AbstractsEarth & Environmental Science

Numerical Modelling of Mid-Crustal Flow Applied to Svecofennian Orogeny

by Hannu Lammi

Institution: University of Helsinki
Year: 2015
Keywords: Geologia
Record ID: 1145585
Full text PDF: http://hdl.handle.net/10138/154825


This work explores the lateral spreading of hot, thick, Paleoproterozoic crust via a series of 2D thermomechanical numerical models based on two geometrical a priori models of the thickened crust: plateau and plateau margin. High Paleoproterozoic radiogenic heat production is assumed. The material viscosity is temperature-dependent following the Arrhenius law. The experiments use two sets of rheological parameters for the crust: dry (granite/felsic granulite/mafic granulite) and wet (granite/diorite/mafic granulite). The results of the modeling are compared to seismic reflection sections and surface geological observations from the Paleoproterozoic Svecofennian orogen. Numerical modelling is performed with Ellipsis, a particle-in-cell finite element code suitable for 2D thermo-mechanical modelling of lithospheric deformation. It uses Lagrangian particles for tracking material interfaces and histories, which allow recording of material P-T-t paths. Plateau-models are based on a 480 km long section of 65 km-thick three-layer plateau crust. In the plateau margin-models, a transition from 65 km thick plateau to 40 km thick foreland is imposed in the middle of the model. The models are extended symmetrically from both ends with slow (1.9 mm/a) and fast (19 mm/a) velocities. Gravitational collapse is simulated with an additional set of fixed boundary plateau margin models. The models are studying the effect of free moving boundaries on the crustal structure and the conditions for mid-crustal flow. Strong mid-crustal channel flow is seen in plateau margin models with dry rheology and slow extension or with fixed boundaries. With fast extension or wet rheology channel flow grows weaker/diminishes. In models with slow extension or fixed boundaries, partial melting controls the style of deformation in the middle crust. Vertical movement of the partially molten material destroys lateral flow structures in plateau regions. According to P-T-t paths, the model materials do not experience high enough temperatures to match HT-LP metamorphic conditions typical for Svecofennian orogenic rocks. Metamorphic conditions in the dry rheology models have counterparts in the LT-LP (>650 °C at ≤600 MPa) amphibolite facies rocks of the Pielavesi area. Plateau margin models with dry rheology and slow extension or fixed boundaries developed mid-crustal channel flow, thinning of middle crust, exhumation of mid-crustal domes and smooth Moho, all of which are found in crustal scale reflection sections. Results of this work suggest plateau margin architecture prior to extension that took place at slow velocities or through purely gravitational collapse, although peak temperature of Svecofennian HT-LP metamorphism was not attained. Tässä työssä tutkitaan myöhäisproterotsooisen paksun ja kuuman kuoren leviämistä kymmenen numeerisen mallin sarjalla. Mallit perustuvat paksuuntuneen kuoren kahteen perusgeometriaan: ylänkö ja ylängön reuna (ylänkö-etumaa). Suureen radiogeeniseen lämmöntuottoon käytetään myöhäisproterotsooiselle tasolle korjattuja…