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The Tasmantid Seamounts: A window into structural ... · The Tasmantid Seamounts: A window into...
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The Tasmantid Seamounts: A window into structural inheritance of ocean floor fabric Fred Richards 1, 2* , Lara Kalnins 3 , Tony Watts 1 , Benjamin Cohen 4 & Robin Beaman 5 1 Department of Earth Sciences, University of Oxford, Oxford, UK 2 Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Cambridge, UK 3 Department of Earth Sciences, Durham University, UK 4 Scottish Universities Environmental Research Centre, East Kilbride, UK 5 College of Science, Technology and Engineering, James Cook University, Cairns, Australia *[email protected] V21A-3034 Introduction The Tasmantid Seamounts extend for >2000 km off the east coast of Australia, constituting one of three contemporaneous, sub-parallel Cenozoic hotspot tracks that traverse the region, locally separated by ≤500 km (Cohen et al., 2013). The sea- mounts young north to south, spanning ~50 - 6 Ma (McDougall & Duncan 1988; Kalnins et al., 2015). At multiple locations, the Tasmantid chain intersects the extinct slow-spreading Tasman Sea ridge system, which was active between 84 - 53 Ma (Mller et al., 2008). Despite the >20 Ma hiatus between spreading cessation and seamount emplacement, palaeo-ridge structure appears to exert significant control on sea- mount morphology and structure. Geophysical datasets acquired on voyage ss2012_v07 have been analysed to answer the following: 1. What mechanism can account for the morphologi- cal diversity of the seamounts? 2. What is the relationship between pre-existing tec- tonic fabric, intraplate magmatism and the strength of the Tasman Sea lithosphere? 3. How has the strength of the Tasmantid melting anomaly evolved through time? 152Ý 152Ý 15Ý 15Ý 10Ý 10Ý −2Ý −2Ý −2Ý −2Ý −24Ý −24Ý −22Ý −22Ý −20Ý −20Ý −1Ý −1Ý −1Ý −1Ý −000 −5000 −4000 −3000 −2000 −1000 0 1000 Bathymetry (m) MAIN STUDY REGION Nova Bank Argo Bank Tweed Brisbane Seamounts Moreton & Mooloolaba Seamounts Recorder Guyots Fraser Guyots Cato Reef Wreck Reefs Britannia Guyots Queensland Guyot Stradbroke Seamount T A S M A N T I D S E A M O U N T C HAI N L O R D H O W E S E A M O U N T C H A I N CORAL SEA Main Range Flinders Glasshouse - Maleny Noosa Fraser E A S T A U S T R A L I A N V O L CA N I C C H A I N LORD HOWE RISE Gifford Guyot Capel Bank Kelso Bank Kenn Reefs Frederick Reef Mellish Plateau Marion Plateau Dampier Ridge Kenn Plateau Middleton Basin Cato Trough TASMAN SEA BRISBANE Chesterfield Reefs AUSTRALIA Bellona Reefs Mellish Reef Bunya Toowoomba Figure 1. Map of the study area. Dotted red line = shiptrack of voyage ss2012_v07. Star = position of Cenozoic seamounts and volcanoes. Solid red line = extinct ridge axis, solid black line = transform fault, dashed black = fracture zone. Bathymetry derived from GBR100 dataset (Beaman 2010). 1 Volcanic Architecture • Tasmantid morphologies fall into four distinct categories: a) conical seamounts char- acterised by steep, smooth flanks; b) elongate and stel- late terraced seamounts with subaerially eroded peaks; c) rugged seamounts con- structed via repeated fissure eruptions along crosscutting volcanic rift zones (VRZs) and d) shield seamounts with shallow slopes and dispersed cinder cones. • The chain exhibits low mass wasting rates and fluctuating edifice volume, with dramat- ic morphological variation between seamounts separated by ≤10 km. • There is a clear link between overall morphology and sea- mount position relative to the extinct spreading centre. 