absolute asthenosphere viscosity from caribbean …...absolute asthenosphere viscosity from...
TRANSCRIPT
Absolute asthenosphere viscosity from Caribbean dynamic topography, mantle structure and magmatismYi-Wei Chen1 ([email protected]), Lorenzo Colli1, Dale E. Bird1, Jonny Wu1 and Hejun Zhu2 1Dept. of Earth and Atmospheric Science, University of Houston2Dept. of Geoscience, University of Texas at Dallas
1
3
4 9
2A Honey Sandwich or a Peanut Butter Sandwich?
Independent constraints
10
8
Conceptual evolution
Flow velocity varies with different viscosities 7 Residual Topography (i.e. Dynamic topography)
A gravity-constrained crustal model (using Oasis Montaj)
Lithospheric age of the Caribbean 5 Sedimentary Thickness (This study) Moho Depth (This study)6
Topography = Thermal subsidence + Dynamic topography + Isostatic topography
Pressure Difference & Velocity Profile
References:1. Müller et al. (2008) Geochemistry, Geophysics, Geosystems 9.4.2. Weismüller et al. (2015) Geophysical Research Letters 42.18: 7429-7435.3. Magnani et al., (2009) Journal of Geophysical Research: Solid Earth 114.B24. Paulson and Richards. (2009) Geophysical Journal International 179.3: 1516-1526.5. Schaber et al. (2009) Geochemistry, Geophysics, Geosystems 10.11.6. Boschman et al. (2014) Earth-Science Reviews 138: 102-136.7. Gazel et al. (2011) Lithos 121.1-4: 117-134. 8. Hoggard et al. (2016) Nature Geoscience 9.6: 456.9. Hu et al. (2016) Nature 538.7625: 368. 10. Sandwell et al. (2014) Science 346.6205: 65-67.
High Pressure Low Pressure
Lesser Antilles Arc
CentralAmerican Arc
Arc extinct
Alkaline basalts withGalapagos signature
The leading edge of slow anomalies
Velocity profile with our preferred asthenospheric viscosity of 5*1018 Pa s
Galapagos plume
Slab window
Dynamic uplift~300m
Tilted Lithosphere
Before 8.5 Ma
Lower Mantle
Lithosphere
Asthenosphere
8.5 Ma
0 Ma
Tortuguero <<0.1 Ma? Guayacan/Azules: 4.5-2 Ma
Victoria dikes: 6.5 MaPearl Lagoon/Cukra Hill: <0.7 Ma
Alkaline basalts (Gazel et al., 2011)
210-2 -1
δln Vs (%)
20ºN
10ºN
0º80ºW90ºW 70ºW 60ºW
Beata Ridge
Slab Windowopened at 8 Ma
AlkalineBasalts:5 cm/yr
Depth: 300 km
Azimuthal anisotropy, proxy for flow direction
A A’
B
B’
Leading edge of the slow anomalies: 15 cm /yr
1000
800
600
400
200
0
1000
800
600
400
200
0
A A’
B B’
Dep
th (k
m)
Dep
th (k
m)
Lesser Antilles Arc
Subducted Atlantic Slab
Subducted Proto-Caribbean Slab ?
