Dynamic Component of Pressure during Metamorphism in a Thrust Zone

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Abstract

In the southeastern fragment of the Raahe-Ladoga suture zone in Russia, within the Meyeri tectonic zone, increased pressures (“overpressure”) were revealed, caused by structural-metamorphic transformations of rocks during collisional interaction of allochthonous and autochthonous blocks. It is assumed that tectonic interaction of the rigid crustal block of the Archean basement of the Karelian craton (autochthon) and the Proterozoic granulite block of the Svecofennian belt (allochthon) controls the conditions for the formation of superlithostatic pressure anomalies. Methods of mineral geobarometry and numerical thermomechanical modeling in the rocks of the thrust zone recorded pressures up to 9–11 kbar, while lithostatic pressure not exceeding 4–6 kbar. The obtained results allow us to consider that the nature of the local superlithostatic pressure up to 7–9 kbar, established by mineral geobarometers and numerical thermomechanical modeling, can be explained by the tectonic interaction of blocks with heterogeneous physical and mechanical properties, and not reflect the error of the applied mineral geobarometry instruments.

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About the authors

Sh. K. Baltybaev

Institute of Precambrian Geology and Geochronology, the Russian Academy of Sciences; St. Petersburg State University, Institute of the Earth Sciences

Author for correspondence.
Email: shauket@mail.ru
Russian Federation, St. Petersburg; St. Petersburg

E. S. Vivdich

Institute of Precambrian Geology and Geochronology, the Russian Academy of Sciences; St. Petersburg State University, Institute of the Earth Sciences

Email: shauket@mail.ru
Russian Federation, St. Petersburg; St. Petersburg

O. P. Polyansky

V. S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences

Email: shauket@mail.ru
Russian Federation, Novosibirsk

V. G. Sverdlova

V. S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences

Email: shauket@mail.ru
Russian Federation, Novosibirsk

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Supplementary files

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2. Fig. 1. Schematic map of the study area showing the metamorphic zoning of the Northern Priladozhye. 1 - Archean rocks of the Karelian Craton; 2 - a protrusion of the Archean basement in the cores of the bordered gneiss domes (1 - Sortavala, 2 - Kiryavolakhtinsky, 3 - Kokkasselsky, 4 - Impilakhtinsky, 5 - Mursulsky, 6 - Pitkyaranta, 7 - Inivara, 8 - Khavussky); 3 - metavolcanics of the Sortavala series; 4-7 - Lower Proterozoic volcanogenic-sedimentary cover of the Ladoga series with metamorphic parageneses of biotite-chlorite (4), staurolite, andalusite (5) and sillimanite-muscovite (6) schists, sillimanite-orthoclase (7) gneisses; 8 – Early Proterozoic rocks of the ultrametamorphic and granitization zone: Priozersk zone (a) and Lakhdenpohja zone (b); 9 – rapakivi granites; 10 – enderbites and unexposed metabasites; 11 – main fault plane and conventional northern and southern boundaries of the Meyer tectonic zone; 12 – tectonic faults; 13 – boundaries between: rocks of different metamorphic zones (a) and rock series (b). Inset: position of Svecofennides in the structures of the region along the AB section line. 1 – Archean rocks of the Fennoscandian Shield (intracratonic Proterozoic structures are not shown to simplify the diagram), 2 – Svecofennides, 3 – Dalslanides, 4 – Caledonides, 5 – Riphean cover, 6 – Raahe-Ladoga suture zone, 7 – study area. The locations and numbers of the studied samples of metapelites (circles) and metabasites (squares) are indicated by symbols of different shapes.

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3. Fig. 2. Photographs of thin sections of metamorphic rocks of the Meyer tectonic zone, taken at parallel (a, c, d, g) and crossed (b, d, e, h) nicols. Samples of metapelites (sample B-20-417 (a, b), sample B-20-466 (c, d)) and metabasites (sample 5284a (d, e), sample B-22-613 (g, h)) are presented.

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4. Fig. 3. BSE images of garnet porphyroblasts from samples of metabasite B-22-526 (a) and metapelite B-20-466 (c) with the corresponding distribution profiles of the end-member contents in them (b, d).

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5. Fig. 4. Compositions of Ca-amphiboles from samples of metabasites of the Meyer tectonic zone on the classification diagram according to (Hawthorne et al., 2012).

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6. Fig. 5. P–T diagrams for samples of garnet-biotite gneisses of the Meyer tectonic zone. The numbers mark the reactions given in Supplementary 2, ESM_2. The diagrams show the sample numbers: B-20-448 (a), B-20-466 (b), the locations of which are indicated in Fig. 1.

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7. Fig. 6. P–T diagrams for the paragenesis Grt + Amp + Bt + Pl + Qz from metabasite samples B-22-526 (a) and B-22-613 (b). The numbers mark the reactions given in Supplementary 2, ESM_2. The location of the samples is shown in Fig. 1.

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8. Fig. 7. P–T parameters calculated in the THERMOCALC program for the paragenesis Grt + Amp + Bt + Pl + Qz from metabasite samples B-22-526 (a) and B-22-613 (b). The numbers indicate the reactions given in Supplementary 2, ESM_2. The location of the samples is shown in Fig. 1.

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9. Fig. 8. BSE photograph of the metabasite sample B-22-613 (a) and an enlarged fragment (b) with contacting garnet and amphibole, for which the highest pressure is recorded according to the garnet-hornblende-plagioclase thermobarometer (Dale et al., 2000).