154Ý 1549Ý 1550Ý 1551Ý 1552Ý −254Ý −253Ý −252Ý −251Ý −250Ý −249Ý −24Ý Perspective view Recorder FZ −4500 −3500 −2500 −1500 −500 Bathymetry (m) 3 identifiable summit benches Hummocky debris deposits with pelagic sediment drape Rugged flanks with narrow embayments Slumped debris deposits coat terraces 1557Ý 155Ý 1559Ý 150Ý 151Ý 152Ý −291Ý −290Ý Perspective view 2 main volcanic centres Steep gullied flanks ENE -WSW trending volcanic ridge Pillow basalts and talus deposits Flank crater −3000 –4500 –1500 Bathymetry (m) 1553Ý 1554Ý −242Ý −241Ý −240Ý Perspective view Carbonate-capped, flat summit Flank cone Hummocky debris deposit Shallow headscars above narrow, steep embayments −4000 −3000 −2000 −1000 Bathymetry (m) Steep, fluted flanks 1550Ý 1551Ý 1552Ý 1553Ý 1554Ý 1555Ý 155Ý 1557Ý 155Ý −235Ý −234Ý −233Ý −232Ý −231Ý −230Ý West Bellona FZ Perspective view Cato FZ KENN PLATEAU CATO TROUGH Volcanic cone field Hummocky debris field with blocks up to 800m across Fissure-fed volcanic ridges Bathymetry (m) −3000 −2000 −1000 0 Gentle slope gradients on E side toward Lord Howe Rise Small satellite guyot Steeper slopes on W side into Cato Trough 2 b a c d Figure 2. (a) N Fraser: conical seamount emplaced off-axis, adjacent to the ocean-con- tinent boundary (OCB). (b) N & S Recorder: both seamounts have terraced upper slopes. N Recorder is off-axis and stellate; S Recorder is emplaced on a fracture zone and is elongated. (c) Stradbroke: rugged seamount straddling an inside corner. (d) Cato: shield seamount located on the Lord Howe Rise microcontinent. Vertical exaggeration is x4.5. N.B. Bathymetry colour scales are different for each seamount. Figure 3. Average slope gradient by sector. (a) South Moreton, a con- ical seamount. (b) Cato, a shield seamount. (c) Britannia, a terraced seamount: (i) basal and (ii) upper, subaerially-eroded slopes. (d) Strad- broke, a rugged seamount. S 5 $ E W N S 5 $ E W N S 5 $ E W N S 5 $ E W N S 5 $ E W N S E W N Slope Analysis • Tectonic setting exerts major control on slope characteristics: conical seamounts with elevated slope gradients but lower inter-sector variance occur off-axis and at outside corners; rugged seamounts with low slope gradient but high inter-sector variability occur at inside corners. • Terraced and shield seamounts have notably variable, low gradient upper slopes suggesting elevated rates of mass wasting due to sub-aerial exposure. • The relatively low slope variance and elevated backscatter on the lower sections of the edifices indicate that large mass-wasting events are rare. This is consistent with minimal shallow deformation and may reflect modest eruption rate with a high intrusive-to-extrusive magmatic budget (Ramalho et al., 2013). Figure 4. Detecting mass wasting. (a) SE-ward view of S Moreton, red dashes = mass wasting deposit with reduced mean slope gradient (b) and low backscatter (e). Slope gradient is fairly constant (c) so bathymetric curvature (e) is ~ 0. The run-out distance: headscar aspect ratio suggests a debris avalanche mechanism (Mitchell et al., 2002). 3 b b a a c c e d d i ii Structural Orientations 4 Figure 7. Effect of cross-transform mechanical coupling on principal stress orientations (modified after Behn et al., 2002). Focal mechanisms show optimal, calculated fault plane solutions at 4 km depth for differing degrees of mechanical coupling. χ = mechanical coupling, OC = inside cor- ner, IC = outside corner, red = ridge axis, black = transform fault. τ dev /σ dev+lith is the ratio of shear to normal stress on the fault plane incorporating both deviatoric and lithostatic components. (a) χ = 0, (b) χ=0.05, (c) χ = 0.1 & (d) χ = 0.15. Horizontal displacement = 100m (10 ka at half-spreading rate of 1 cm a -1 ). Figure 5. Linear feature orientation vs. tectonic setting. (a) Stradbroke: (i) trends oblique-to-spreading; (ii) inside-corner setting. (b) S & N Recorder: (i) S Recorder: volcanic trends parallel to Recorder Fracture Zone and (iii) fracture zone setting (ii) N Re- corder has ridge-parallel features and (iii) off-axis setting. (c) N Fraser: (i) trends ridge-parallel or subparallel to fracture zones; (ii) ocean-continent