South Americacontinental root
Subducted Caribbean slab
Asthenosphere
Asthenosphere
The Galapagos Beata Ridge
The AtlanticThe CaribbeanThe Pacific
Robust Residual topography (This study) Hoggard et al., 2016
Residual topography (This study)
slab
win
dow
Less
er A
ntill
es tr
ench
Res
idua
l Top
ogra
phy
(RT)
(km
)
-2
-1
1
2
0
h = 300 m
d = 2500 km
P= ρgh
ρ = 3300 g/ccg= 9.8 m/s2
0
40
80
120
160
2000 5-5 10 15 Velocity (cm/yr)
Dep
th b
elow
lith
osph
ere
(km
)
Leading edge of
3*10185*101810195*10195*1020
the hot material
v(y) = * * y * (y - H) + u *
Pressure gradient Velocity as a function ofdepth
Viscosity values from 1 Thickness from 9
from 9
ρgß 12η
yH
Best-fit regression line:
Alkaline basalts from 9
0 500 1000 1500-500-1000-1500 2000 2500 Distance (d) fromslab window (km)
RT = α + ß*dα = 0.476 ± 0.012 ß = 1.4*10-4 ± 9*10-6
The uncertainty of velocity profile from the uncertainty of pressure gradient
Our preferred velocity profile with our proposed viscosity
Density (g/cc)Thickness (km)
Crust
Mantle
SedimentWater
Moho (see 6 )
Basement (see 5 )
Topography
Free air gravity anomaly
- GTOPO30 (USGS, 1998)- Terrainbase (Row et al., 1995)
- Refraction / Reflection- Global Sedimentary Thickness (Divins, 2003)
- Refraction- Gravity inversion
Oceanic Mantle:age-dependent(see 3 )= 3.25-3.31
Continental Mantle= 3.3
Crust of oceanic plateaus= 2.85
Sediment: thickness-dependent = 2.15-2.6
Water = 1.03Sea level
GravityFree air gravity anomaly ∑Mass = ∑ ( Thickness x Density)
Topography & Bathymetry (km)
-8 50
Residual Topography (m)
-150 8500
20ºN
10ºN
0º80ºW90ºW 70ºW 60ºW
BR
Cross-section in 8
Moho depths (km)
-31-55 -43 -19
Slope-intercept Synthetic
Rough & Acoustic Basement Boundary
?
?
80ºW90ºW
20ºN
10ºN
0º
2010 30 40
20
10
30
40
Inversion Moho (km)
Ref
ract
ion
Moh
o (k
m) 1 t
o 1 lin
e
Slope-intercept
Synthetic
Seismic refraction
Sediment Thickness (km)
0 5 10 15
20ºN
10ºN
0º
80ºW90ºW 70ºW 60ºW
seismic reflection
seismic refraction
reflection + refraction
wells
DSDP
IODP
Cocos-Nazca spreading center Basalt samples of
the Caribbean crust(~100-80 Ma)
Cayman trough
spreading center
Age (Ma) (from Muller et al., 2008)
0 50 100 150
20ºN
10ºN
0º
80ºW90ºW 70ºW 60ºW
Mantle flow
Dynamic topography Isostatic topography
Thick Crust Thin Crust
3.25 3.27 3.29 3.31Density
(g/cc)
4
Dep
th (k
m)
12
20
0 100Age (Ma) 200
Thermal subsidence(Richards et al., 2018)
++=
Galapagos
Cocos ridge
Beata Ridge
Colombiabasin
Aves ridge Lesser Antilles
LNR
UNR
Cayman Trough
Carnegie ridge
Venezuelabasin
Sandra riftPFZ
NPDB
SCDB
Topography and Bathymetry (km)
0-4-6 -2 2 4
20ºN
10ºN
0º
80ºW90ºW 70ºW 60ºW
S. AmericaContinent
N. America Continent
Continental Terranes (Boschman et al., 2014)
3. Calculate flow velocities with different viscosity values.
4. Compare to independent velocity constraints from geology.
2. Convert it to an average pressure gradient.
1. Quantify the dynamic topography across the Caribbean region.velocity profile
low viscosityupper mantle high
Pressure lowPressure
velocity profile
Geological constraints
Geological constraints
high viscosityupper mantle
warped lid
Strategy:
Viscosity (Pa s)1018 10211019 1020
PSR at Indonesia (Hu et al., 2016)
(Han et al., 2019)
Isostatic rebound at Western US
GIA at Iceland
PSR at Samoan islands
PSR: Postseismic Rebound
GIA:Glacial Isostatic Adjustment
(Auriac et al., 2013)
(Bills et al., 2007)
GIA at Canada & Scandinavia
(Paulson & Richards, 2009)
Global Geoid (Schaber et al., 2009)
Viscosity: 5*1018 Pa s (This study)
(2*103 mPa s) (2*105 mPa s)
←The Caribbean is bounded by subduction zones and continental terrains (transparent yellow polygons); therefore, the subducted slabs and the continental roots hinder the asthenosphere to flow freely, but confine the flow within a narrow gateway beneath the Caribbean, a scenario similar to the simplified 2D model shown in 2 .
We used seismic refraction, reflection and borehole data to build the velocity model of the upper most crust as a function of depth. The velocity model and the 2-way travel time from seismic reflection help us constrain the sediment thickness. Sub-tracting the sediment thickness from the bathymetry, the basement depth is ob-tained.
