Analog modeling of one-way gravitational spreading of hot orogens: A case study from the Svecofennian orogen, Fennoscandian Shield

Geodynamic models offer insight into deformation styles of syn- and post-collisional extensional pro-cesses. Such models often ignore the effect of lateral viscosity contrast between accreted terranes andinherited tectonic boundaries. This paper presents a set of crustal-scale analog experiments of one-waypost-collisional gravitational spreading testing the effect of crustal heterogeneity by using two mechani-cally contrasting blocks, pre-existing cuts with varying dips, and three layers with contrasting rheologies.Two blocks represent strong, mature, and weak, juvenile crustal parts. Both blocks have three horizontalcrustal layers: brittle upper, weak partially molten middle and dry strong lower layer. Cuts representpre-existing crustal-scale shear zones and tectonic boundaries.

The experiments indicated that tectonic boundaries may facilitate exhumation and increase exten-sional strain, when the boundaries dip opposite to the flow direction. The tectonic boundaries may alsosplit and divide the crust into separate blocks with independent deformation signatures: shortening, elongation or rotation. The tectonic boundaries rotated along vertical axes toward the flow direction. Themodeling results suggest that in areas with inherited tectonic boundaries and compositional differencesthe amount of extension is unevenly distributed between the different parts during the widespread uni-lateral gravitational spreading and that the flow has different effects on the contrasting crustal units, inboth lateral and vertical directions.

To validate the modeling, the results are compared to geophysical and geological data from the Paleo-proterozoic Svecofennian orogen in the Fennoscandian Shield. The comparison suggests that the orogenunderwent 50% extension and was thinned by maximum of 20% via westward gravitational spread-ing. This spreading rotated the crustal-scale blocks, shear zones and tectonic contacts. It also broughtexhumed, high grade middle crustal blocks into contact with subsided, low grade, upper crustal blocks.

We suggest that results of the analog models can be used as proxies in both ancient and modern hotaccreted orogens, which have undergone post-convergence continental mid-crustal weakening and havecomprised of juxtaposed terranes with varying rheological compositions.