boundary setting. Red = major VRZs, blue = minor VRZs and black = faults. IC = inside corner, OC = outside corner. • Alignments suggest deep faulting of the oceanic lithosphere allowing channelisation of magma along pre-existing structures despite emplacement post-dating active extension by >20 Ma. • Dominance of the tectonic signal points to low melt production, implying that the Tasmantid “plume” was a relatively weak melting anomaly. • Seamounts located at or adjacent to ridge-transform intersections have dominant trends oblique to spreading, implying strong mechanical coupling across the transforms. • The high degree of mechanical coupling is consistent with slow Tasman Sea spreading rates and low rates of melt production. 0.15 0 0.1 0.05 Ѭ dev / ѫ dev+litho 125 100 75 50 25 Y (km) OC IC 50 100 X (km) 50 100 X (km) 50 100 X (km) 50 100 X (km) INCREASING CROSS-TRANSFORM MECHANICAL COUPLING 500 km 100 km Ridge Axis & FZs 25 km 2II−$[LV 25 km Fracture Zone 25 km Inside Corner 25 km Outside Corner 25 km Continental S E W N S E W N S E W N S E W N S E W N S E W N 10 km 10 km 10 km 10 km S E W N S E W N S E W N S E W N C A T O F Z RECORDER FZ IC IC OC IC OC OC IC IC IC OC OC 10 km 10 km 10 km OC IC b b a a c c d i i i ii ii ii d f Figure 6. Linear feature orientations of all seamounts. (a) Axial ridges (red) and fracture zones (black). (b) Off-axis sea- mounts. (c) Fracture zone seamounts. (d) Inside corner seamounts. (e) Outside corner seamounts. (f) Continental seamounts. b a c e iii Deep Structure • Bouguer anomalies are calculated by removing the combined gravity effect of the water-sediment and and sediment-seamount interfaces to investigate gravity signals related to deeper density interfaces. • Calculations assume that seamount topography is uncompensated, i.e. the lithosphere is infinitely rigid. • The reduction density of a seamount refers to the input seamount density that minimises spectral coherence between calculated Bouguer anomalies and the bathymetry. • 20-50 mGal Bouguer highs over many seamounts, coupled with reduction densities up to ~3100 kg m -3 , suggest extensive intra-basement intrusion of primary magmas (cf. Contreras-Reyes et al., 2010). • Low reduction densities over Cato and Wreck are consistent with both seamounts being emplaced on continental basement and significant fractionation of parent magmas (Hammer et al. 1990). 5 −222Ý 720000P 750000P 70000P −3000 −2000 −1000 0 %DWK\PHWU\ P i ii 7530000P 750000P 720000P 750000P 70000P −200 −100 0 100 200 7RSRJUDSKLF $QRPDO\ P*DO i ii 1551Ý 1554Ý 1557Ý −222Ý −200 −100 0 100 200 )UHH−$LU $QRPDO\ P*DO i ii 1551Ý 1554Ý 1557Ý 7530000P 750000P 0 5 2 5 2 2 0 0 2 5 7 1 %RXJXHU $QRPDO\ P*DO i ii −300 −200 −100 0 100 200 300 400 500 *UDYLW\ $QRPDO\ P*DO 700000 725000 750000 775000 (DVWLQJ P −5000 −4000 −3000 −2000 −1000 0 1000 7RSRJUDSK\ P i ii 00 01 02 03 04 05 0 07 0 09 10 &RKHUHQFH 10 2 10 3 10 4 10 5 :DYHOHQJWK P ρ U %DVHPHQW 'HQVLW\ 2400 NJ P −3 200 NJ P −3 200 NJ P −3 3000 NJ P −3 3200 NJ P −3 2300 NJ P 2500 NJ P 2700 NJ P 2900 NJ P 3100 NJ P 3300 NJ P −3 −3 −3 −3 −3 −3 2500 NJ P −3 6HGLPHQW−%DVHPHQW $QRPDO\ &RPELQHG $QRPDO\ %RXJXHU $QRPDO\ :DWHU6HGLPHQW $QRPDO\ %DVHPHQW ,QWHUIDFH )UHH$LU $QRPDO\ :DWHU6HGLPHQW ,QWHUIDFH Figure 8. Gravity reduction over Wreck seamount. (a) Bathymetry. (b) Combined basement-seafloor inter- face topographic anoma- ly (sediment density (ρ sed ) = 2000 kg m -3 ; seamount density (ρ sea ) = 2500 kg m -3 ). (c) Free-air anomaly. (d) Bouguer anomaly. (e) Gravity anomaly and to- pography comparison. (f) Coherence between Bouguer anomaly and bathymetry for range of ρ sea (2300 - 3300 kg m -3 ). Seamount Reduction Density (kg m -3 ) Mean of Error (kg m -3 ) Stradbroke 2760 12.5 Queensland & Britannia 2715 15.0 North & South Brisbane 3050 35.0 South Moreton 3135 30.0 North Moreton 2800 7.5 North & South Recorder 2805 37.5 