

ages of microfossils and volcanism

Calming of Philippine Sea subduction

10 Ma~6 Ma

Absence of subduction-related magmatism

Initial opening of the Mariana Trough

8 Ma~6 Ma

Longjing movement within the East

China Sea shelf basin

Late Miocene

Analysis of seismic data

and well logs

Resumption of the subduction of the Philippine Sea plate beneath East Asia

6 Ma

Radiometric dating of magmatism

Subduction initiation at the Philippine Trench

8 Ma~9 Ma

Radiometric dating of magmatism

Second and third phases of rifting at the Okinawa Trough

Late Pliocene~present

Analysis of seismic reflection data

Opening of the southern Okinawa Trough

Early Pleistocene~present

Interpretation of seismic reflection data

Taiwan Orogeny

3 Ma~present

Table S2. Case-specific parameters

Case

Viscosity pre-factor ()1

Reconstruction2

Clapeyron slope ()

Start age (A)

Case 1-1

1, 0.2, 1, 50

N

-1.5 MPa∙K-1

200 Ma

Case 1-2

1, 0.2, 1, 50

N

-1.5 MPa∙K-1

55 Ma

Case 1-3

1, 0.2, 1, 100

N

-1.5 MPa∙K-1

200 Ma

Case 1-4

1, 0.2, 1, 50

N

-3 MPa∙K-1

200 Ma

Case 1-5

1, 0.2, 1, 30

N

-1.5 MPa∙K-1

200 Ma

Case 1-6

1, 1, 1, 50

N

-1.5 MPa∙K-1

200 Ma

Case 2-1

1, 0.2, 1, 50

Z

-1.5 MPa∙K-1

200 Ma

1 Values of the pre-factor in equation (2) for the mantle above 150 km, between 150 km and 410 km, between 410 km and 660 km, and below 660 km.

2 N indicates the cases based on the new reconstruction, and Z indicates the case based on Zahirovic et al. (2016).

Table S3. Constant parameter settings in the geodynamic modelling; the subscript “0” denotes a reference value

Parameter

Symbol

Value

Unit

Thermal expansion coefficient

Density

Temperature change

2825

Universal gas constant

R

8.314

Activation energy (upper mantle)

Activation energy (lower mantle)

Viscosity

Thermal diffusivity

Gravity acceleration

Background mantle temperature

Activation temperature

452

Earth’s radius

Mantle thickness

Rayleigh number

Ambient depth of phase change

660

Temperature of phase change

1667

Width of phase change

40

Table S4. Intersection (in %) between predicted present-day temperature of Case 1-1 and seismic tomography models MIT-P08 and GAP-P4 in profiles (a-d).

Profile

GAP-P4

MIT-P08

a

85

76

b

92

74

c

90

77

d

89

71

Table S5. The paleo pole position for Philippine Sea plate (PSP) in relation with the Eurasia plate (EUR) utilized in this reconstruction.

Plate ID

Time (Ma)

Lat

Lon

Angle

Relative plate

609 (PSP)

0

0

0

0

301 (EUR)

609 (PSP)

6

-48.1484

-21.9982

-6.5912

301 (EUR)

609 (PSP)

10

-39.9029

-29.5407

-10.2632

301 (EUR)

609 (PSP)

15

-26.0539

-31.9011

-21.2257

301 (EUR)

609 (PSP)

35

-30.1713

-8.3237

-28.5938

301 (EUR)

609 (PSP)

40

-26.4227

-9.1886

-33.8613

301 (EUR)

609 (PSP)

45

-22.1511

-14.5166

-42.5005

301 (EUR)

609 (PSP)

50

-16.6388

-20.6924

-60.2846

301 (EUR)

Figure S1. Paleolatitude (a-e) and rotation (f) predictions since 50 Ma by the reconstruction models at scattered sampling sites around the Philippine Sea plate. The black boxes with error bars in subfigures (a-e) represent the paleomagnetic data, and the white boxes represent alternative paleolatitudes for paleomagnetic data near the equator. The coloured curves are predictions by different reconstruction models. The green curve corresponds to Seton et al. (2012), the yellow curve represents the results of model 1a in Wu et al. (2016), the purple curve represents model 2 from Wu et al. (2016), the blue curve represents Zahirovic et al. (2016), and the red lines are the predictions by our model. In subfigure f, the colouring rule is the same as that for subfigures (a-e). Additionally, the dashed line in subfigure f is the reference line, which indicates no rotation. CW and CCW mean clockwise and counter-clockwise rotation, respectively. The locations of the sampling sites are plotted as red dots in Fig. 1, although the sources of the paleomagnetic data may have been scattered around the sites.

Figure S2. Trench migration history since 50 Ma for the reconstruction in the absolute reference system. The trench locations are coloured according to their corresponding age measured from 55 Ma. The values along the black arrows denote the trench-retreat distance during the corresponding time. The thin black line delineates the present-day coastline. RT-Ryukyu Trench; JS-Japan Subduction Zone; IBS-Izu Bonin Subduction Zone.

Figure S3. a) Average viscosity-depth relationship of Case 1-1 (red line), Case 1-3 (orange line), Case 1-5 (green line) and Case 1-6 (light-blue dashed line). b) and c) Predicted temperature field and corresponding viscosity field, respectively, for Case 1-1 in a vertical profile across Northeast Asia for comparison.

Figure S4. Comparison between the present mantle structure above 900 km as predicted by Cases 1-1 (red lines) and 1-6 (black lines) and the regional high-resolution tomography model FWEA18 (Tao et al., 2018) along profiles a to d. “dvp” and “dvs” denote S- and P-wave velocity anomalies, respectively.

Figure S5. Temporal evolution of subducted slabs along profiles c and e in Case 1-2. The segmented slabs associated with the migrating-trench model are compared to seismic-tomography images in both vertical and horizontal slices for mutual identification. The figure arrangements and all the symbols are the same as those in Figure 4.

Figure S6. Temporal evolution of subducted slabs along profiles c and e in Case 1-2. The segmented slabs that are associated with the migrating-trench model are compared to seismic-tomography images in both vertical and horizontal slices for mutual identification. The figure arrangements and all the symbols are the same as those in Figure 4.

Figure S7. Slab structure as predicted by Case 1-3 (red lines), Case 1-4 (purple lines), Case 1-5 (yellow lines) and Case 1-6 (black lines) along profiles a to d, with seismic images from MITP-08 and GAP-P4 as background. The figure arrangements and all the symbols are the same as those in Figure 3.

Figure S8. Predicted stagnant age of the subducted Pacific slab within the transition zone beneath East Asia in map view. The belts consist of projections of the flavoured particles that represent subducted slab materials and are coloured according to their stagnation age in the mantle transition zone.

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