Transcript Geological implications from complete

Geological implications from
complete Gondwana GOCE-products
reconstructions
Carla Braitenberg
Department of Mathematics and Geosciences
University of Trieste (Italy)
Contents
Continent wide Earth exploration with GOCE gravity
1) Positive gravity anomalies to identify crustal
densification
2) Cratons and deep mantle control of tectonic
activity
3) Gondwana reconstructions of mantle
tomography and GOCE gravity
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)
Geologic units and gravity
Identify Productive lineaments
Geological Map
GOCE Bouguer map
Braitenberg, 2014
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
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
Cratons are stable platforms
• Oldest basement with ages between Archean
(3.8-2.5 Ga) and Mesoproterozoic (1.6-1.0Ga)
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)
Basement age of the continental crust
Mooney, 2009.
Increased velocity
stacked and averaged layers 185km-355km
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
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
Gondwana assemblement
Vp variations averaged 185 to 355km depth
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
GOCE FA Res variations with
deep Vp anomalies
Isolines: 0.5 Br, 1.2 Y, 1.3 W % average variation Vp
GOCE FA Res variations with
deep Vp anomalies
Isolines: 0.5 Br, 1 Y, 1.2 W, 2 Gr % average variation Vp
GOCE FA Res variations with
deep Vp anomalies
Isolines: 1.2 Y, 1.3 W, 1.6 Gr % average variation Vp
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)
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
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
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.
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.
Thank you for the attention!