South Fraser 2860 15.0 North Fraser 2825 60.0 Cato 2510 37.5 Wreck 2500 42.5 Table 1. Reduction density (i.e. seamount density, ρ sea , that minimises weighted average coherence between gravity and bathymetry) for each of the Tasmantids with associated mean of error. Coherence and associat- ed error are weighted using the power spectrum of the bathymetry. a b c d e f Gravity Modelling to Determine Lithospheric Strength • Models assume a finite-strength lithosphere and that the seamounts constitute loads, potentially associated with crustal roots and flexural moats. • Inclusion of large sediment loads and extensive intra-basement intrusion of mafic magmas is required to achieve acceptable fits to observed gravity anomalies. • There is no observable elastic thickness vs. age relationship along the chain. • Seamounts emplaced at inside corners, fracture zones and on highly-extended continental basement have the lowest elastic thicknesses suggesting that structural inheritance, not age, is the dominant control on lithospheric strength. −500 −400 −300 −200 −100 0 100 200 300 400 500 −40 −35 −30 −25 −20 −15 −10 −5 0 Distance (km) Depth (km) Water Depth Basement TTS Backstrip Moho Flexed Moho Seamount Infill Dense Core 2900 2700 3000 2900 2700 2000 2000 2000 1030 2500 2500 3300 −200 −100 0 100 200 Gravity Anomaly (mGal) Satellite FAA Modelled FAA Shipboard FAA Figure 9. (a) Te vs. age of crust at time of loading with predicted isotherms for half-space plate cooling model. Data are coloured by tectonic context. (b) Best-fitting crustal model for Stradbrokeseamount from process-ori- ented gravity and flexure modelling. Values indicate densities (kg m -3) ). TTS = Total Tectonic Subsidence. 0 10 20 30 Elastic Thickness (km) 0 10 20 30 40 50 60 Age Differential (t seafloor - t load (Ma)) 0 10 20 30 0 10 20 30 40 50 60 150° 300° 450° 600° 900° Off-Axis Fracture Zone Inside Corner Ocean-Continent Continental 6 a b 7 Conclusions 1. The modest rates of mass wasting revealed by slope analysis, combined with the prevalence of dense cores indicated by gravity signatures and lithospheric modelling, suggest that subsurface intrusion, rather than sub-aqueous eruption, was the dominant magmatic growth mechanism. 2. Low overall Te and the >20 Ma time separation between seamount emplacement and spreading cessation suggest deep intra-lithospheric faulting must have accompanied spreading in order to allow Tasmantid magmas to exploit and align with pre-existing structural weaknesses. 3. Slow rate of magma supply, as indicated by the dominance of tectonic controls, high intrusive:extrusive ratios and scarcity of large mass-wasting deposits, points to a relatively weak Tasmantid melting anomaly. 4. Structural inheritance dominates the magmatic evolution of the Tasmantids as demonstrated by: i) dependence of morphology on tectonic setting; ii) lack of a Te-age relationship and iii) strong alignment of volcanic features with principal stress directions predicted for the Tasman Sea ridge system. 5. The strong dependence of intraplate magmatic fabric on extinct ridge structure demonstrates the importance of understanding tectonic inheritance and its influence on magmatic systems in both continental and oceanic settings. References: Beaman, 2010, MTSRF Project 2.5i.1a Final Report, pp. 13 & Appendix 1; Behn et al., 2002, Geophys. Res. Lett., 29(24), 2207; Cohen et al., 2013, Tectonics, 32, pp.1371–1383. Contreras-Reyes et al., 2010, EPSL, 289(3-4), pp. 323-333. Hammer et al., 1990, Geophysics, 56(1), pp. 68–79. Kalnins et al., 2015, DI41A-2591, AGU Fall Meeting 2015. McDougall & Duncan (1988), Earth Plan. Sci. Lett., 89, pp. 207-220; Mitchell et al., 2002, Journ. Volc. & Geo. Res., 155(1-2), pp. 82–107; Müller et al., 2008, G 3 ,9, Q04006; Ramalho et al., 2013, Earth Sci. Rev., 127, pp.140–170; Watts & Zhong, 2000, Geophys. J. Int., 142, pp. 855-875.
