Crustal structure of Sri Lanka derived from joint inversion of SWD and RF using a Bayesian approach
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Poster Presentation EGU 2020–11544 May 5, 2020 Crustal structure of Sri Lanka derived from joint inversion of SWD and RF using a Bayesian approach J. Dreiling1 , F. Tilmann1 , X. Yuan1 , C. Haberland1 , S.W.M. Seneviratne2 1 German Research Centre for Geosciences (GFZ), Potsdam 2 Geological Survey and Mines Bureau (GSMB), Sri Lanka
Citation
The content of this presentation is published. For citing, consider:
• Dreiling et al., 2020: Dreiling, J., Tilmann, F., Yuan, X., Haberland, C., &
Seneviratne, S.W.M. (2020). Crustal structure of Sri Lanka derived from joint
inversion of surface wave dispersion and receiver functions using a Bayesian
approach. Journal of Geophysical Research: Solid Earth.
https://doi.org/10.1029/2019JB018688.
• Dreiling and Tilmann, 2019: Dreiling, J., & Tilmann, F. (2019). BayHunter –
McMC transdimensional Bayesian inversion of receiver functions and surface wave
dispersion. GFZ Data Services. https://doi.org/10.5880/GFZ.2.4.2019.001 and
https://github.com/jenndrei/BayHunter.Abstract
We study the crustal structure of Sri Lanka by analyzing data from a temporary
seismic network deployed in 2016–2017 to shed light on the amalgamation process from
a geophysical perspective. Rayleigh wave phase dispersion curves from ambient noise
cross-correlation and receiver functions were jointly inverted using a transdimensional
Bayesian approach.
The Moho depths in Sri Lanka range between 30 and 40 km, with the thickest crust
(38–40 km) beneath the central Highland Complex (HC). The thinnest crust (30–35 km) is
found along the west coast, which experienced crustal thinning associated with the formation
of the Mannar Basin. VP /VS ratios lie within a range of 1.60–1.82 and predominantly favor
a felsic to intermediate bulk crustal composition with a significant silica content of the rocks.
A major intra-crustal (18–27 km depth), slightly westward dipping (∼4.3◦ ) interface
with high VS (∼4 km/s) underneath is prominent in the central HC, continuing into the
western Vijayan Complex (VC). The discontinuity might have been part of the respective
units prior to the collision and could be an indicator for the proposed tilting of the WC/HC
crustal sections. It might also be related to the deep crustal HC/VC thrust contact with
the VC as an indenting promontory of high VS . A low velocity zone in the central HC could
have been caused by fluid influx generated by the thrusting process.Geologic Background
Gondwana’s assembly and break-up
TC
South
India
Madagascar
WDC
Africa EDC
SGT
KKB
Sri Lanka
LHC
(Meert and Lieberman, 2008)
WTHM East Antarctica
Pan-African Orogeny (PAO) Gondwana break-up
KM
DML
• stepwise collision of cratons • stepwise break-up
(750–490 Ma) to form Gondwana between 185–100 MaGeologic Background
Gondwana’s assembly and break-up
10˚
Cenozoic Cover
Wanni Complex (WC)
Kadugannawa Complex (KC)
Highland Complex (HC) Geology
Vijayan Complex (VC)
• Amalgamation during PAO
9˚
SL24
(Kehelpannala, 2004)
Gulf of
Mannar SL25
SL21
MALK
SL22
?
8˚ SL16
SL17 SL23
SL18
?
SL02 SL19
SL04
SL01 SL03 SL06 SL20
SL31
PALK
SL05 SL09
SL08 SL10 SL11
SL26 SL13
7˚ SL15
SL12
SL14
? SL27
SL28
Seismic network
SL30
SL29 • 33 seismic stations
HALK
• continuous seismic data between
6˚
2016–2017
79.5˚ 80˚ 80.5˚ 81˚ 81.5˚ 82˚Seismic Data
Surface wave dispersion
Dispersion measurements 300
# Picks
• ambient noise cross-correlation 100
4.0
between station pairs Rayleigh wave
3.8 mean
Velocity in km/s
• phase velocities were measured 3.6
(ZZ, ZR, RZ, RR) and stacked to 3.4
receive Rayleigh wave dispersion 3.2
3.0
curves
1 2 4 8 16 32
Period in s
Tomography 3.8
• tomography was performed at 3.6
Phase velocity in km/s
40 discrete periods 3.4
• tomographic dispersion curves 3.2
were assembled at locations of 3s 12 s 21 s 3.0
seismic stationsSeismic Data
Receiver functions
Receiver functions
• events M>5.5, distances 30–90◦
• filtering, trimming, downsampling, rotation to LQT,
and deconvolution of event traces to compute RFs
• RFs from each station were stacked to a final RF
5
0 SL01 SL02 SL03 SL05 SL06 PALK SL08 SL09 SL10 SL11 SL12 SL13 SL15
5
Delay time in s
10
15
20
25
30
RF stacks along profile (location on map). – Moho phases; – intra-crustal phasesBayesian Inversion
Principles and application
BayHunter – A Python tool
Bayes’s theorem for Bayesian joint inversion.
p(m|dobs )p(dobs ) = p(dobs |m)p(m) Available on GitHub.