The thermal age of the Caribbean lith-osphere was was rescaled from 100 Ma to 80 Ma based on the basalts.
← To obtain dynamic topog-raphy, we subtract the total topography from thermal subsidence and isostatic to-pography. Thermal subsid-ence is obtained from the plate cooling model of Rich-ards et al. (2018) using the age model from Müller et al. (2008) (see 4 ). Isostatic effect was corrected using seismic-constrained sedi-mentary thickness (see 5 ) and gravity constrained Moho (see 6 ). The residual topography (see 7 ) is gen-erated by mantle flow and is our best-estimate for dy-namic topography.
Due to the limited refraction constraints of the Moho (red lines and blue open boxes), we used free air gravity anomaly to constrain the Moho. Our regional gravity-con-strained Moho model fits local Moho depth estimates from refraction studies (see inset).
Independent constraints come from (1) back arc magmatism which shows clear Galapagos isotopic signatures, initiating at ~6.5 Ma near the slab window (black circle) which opened at ~8 Ma and propagating at a rate of 5 cm/yr northward. (2) Full waveform tomography shows a slow velocity anomaly in the western Caribbean (west of Beata Ridge). If we assume the anomaly is hot mantle material flowing through the slab window, the propagation rate is ~15 cm/yr. The two cross sections suggest that the asthenosphere is ~200 km thick.
The dynamic topography across the Caribbean region constrains the pressure gradient that drives Galapagos hot mantle material flowing eastward through the slab window. Given the driving pressure gradient and the thickness of the asthenosphere, flow speeds depend only on the viscosity of the as-thenosphere. A value of ~5*1018 Pa s best fits the propagation rates of the back-arc basalts and the imaged seismic structure.
The concept of a weak asthenosphere sandwiched between mobile tectonic plates and mechanically strong sub-asthenospheric mantle is fundamental to understand plate tectonics and mantle convection. However, the quantitative estimation of absolute asthenospheric viscosity is highly uncertain and it varies by 1 to 3 orders of magnitude (see 1 ). Wide-range viscosity estimations come mainly from two reasons: (1) Most observations can be satisfied with proper viscosity contrasts –instead of the absolute viscosity– between asthenosphere and sub-asthe-nospheric mantle. (2) The viscosity contrast trades off with the thickness of the asthenospheric layer.
Here, we estimate absolute viscosity of asthenosphere for the first time from the pressure gradient and the asthenospheric flow velocity at the Caribbean, using the well-established analytical solution (see 8 ). Funnelled by subduction zones and continental lithospheric roots (see 3 ), the Caribbean region provides a unique tectonic setting reminiscent of a plane channel (see 2 ) that closely approximates the conditions of the analytical solution. The asthenesphric flow at Caribbean from the Pacific towards the Atlantic has been a long-standing hypothesis. We in-ferred the flow velocity from regional magmatism (see 9 ) and tomographic imaging with the onset time of the flow at ~8.5 Ma constrained by the opening of Panama slab window (see 10 ). This flow is mainly pressure-driven (i.e. Poiseuille flow), since the Caribbean has been fixed in mantle reference frame since Eocene. The pressure gradient (see 7 ) is calculated from the dynamic topography (see 3 ) across the Carib-bean, obtained by removing the isostatic effect of sediments (see 5 ) and crust (see 6 ). Independent constraint of asthenosphere thickness of ~200km is suggested by tomography ( see 9 ).
The importance and improvement of this study are three folded. First, we provided a better quantified pressure gradi-ent based on a proper isostatic correction and uncertainty es-timations. With reasonable assumptions, we are the first study that use the stain rate (i.e. flow velocity) directly from the asthenosphere, instead of an average strain rate of the upper mantle from surface constraints. Finally, we show for the first time an on-site estimation of the viscosity where the asthenosphere thickness is independently constrained. Our results suggest the viscosity of the asthenosphere at the Ca-ribbean is ~5*1018 Pa s, which is in line with estimates for non-cratonic and oceanic regions, but is significantly lower than post-glacial rebound estimates for cratonic regions. This further supports the notion that the stronger asthenosphere estimates are relatively limited to cratonic regions.