Transcript of The Tasmantid Seamounts: A window into structural ... · The Tasmantid Seamounts: A window into...
The Tasmantid Seamounts: A window into structural inheritance of ocean floor fabricFred Richards1, 2*, Lara Kalnins3, Tony Watts1, Benjamin Cohen4 & Robin Beaman5
1Department of Earth Sciences, University of Oxford, Oxford, UK 2 Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Cambridge, UK 3Department of Earth Sciences, Durham University, UK 4Scottish Universities Environmental Research Centre, East Kilbride, UK 5College of Science, Technology and Engineering, James Cook University, Cairns, Australia *[email protected]
V21A-3034
Introduction The Tasmantid Seamounts extend for >2000 km off the east coast of Australia, constituting one of three contemporaneous, sub-parallel Cenozoic hotspot tracks that traverse the region, locally separated by ≤500 km (Cohen et al., 2013). The sea-mounts young north to south, spanning ~50 - 6 Ma (McDougall & Duncan 1988; Kalnins et al., 2015).
At multiple locations, the Tasmantid chain intersects the extinct slow-spreading Tasman Sea ridge system, which was active between 84 - 53 Ma (Muller et al., 2008). Despite the >20 Ma hiatus between spreading cessation and seamount emplacement, palaeo-ridge structure appears to exert significant control on sea-mount morphology and structure.
Geophysical datasets acquired on voyage ss2012_v07 have been analysed to answer the following:
1. What mechanism can account for the morphologi-cal diversity of the seamounts?
2. What is the relationship between pre-existing tec-tonic fabric, intraplate magmatism and the strength of the Tasman Sea lithosphere?
3. How has the strength of the Tasmantid melting anomaly evolved through time?
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Figure 1. Map of the study area. Dotted red line = shiptrack of voyage ss2012_v07. Star = position of Cenozoic seamounts and volcanoes. Solid red line = extinct ridge axis, solid black line = transform fault, dashed black = fracture zone. Bathymetry derived from GBR100 dataset (Beaman 2010).
1 Volcanic Architecture• Tasmantid morphologies fall
into four distinct categories: a) conical seamounts char-acterised by steep, smooth flanks; b) elongate and stel-late terraced seamounts with subaerially eroded peaks; c) rugged seamounts con-structed via repeated fissure eruptions along crosscutting volcanic rift zones (VRZs) and d) shield seamounts with shallow slopes and dispersed cinder cones.
• The chain exhibits low mass wasting rates and fluctuating edifice volume, with dramat-ic morphological variation between seamounts separated by ≤10 km.
• There is a clear link between overall morphology and sea-mount position relative to the extinct spreading centre.
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3 identifiable summit benches
Hummocky debris deposits with pelagic
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Figure 2. (a) N Fraser: conical seamount emplaced off-axis, adjacent to the ocean-con-tinent boundary (OCB). (b) N & S Recorder: both seamounts have terraced upper slopes. N Recorder is off-axis and stellate; S Recorder is emplaced on a fracture zone and is elongated. (c) Stradbroke: rugged seamount straddling an inside corner. (d) Cato: shield seamount located on the Lord Howe Rise microcontinent. Vertical exaggeration is x4.5. N.B. Bathymetry colour scales are different for each seamount.
Figure 3. Average slope gradient by sector. (a) South Moreton, a con-ical seamount. (b) Cato, a shield seamount. (c) Britannia, a terraced seamount: (i) basal and (ii) upper, subaerially-eroded slopes. (d) Strad-broke, a rugged seamount.
