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 Positive gravity anomalies to identify crustal densification Cratons and deep mantle control of tectonic activity 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!