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North Atlantic 2016 – Durham
Mapping Crustal Thickness, OCT Structure and Crustal Type Using
Satellite Gravity Anomaly Inversion for the North Atlantic: Some
Answers but Many Questions
Nick KusznirEarth and Ocean Sciences, University of Liverpool, Liverpool, L69 3BX, UK
Email: [email protected]
Aim
•To present evidence for Iceland being
underlain by lithosphere with some
continental component?
•To make some observations of analogous
phenomena using global crustal thickness
mapping from satellite gravity inversion
NJK /Sept 2016
2Abstract
Gravity anomaly inversion of satellite derived free-air gravity incorporating a lithosphere thermal gravity anomaly correction data now provides a useful and reliable methodology for mapping global crustal thickness in the marine domain (Chappell & Kusznir, GJI, 2008). The resulting maps of crustal thickness and continental lithosphere thinning factor may be used to determine continent-ocean boundary location, and the distribution of oceanic lithosphere, micro-continents and oceanic plateaux (e.g. Alvey et al., EPSL 2008). Crustal cross-sections using Moho depth from gravity inversion allow continent-ocean transition structure and magmatic type (magma poor, “normal” or magma rich) to be determined. Using crustal thickness and continental lithosphere thinning factor maps with superimposed shaded-relief free-air gravity anomaly, we can improve the determination of pre-breakup rifted margin conjugacy and sea-floor spreading trajectory during ocean basin formation.
Gravity anomaly inversion of satellite derived free-air gravity incorporating a lithosphere thermal gravity anomaly correction data now provides a useful and reliable methodology for mapping global crustal thickness in the marine domain (Chappell & Kusznir, GJI, 2008). The resulting maps of crustal thickness and continental lithosphere thinning factor may be used to determine continent-ocean boundary location, and the distribution of oceanic lithosphere, micro-continents and oceanic plateaux (e.g. Alvey et al., EPSL 2008). Crustal cross-sections using Moho depth from gravity inversion allow continent-ocean transition structure and magmatic type (magma poor, “normal” or magma rich) to be determined. Using crustal thickness and continental lithosphere thinning factor maps with superimposed shaded-relief free-air gravity anomaly, we can improve the determination of pre-breakup rifted margin conjugacy and sea-floor spreading trajectory during ocean basin formation.
Crustal thickness mapping (figure 1b) shows large crustal thicknesses (> 30 km) under SE Iceland (Torsvik et al., PNAS, 2015) extending offshore to the NE and consistent with SE Iceland being underlain by continental crust associated with a southern continuation of the Jan Mayen micro-continent. This interpretation is supported by geochemical evidence.
Plate restoration to 83 Ma of crustal thickness derived from gravity inversion for the S Atlantic (figure 2) shows the Rio Grande Rise and Walvis Ridge forming a single feature which is analogous to Iceland. Some continental component has been proposed for the Rio Grande Rise. Similar features with anomalously thick crust within the ocean domain with continental affinity are also observed within the Indian Ocean (Torsvik et al., Nature Geoscience, 2014) and appear to be attractors for ocean ridge jumps. Some of many questions are whether these regions clearly within the oceanic domain are underlain by lithosphere with some continental compositional component and whether the ridge jumps are attracted by rheological weaknesses controlled by compositional or thermal anomalies.
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•3D spectral inversion for Moho depth - Parker (1972)- Low pass Butterworth filter ( λ = 100 km)
•Smith’s theorem – unique solution for assumptions made
•Lithosphere thermal gravity anomaly correction
- Oceanic and rifted continental margin lithosphere have elevated geothermal gradients- Large negative thermal gravity anomaly (< -350 mgal) - Correction needed to determine Moho depth from gravity inversion
•Magmatic addition prediction uses decompression melting model of White & McKenzie (1989)
•Sediment density model assumes normal
compaction
Global Oceanic Crustal Thickness Mapping using Satellite Gravity Inversion
•Greenhalgh & Kusznir, Geophys. Res. Lettr., 2007
•Chappell & Kusznir, Geophys. J. Int., 2008
•Alvey, Gaina, Kusznir & Torsvik, EPSL, 2008
•Cowie & Kusznir, J. Petrol Geol., 2012
•Torsvik et al. Nature Geoscience, 2013
•Torsvik et al., PNAS, 2015
•Cowie, Kusznir & Manatschal et al., GJI, 2015
•Cowie, Angelo, Kusznir, Manatscahl & Horn,
Petroleum Geoscience, 2016
Example validation
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Crustal Thickness (km)
0
5
10
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25
30
35
40
45
50
Global Oceanic Crustal
Thickness Mapping using
Satellite Gravity Inversion
(Kusznir, Cowie & Alvey, 2014)
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Crustal Basement Thickness from Gravity Inversion
North Atlantic & Iceland
(Kusznir, 2015; in Torsvik et al.,
PNAS, 2015)http://www.pnas.org/content/112/15/E1818.abstract
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Crustal Basement Thickness from Gravity Inversion
North Atlantic & Iceland Comparison of Gravity and Seismic Moho Depths
(Kusznir, 2015; in Torsvik et al.,
PNAS, 2015)
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(Torsvik et al., PNAS, 2015)
North Atlantic & Iceland
(Kusznir, 2015; in Torsvik et al.,
PNAS, 2015)
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(Torsvik et al., PNAS, 2015)
North Atlantic & Iceland
Crustal Basement Thickness from Gravity Inversion (m)
•Thick crust under SE Iceland extending to NE
•Skjaldarsgrunn
•Distinct from FIR
•Continental component from geochemical
•Fragment of Jan Mayen
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Rifted Continental Margins
• Magma Poor
• “Normal”
• Magma Rich
(courtesy Manatschal)
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•Reference crustal thickness = 35 km•Volcanic margin (ocean crust thickness = 10 km, γcrit = 0.5)•Breakup age = 54 Ma•Oldest isochron used = 54 Ma, defines cooling age only•Lithosphere thermal correction on•With sediments: Density compaction controlled•Sediments: merged NOAA-NGDC & Laske et al.