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Slope Analysis• Tectonic setting exerts major control on slope characteristics: conical seamounts with elevated slope gradients but lower inter-sector variance occur off-axis and
at outside corners; rugged seamounts with low slope gradient but high inter-sector variability occur at inside corners. • Terraced and shield seamounts have notably variable, low gradient upper slopes suggesting elevated rates of mass wasting due to sub-aerial exposure.• The relatively low slope variance and elevated backscatter on the lower sections of the edifices indicate that large mass-wasting events are rare. This is consistent
with minimal shallow deformation and may reflect modest eruption rate with a high intrusive-to-extrusive magmatic budget (Ramalho et al., 2013).
Figure 4. Detecting mass wasting. (a) SE-ward view of S Moreton, red dashes = mass wasting deposit with reduced mean slope gradient (b) and low backscatter (e). Slope gradient is fairly constant (c) so bathymetric curvature (e) is ~ 0. The run-out distance:headscar aspect ratio suggests a debris avalanche mechanism (Mitchell et al., 2002).
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Structural Orientations4
Figure 7. Effect of cross-transform mechanical coupling on principal stress orientations (modified after Behn et al., 2002). Focal mechanisms show optimal, calculated fault plane solutions at 4 km depth for differing degrees of mechanical coupling. χ = mechanical coupling, OC = inside cor-ner, IC = outside corner, red = ridge axis, black = transform fault. τdev/σdev+lith is the ratio of shear to normal stress on the fault plane incorporating both deviatoric and lithostatic components. (a) χ = 0, (b) χ=0.05, (c) χ = 0.1 & (d) χ = 0.15. Horizontal displacement = 100m (10 ka at half-spreading rate of 1 cm a-1).
Figure 5. Linear feature orientation vs. tectonic setting. (a) Stradbroke: (i) trends oblique-to-spreading; (ii) inside-corner setting. (b) S & N Recorder: (i) S Recorder: volcanic trends parallel to Recorder Fracture Zone and (iii) fracture zone setting (ii) N Re-corder has ridge-parallel features and (iii) off-axis setting. (c) N Fraser: (i) trends ridge-parallel or subparallel to fracture zones; (ii) ocean-continent boundary setting. Red = major VRZs, blue = minor VRZs and black = faults. IC = inside corner, OC = outside corner.
• Alignments suggest deep faulting of the oceanic lithosphere allowing channelisation of magma along pre-existing structures despite emplacement post-dating active extension by >20 Ma. • Dominance of the tectonic signal points to low melt production, implying that the Tasmantid “plume” was a relatively weak melting anomaly.• Seamounts located at or adjacent to ridge-transform intersections have dominant trends oblique to spreading, implying strong mechanical coupling across the transforms.• The high degree of mechanical coupling is consistent with slow Tasman Sea spreading rates and low rates of melt production.
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Figure 6. Linear feature orientations of all seamounts. (a) Axial ridges (red) and fracture zones (black). (b) Off-axis sea-mounts. (c) Fracture zone seamounts. (d) Inside corner seamounts. (e) Outside corner seamounts. (f) Continental seamounts.
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Deep Structure • Bouguer anomalies are calculated by removing the combined gravity effect of the water-sediment and
and sediment-seamount interfaces to investigate gravity signals related to deeper density interfaces.• Calculations assume that seamount topography is uncompensated, i.e. the lithosphere is infinitely rigid.• The reduction density of a seamount refers to the input seamount density that minimises spectral
coherence between calculated Bouguer anomalies and the bathymetry.• 20-50 mGal Bouguer highs over many seamounts, coupled with reduction densities up to ~3100 kg m-3,
suggest extensive intra-basement intrusion of primary magmas (cf. Contreras-Reyes et al., 2010).• Low reduction densities over Cato and Wreck are consistent with both seamounts being emplaced on
continental basement and significant fractionation of parent magmas (Hammer et al. 1990).
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Figure 8. Gravity reduction over Wreck seamount. (a) Bathymetry. (b) Combined basement-seafloor inter-face topographic anoma-ly (sediment density (ρsed) = 2000 kg m-3; seamount density (ρsea) = 2500 kg m-3). (c) Free-air anomaly. (d) Bouguer anomaly. (e) Gravity anomaly and to-pography comparison. (f) Coherence between Bouguer anomaly and bathymetry for range of ρsea (2300 - 3300 kg m-3).
