1. Geological implications from complete Gondwana GOCE-products reconstructions
Carla Braitenberg
Department of Mathematics and Geosciences
University of Trieste (Italy)

2. Contents Continent wide Earth exploration with GOCE gravity
Positive gravity anomalies to identify crustal densification
Cratons and deep mantle control of tectonic activity
Gondwana reconstructions of mantle tomography and GOCE gravity

3. Data and tools GOCE gravity and gradients – TIM R5 (Pail et al 2011)
GUT: GOCE User Toolbox
Global topo-reduction available in SH (e.g. Hirt et al., 2012)
Seismic tomography of the mantle (Simmons et al., 2012).
Already demonstrated:
Resolution and precision of GOCE adequate for geologic lineaments (Alvarez et al.,2014 Tectonophysics; Braitenberg et al., 2014 IAG Symposia; 2014 J. Applied Geoinformatics)

4. Geologic units and gravity Identify Productive lineaments
Geological Map
GOCE Bouguer map
Braitenberg, 2014

5. Crust densification - gravity highs
Biunivocal relation between crustal densification and positive gravity anomaly
Crustal scale geodynamic event lead to increased density:
Magmatism Metamorphism
Crustal scale positive gravity anomaly observed

6. Example
Crustal scale geodynamic event (e.g. Ediacaran 2500km extension) with Ultra High Pressure Metamorphism is claimed from interpolation of single rock samples
No gravity signal observed: interpolation must be questioned
Gravity signal is observed:
Interpolation strongly supported
Ganade de Araujo et al., 2014, Nature Comunications

7. Cratons are stable platforms
Oldest basement with ages between Archean ( Ga) and Mesoproterozoic ( Ga) define Cratons
Identify craton: age > 1Ga. Not affected by younger tectonic events.
Underlain by relatively high velocity to great depths (300km)
Flexure studies: high flexural rigidity
Density: probably decreased with respect to younger lithosphere (Artemieva, 2009)

8. Basement age of the continental crust
Mooney, 2009.

9. Increased velocity
stacked and averaged layers 185km-355km

11. Hypothesis testing
Gravity or gradient from GOCE has been demonstrated to distinguish crustal scale events that produced magmatic alteration or metamorphism
Cratons stable, least effected by events
-> gravity and gradient should positively correlate with craton outlines
-> gravity should anticorrelate with mantle velocities
-> gravity should be subdued over cratons

12. Gondwana reconstruction
Assemble cratonic units to form Gondwana
Rotations based on paleomagnetic observations on continent, ocean spreading reconstructions
We refer to models of Torsvik et al., 2009
Hypothesis: continental fragments can be rotated together with mantle to a depth of 350 km. This is certainly valid for cratons.
Rotate gravity field and seismic tomography models

13. Gondwana assemblement Vp variations averaged 185 to 355km depth

14. GOCE FA residual with Vp 185 to 355km depth
Isolines: 0.8 B, 1 Y, 1.2 Gr, 1.3 W, 1.6 B % average variation Vp

15. GOCE FA Res variations with deep Vp anomalies
Isolines: 0.5 Br, 1.2 Y, 1.3 W % average variation Vp

16. GOCE FA Res variations with deep Vp anomalies
Isolines: 0.5 Br, 1 Y, 1.2 W, 2 Gr % average variation Vp

17. GOCE FA Res variations with deep Vp anomalies
Isolines: 1.2 Y, 1.3 W, 1.6 Gr % average variation Vp

18. Observational Results 1/2
Cratons defined from rock age maps correlate to increased velocity up to 355 km or more. Average increase up to 2% lateral increase with respect to depth average
Outlines of cratons defined by increased velocity, e.g. 0.5% outline, downscale gravity values largely.
North America: negative gravity modelled from Vs variations (Forte et al., 2010)

19. Observational Result 2/2
Absence of high velocity root coincides with increased gravity, defined by 0.5% isoline, partially
SAM-AFR conjugate passive margins with magmatism or underplating
Andean orogen
Exception to rule: Zimbabwe Greenstone belt – gravity high crosses high velocity root

20. Conjecture on magmatic events
Border of high velocity lithosphere roots appear as gravity highs due to metamorphic events and magmatism
Cratons with lithosphere roots have little large scale positive gravity lineaments due to stability through Earth history
Exception: event that is older than craton consolidation.
Older events, unique to early Earth evolution
Greenstone belts

21. Discussion
It seems: Cratons focus deformation, magmatism, subduction, rifting, along the borders
Possible cause: mantle flow- cratonic roots channeling (Tirone et al., 2009,G3)
Could be first order physical process conditioning crustal evolution.

22. Conclusion
Correlation found: often the positive gravity variations positioned outboard of deep lithosphere roots
Cratons have preferentially low gravity
Before GOCE this study was not possible due to lack of resolution and precision of global fields.
Possibly demonstrates conditioning of mantle flow through cratonic root.

23. Thank you for the attention!