Mineral transformation, development of layering of weak minerals Grain size reduction accommodated by grain boundary sliding Quantifier la chute de viscosité dans les zones déformées? Gueydan et al., 2014
and ed quantitative descrip- (TMoho > 650–700 °C). In intraplate deformation zones, the contr tion of crust and mantle weakening are both important. Thus, niﬁcant strain rate variations can be expected intraplate deforma spheric strength. A–D: Yield stress proﬁles computed for a medium continental geotherm (TM = 600 °C) and two bulk s itance (TI) weakening for a reference crust-mantle rheology (B). Cf. Sections 3.1 and 3.3 for rheology laws of the weak/s ch yield stress proﬁle (A–D). Quantification la chute de viscosité dans les zones déformées Mazzotti & Gueydan., 2018
weakening factors such as low fault friction or mantle viscosity are frequently proposed to explain the observed strain and seismicity concentrations, although with limited quantitative descrip- tion of their eﬀects (Baird et al., 2009; Kenner and Segall, 2000; Wu and Mazzotti, 2007). 4.2. Tectonic inheritance impact on strain rates Despite their narrow application range, the strain-weakening me- chanisms share two fundamental characteristics: They are irreversible (i.e., no annealing) and they result in signiﬁcant lithosphere strength reduction (and thus strain rate increase). This eﬀect is illustrated in Fig. 5 with strain-rate predictions for the reference (no weakening) and inherited strain-weakening models. For a given geotherm, inherited weakening in the upper – mid crust and upper mantle results in an increase of the predicted strain by two to four orders of magnitude. The eﬀect of tectonic inheritance allows our model predictions to reach empirical strain estimations for an average net force of 3 × 1012 N m−1. This is particularly signiﬁcant for intraplate de- formation zones in SCR, where strain rates of the order of 10−11–10− 9 a− 1 are predicted for cold geotherms (TMoho = 500–600 °C), thus providing a general mechanism for strain concentration in intraplate regions aﬀected by old tectonics and sig- niﬁcant inherited deformation (paleo-PBZ). In contrast, model strain (TMoho > 650–700 °C). In intraplate deformation zones, the cont tion of crust and mantle weakening are both important. Thus niﬁcant strain rate variations can be expected intraplate deform zones, depending on their spatial relationships to the paleo-tec crustal and mantle structures (Fig. 2). To ﬁrst-order, our results ind that the zones of highest present-day deformation and seismicity l require either weakening of the whole lithosphere at an averag sultant force, or a combination of local force concentr (FT ≥ 6 × 1012 N·m) and weakening limited to the crust or m (Fig. 5). 5. Discussion 5.1. Strain rate sensitivity to model parameters Although based on simple assumptions, our 1D model provid useful quantitative description of how various mechanisms aﬀect tinental strain rates. Within the framework of this model, this allow to categorize these mechanisms in terms of eﬀective strength decr strain-rate increase. The highest impact on strain rates is assoc with inherited strain-weakening and geotherm variation (chang Moho temperature ca. 100 °C), which both result in strain rate v tions of at least 2–3 orders of magnitude (Fig. 5). In contrast, composition and rheology variations lead to strain rate variatio Fig. 3. Examples of yield stress proﬁles and integrated lithospheric strength. A–D: Yield stress proﬁles computed for a medium continental geotherm (TM = 600 °C) and two bulk rates of 10–15 s-1 and 10–17 s-1. D: Impact of tectonic inheritance (TI) weakening for a reference crust-mantle rheology (B). Cf. Sections 3.1 and 3.3 for rheology laws of the weak/ crust and mantle. E: Integrated lithospheric strength for each yield stress proﬁle (A–D). 5.2. Model limitations The model presented here is limited by two main assumptions: 1D nature and near-failure equilibrium of the whole lithosphere column. The former precludes the application of our model predictions to spe- ciﬁc cases such as geometrical relationship between tectonic stress, (e.g., Baird, 2010; M their complex inter merical models. The assumption column implies the and may not apply Mazzotti & Gueydan., 2018
Schematic forces and tectonic inheritance in a paleo-graben system. 1D litho- sphere columns with diﬀerent levels of tectonic inheritance and strain weakening are subjected to a net tectonic force (FT). Additional forcing processes such as erosion (FE), glacial isostatic adjustment (FGIA), crustal density contrasts (Δρ) are not considered di- rectly can modify the local resultant force acting on the lithosphere. Solid and dashed lines represent inherited faults and shear zones, respectively. Gueydan & Précigout, Tectonophysics, 2014
weakening d Mantle weakening e Entire crust & mantle weakening 0 125 250 Strength (MPa) Analytic Model Strain rates (s-1) 0 1.4x10-16 2.8x10-16 Bulk Model b Brittle crust weakening c Entire crust weakening d Mantle weakening e Entire crust & mantle weakening Analytic Model Bulk Mod Numerical modelling: PhD of Alizia Taryoun (with S. Mazzotti) b Brittle crust weakening c Entire crust weakening d Mantle weakening e Entire crust & mantle weakening Analytic Model Bulk Model Quantification la déformation actuelle dans les zones d’heritage
zone de 10-30 km de large Localement def. Finie. > 5-6 Rift continentale Déformation sur une zone de 30-50 km de large Def. Finie limitée (1-2) Collision/Subduction Déformation sur une zone de >50 km de large Croute uniquement ou manteau impliquée?