Norwegian Margin Crustal Thickness from Gravity InversionMagma Rich Palaeocene Breakup - Moere, Voering & Jan Mayen Margins
Crustal Basement Thickness (m)
& Superimposed Shaded Relief Free Air Gravity
(Watson & Kusznir, 2010)
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•Reference crustal thickness = 35 km•Volcanic margin (ocean crust thickness = 10 km, γcrit = 0.5)•Breakup age = 54 Ma•Oldest isochron used = 54 Ma, defines cooling age only•Lithosphere thermal correction on•With sediments: Density compaction controlled•Sediments: merged NOAA-NGDC & Laske et al.
Norwegian Margin Crustal Thickness from Gravity InversionMagma Rich Palaeocene Breakup - Moere, Voering & Jan Mayen Margins
Crustal Basement Thickness (m)
& Superimposed Shaded Relief Free Air Gravity
•Reference crustal thickness = 35 km•Volcanic margin (ocean crust thickness = 10 km, γcrit = 0.5)•Breakup age = 54 Ma•Oldest isochron used = 54 Ma, defines cooling age only•Lithosphere thermal correction on•With sediments: Density compaction controlled•Map sediments – merged NOAA-NGDC & Laske et al.•Xsection sediments – NPD Bulletin 8 (Blystad et al.), iSIMM
(Watson & Kusznir, 2010)
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•Reference crustal thickness = 35 km•Volcanic margin (ocean crust thickness = 10 km, γcrit = 0.5)•Breakup age = 54 Ma•Oldest isochron used = 54 Ma, defines cooling age only•Lithosphere thermal correction on•With sediments: Density compaction controlled•Sediments: merged NOAA-NGDC & Laske et al.
Norwegian Margin Crustal Thickness from Gravity InversionMagma Rich Palaeocene Breakup - Moere, Voering & Jan Mayen Margins
Crustal Basement Thickness (m)
& Superimposed Shaded Relief Free Air Gravity
•Reference crustal thickness = 35 km•Volcanic margin (ocean crust thickness = 10 km, γcrit = 0.5)•Breakup age = 54 Ma•Oldest isochron used = 54 Ma, defines cooling age only•Lithosphere thermal correction on•With sediments: Density compaction controlled•Map sediments – merged NOAA-NGDC & Laske et al.•Xsection sediments – NPD Bulletin 8 (Blystad et al.), iSIMM
(White et al., Nature, 2008)
Faeroes-Shetland, Moere &
Voering Basins separated from
Norwegian Sea Oceanic Crust
by Thicker Crust with
Continental Component
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Norwegian Margin Crustal Thickness from Gravity InversionMagma Poor Oligocene Sea-floor Spreading on the Aegir Ridge
& Breakup West of Jan Mayen
(Breivik et al. JGR, 2006)(Kodaira et al. JGR, 1998)
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(a)
(b)
0
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35
40
45
50
Cru
stal
thic
knes
s (k
m)
0 Ma
83 Ma
South Atlantic: Rio Grande Rise & Walvis Ridge
Plate
Reconstructions
Restoration of
Crustal Thickness
using GPlates v1.5
(Kusznir, Cowie & Alvey, 2014)
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(a)
(b)
0
5
10
15
20
25
30
35
40
45
50
Cru
stal
thic
knes
s (k
m)
0 Ma
83 Ma
South Atlantic: Rio Grande Rise & Walvis Ridge
Plate
Reconstructions
Restoration of
Crustal Thickness
using GPlates v1.5
Recovered Granite Recovered Granite on RGRon RGR
(Kusznir, Cowie & Alvey, 2014)
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16Indian Ocean: Seychelles, Mascarene & Nazareth Banks & Mauritius
Repeated Ocean
Ridge Jumps
Creating Thicker
Oceanic Crust
Mauritius Pre-Cambrian Zircons
(Continental Lithosphere?)
(Torsvik et al. Nature, 2013)
(Alvey & Kusznir, 2011)
(Alvey & Kusznir, 2011)
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Questions
Iceland, Rio Grande Rise, Mauritius, Conrad Rise, Crozet Plateau etc
• Are these regions underlain by lithosphere (or deeper tectosphere)
with some continental compositional component?
• They are associated with rift/ridge jumps
• Are these intra-ocean ridge jumps attracted by rheological
weaknesses controlled by compositional or thermal anomalies (or
both)?
• Can these ocean ridge jumps (and hot spots) be explained by upper
mantle chemical heterogenity and thermal “weather” (+/- a few
tens of oC)?