Parameters
p(m|dobs ) ∝ p(dobs |m)p(m)
• VS -depth structure
posterior ∝ likelihood · prior • crustal VP /VS
• data noise (σ)
• number of layers
prior distribution
Parameter space
likelihood
• exploration based on
Bayes’s theorem
posterior distribution
• Markov chain Monte
Carlo sampling
• two sampling phasesBayesian Inversion Results
Posterior distribution (100,000 models) for SL21
SL21
0
Velocity in km/s
3.8
10 3.5
observed
3.2
20
2 4 8 16 32
Period in s
30
Depth in km
Amplitude
Moho
40
0 10 20 30
50 Time in s
2017 0.026
Joint Misfit
median
Likelihood
60
all models
best 50 %
70 best 25 %
3.5 4.0 4.5
Vs in km/s 2000 2020 0.025 0.030
0.007 0.011 6 1.73
Sigma SWD
Sigma RF
# Layers
Vp/Vs
0.005 0.010 0.015 0.0100 0.0125 5 10 1.6 1.8Bayesian Inversion Results
Estimation of Moho depth for SL10
0
(a) (b)
10 median:
3.79 km/s
20
30
Depth in km
Moho
40 median:
Moho depth in km
37.5 38.02 km
50 38.0
60 38.5
3.77 3.79 3.81
70 Vs crustal mean
mode
3.5 4.0 4.5
Vs in km/sBayesian Inversion Results
for Sri Lanka
Observations
• Moho interface depth between 30 and 40 km
• VP /VS between 1.60 and 1.82 (felsic/intermediate)
30 32 34 36 38 40 1.6 1.7 1.8
Moho depth (km) Vp / Vs
4
37
36 36
39
38
37Bayesian Inversion Results
for Sri Lanka
Observations
• central low velocity zone at ∼10 km depth
?
• mid-crustal interface dipping between 18 and 27 km
?
• Moho interface depth between 30 and 40 km
?
WC HC VC
0 4.5
10
20
Depth in km
Vs in km/s
4.0
30
40
50 3.5
60
70 SL01 SL02 SL03 SL05 SL06 PALK SL08 SL09 SL10 SL11 SL12 SL13 SL15 3.0
0 50 100 150 200
Kilometer along profileBayesian Inversion Results
for Sri Lanka
Interpretations
• low velocity zone might represent a different rock
?
composition through fluid influx
?
• mid-crustal feature in HC might represent a tilted
crustal section or the HC/VC thrust contact ?
WC HC VC
0 4.5
17.5 20.0 22.5 25.0
Mid−crustal int. (km)
10 ?
20
Depth in km
Vs in km/s
4.0
30
40
50 3.5
60
70 SL01 SL02 SL03 SL05 SL06 PALK SL08 SL09 SL10 SL11 SL12 SL13 SL15 3.0
0 50 100 150 200
Kilometer along profileBayesian Inversion Results
in context of Gondwana deformation
Comparison to Pan-African terranes Madagascar
• Moho depths: 32–40 km Africa
SGT
• felsic/intermediate bulk crustal composition
• upper crustal LVZ in some terranes
→ similarities hint toward strong crustal
LHC
unification during Pan-African Orogeny
1.9 1.9
mafic East Anta
1.8 1.8
intermediate
VP/VS
felsic
1.7 1.7
Sri Lanka
Madagascar
Southern India
Pan-African Terranes
1.6 1.6
3.5 3.6 3.7 3.8 3.9 4.0 25 30 35 40 45
VS in km/s Moho depth in kmConclusion and Summary
Crustal structure of Sri Lanka
• felsic/intermediate bulk crustal composition
• upper crustal LVZ indicates a different rock composition
• mid-crustal interface represents an inherited structure in the HC
or the HC/VC thrust contact
• Moho discontinuity shows mostly isostatically compensated crust
Comparison to other Pan-African terranes
• seismic similarities due to region spanning unification during PAO
• differences through autochtone terrane compositions, positions within
the orogen, and individual reworking processes after PAOReferences
• Dreiling et al., 2020: Dreiling, J., Tilmann, F., Yuan, X., Haberland, C., &
Seneviratne, S.W.M. (2020). Crustal structure of Sri Lanka derived from joint
inversion of surface wave dispersion and receiver functions using a Bayesian
approach. Journal of Geophysical Research: Solid Earth.
https://doi.org/10.1029/2019JB018688.
• Dreiling et al., 2020: Dreiling, J., Tilmann, F., Yuan, X., Haberland, C., &
Seneviratne, S.W.M. (2020). Seismic crustal model of Sri Lanka. GFZ Data
Services. https://doi.org/10.5880/GFZ.2.4.2020.001.
• Dreiling and Tilmann, 2019: Dreiling, J., & Tilmann, F. (2019). BayHunter –
McMC transdimensional Bayesian inversion of receiver functions and surface wave
dispersion. GFZ Data Services. https://doi.org/10.5880/GFZ.2.4.2019.001 and
https://github.com/jenndrei/BayHunter.
Figures are taken from:
• Meert and Lieberman, 2008: Meert, J. G. and Lieberman, B. S. (2008). The
Neoproterozoic assembly of Gondwana and its relationship to the
Ediacaran–Cambrian radiation. Gondwana Research.
• Kehelpannala, 2004: Kehelpannala, K. (2004). Arc accretion around Sri Lanka
during the assembly of Gondwana. Gondwana Research.You can also read