Seamount Reduction Density (kg m-3)
Mean of Error(kg m-3)
Stradbroke 2760 12.5
Queensland& Britannia 2715 15.0
North & South Brisbane 3050 35.0
South Moreton 3135 30.0
North Moreton 2800 7.5
North & SouthRecorder 2805 37.5
South Fraser 2860 15.0
North Fraser 2825 60.0
Cato 2510 37.5
Wreck 2500 42.5
Table 1. Reduction density (i.e. seamount density, ρsea, that minimises weighted average coherence between gravity and bathymetry) for each of the Tasmantids with associated mean of error. Coherence and associat-ed error are weighted using the power spectrum of the bathymetry.
a b
c d
e f
Gravity Modelling to Determine Lithospheric Strength• Models assume a finite-strength lithosphere and that the seamounts constitute loads, potentially associated with crustal roots and flexural moats. • Inclusion of large sediment loads and extensive intra-basement intrusion of mafic magmas is required to achieve acceptable fits to observed gravity anomalies.• There is no observable elastic thickness vs. age relationship along the chain.• Seamounts emplaced at inside corners, fracture zones and on highly-extended continental basement have the lowest elastic thicknesses suggesting that structural
inheritance, not age, is the dominant control on lithospheric strength.
−500 −400 −300 −200 −100 0 100 200 300 400 500−40
−35
−30
−25
−20
−15
−10
−5
0
Distance (km)
Dep
th (k
m)
Water DepthBasementTTSBackstrip MohoFlexed Moho
Seamount InfillDense Core
2900
2700
3000
2900 2700
2000
2000
2000
1030
2500 2500
3300
−200
−100
0
100
200
Gra
vity
Ano
mal
y (m
Gal
)
Satellite FAAModelled FAAShipboard FAA
Figure 9. (a) Te vs. age of crust at time of loading with predicted isotherms for half-space plate cooling model. Data are coloured by tectonic context. (b) Best-fitting crustal model for Stradbrokeseamount from process-ori-ented gravity and flexure modelling. Values indicate densities (kg m-3)). TTS = Total Tectonic Subsidence.
0
10
20
30
Ela
stic T
hic
kness (
km
)
0 10 20 30 40 50 60
Age Differential (tseafloor − tload (Ma))
0
10
20
30
0 10 20 30 40 50 60
150°
300°
450°
600°
900°
Off−Axis
Fracture Zone
Inside Corner
Ocean−Continent
Continental
6
a b
7 Conclusions1. The modest rates of mass wasting revealed by slope analysis, combined with the prevalence of dense cores indicated by gravity signatures and lithospheric
modelling, suggest that subsurface intrusion, rather than sub-aqueous eruption, was the dominant magmatic growth mechanism.
2. Low overall Te and the >20 Ma time separation between seamount emplacement and spreading cessation suggest deep intra-lithospheric faulting must have accompanied spreading in order to allow Tasmantid magmas to exploit and align with pre-existing structural weaknesses.
3. Slow rate of magma supply, as indicated by the dominance of tectonic controls, high intrusive:extrusive ratios and scarcity of large mass-wasting deposits, points to a relatively weak Tasmantid melting anomaly.
4. Structural inheritance dominates the magmatic evolution of the Tasmantids as demonstrated by: i) dependence of morphology on tectonic setting; ii) lack of a Te-age relationship and iii) strong alignment of volcanic features with principal stress directions predicted for the Tasman Sea ridge system.
5. The strong dependence of intraplate magmatic fabric on extinct ridge structure demonstrates the importance of understanding tectonic inheritance and its influence on magmatic systems in both continental and oceanic settings.
References: Beaman, 2010, MTSRF Project 2.5i.1a Final Report, pp. 13 & Appendix 1; Behn et al., 2002, Geophys. Res. Lett., 29(24), 2207; Cohen et al., 2013, Tectonics, 32, pp.1371–1383. Contreras-Reyes et al., 2010, EPSL, 289(3-4), pp. 323-333. Hammer et al., 1990, Geophysics, 56(1), pp. 68–79. Kalnins et al., 2015, DI41A-2591, AGU Fall Meeting 2015. McDougall & Duncan (1988), Earth Plan. Sci. Lett., 89, pp. 207-220; Mitchell et al., 2002, Journ. Volc. & Geo. Res., 155(1-2), pp. 82–107; Müller et al., 2008, G3,9, Q04006; Ramalho et al., 2013, Earth Sci. Rev., 127, pp.140–170; Watts & Zhong, 2000, Geophys. J. Int., 142, pp. 855-875.