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10. Fig. 9. Results of computer modeling of MTZ metapelites. The red line indicates the granite minimum for metapelites under the action of carbon dioxide-water fluid: at XCO2 = 0 (dashed line) and XCO2 = 0.3 (solid line).

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11. Fig. 10. Results of computer modeling of the mineral composition of metapelite samples B-20-448 (a–c) and B-20-466 (d–e). (a, d) – stability fields of mineral parageneses calculated in the PERPLE_X program, highlighting the region of existence of mineral paragenesis in samples B-20-448 and B-20-466, respectively; (b, d) – detailing the region with the required paragenesis and isomodes (solid lines) reflecting the content (in vol. %) of garnet (red lines), biotite (brown lines), plagioclase (blue lines), quartz (light blue lines); (c, e) – diagrams with isopleths (dashed lines) corresponding to the content (in fractions of a unit) of almandine (red lines), pyrope (purple lines), grossular (green lines), anorthite (blue lines) and annite (brown lines). The “+” and “–” signs indicate the presence or absence of the mineral phase indicated by the arrow in the pseudo-section. The diagram excludes fields containing albite (Ab), paragonite (Pg), and does not show phases whose content does not exceed 1 vol.%. The following pseudo-sections are designated by numbers in circles: 1 – Bt + Pl + Kfs + M(melt) + Grt + Ky + Qz + Rt; 2 – Bt + Pl ± Kfs + M(melt) + Grt + Crd + Sil + Qz; 3 – Bt + Pl ± Kfs + M(melt) + Grt + Crd + Qz.

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12. Fig. 11. Results of computer modeling of mineral compositions of metabasite samples 6066k (a–c) and B-20-450 (d–e). (a, d) – stability fields of mineral parageneses calculated in the PERPLE_X program, highlighting the region of existence of mineral paragenesis in samples 6066k and B-20-450, respectively; (b, d) – detailing of the region with the required paragenesis and isomodes (solid lines) reflecting the content (in vol. %) of clinopyroxene (gray lines), hornblende (orange lines), garnet (red lines), plagioclase (blue lines), biotite (brown lines); (c, e) – diagrams with isopleths (dashed lines) corresponding to the content (in fractions of a unit) of almandine (red lines), pyrope (purple lines), grossular (green lines), anorthite (blue lines) and annite (brown lines), diopside (gray lines) and Ca²⁺ (a.f.u.) in amphibole (orange lines). The “+” and “–” signs show the presence or absence of the mineral phase indicated by the arrow in the pseudo-section. The diagram excludes fields containing albite (Ab), paragonite (Pg), and phases whose content does not exceed 1 vol.% are not shown. The following pseudo-sections are designated by numbers: (a) 1 – Grt + Bt + Pl + Amp + Qz + Rt + Dol + Ank; 2 – Grt + Bt + Pl + Amp + Qz + Cal + Dol + Ank; 3 – Grt + Bt + Pl + Amp + Qz + Rt + Cal + Dol + Ank; 4 – Grt + Bt + Pl + Amp + Qz + Rt + Ilm + Dol + Ank; 5 – Grt + Bt + Pl + Amp + Qz + Cal + Dol; 6 – Grt + Bt + Pl + Amp + Qz + Ilm + Dol + Ank; 7 – Grt + Bt + Pl + Amp + Qz + Ilm + Cal + Dol ± Ank; 8 – Grt + Bt + Pl + Amp + Qz + Rt + Cal + Dol. (d) 1 – Grt + Bt + Pl + Ms + Qz + Dol + Ank; 2 – Grt + Bt + Pl + Ms + Amp + + Qz + Sd + Ank; 3 – Grt + Bt + Pl + Ms + Chl + Qz + Sd + Ank; 4 – Grt + Bt + Pl + Amp + Qz + Sd + Ank; 5 – Grt + Bt + Pl + Amp + Chl + Qz + Sd + Ank.

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13. Fig. 12. Pseudo-section diagrams for a given composition of metabasite sample B-22-613. (a, b) µAl₂O₃–µK₂O; (c, d) µFeO–µK₂O; (d, f) µCaO–µK₂O; (g, h) µMgO–µK₂O. (a, c, d, g) – diagrams with isomodes (solid lines) showing the content (in vol. %) of rock-forming minerals: amphibole (orange lines), garnet (red lines), plagioclase (blue lines), quartz (light blue lines); (b, d, f, h) – diagrams with isopleths (dashed lines) corresponding to the content (in fractions of units) of almandine (red lines), pyrope (purple lines), grossular (green lines) and Ca²⁺ (a.f.u.) in amphibole (orange lines). The signs “+” and “–” show the presence or absence of the mineral phase indicated by the arrow in the pseudo-section. The modeling was performed at fixed values of temperature, pressure and mole fraction of carbon dioxide in the fluid: T = 700°C, P = 7.3 kbar, XCO₂ = 0.3. Initial composition of the rock, in wt. %: SiO₂ 49.55, TiO₂ 1.04, Al₂O₃ 18.07, Fe₂O₃t 13.55, MnO 0.22, MgO 6.2, CaO 5.89, Na₂O < 0.1, K₂O 3.12, P₂O₅ 0.32. Pseudosections with suitable paragenesis are highlighted in gray.

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14. Fig. 13. Graphical representation of the rheological law of dislocation creep for the materials used in the model: (a) – dependence of stress on the strain rate at a fixed temperature of 700°C; (b) – dependence of stress on temperature at a fixed strain rate of 1.e-15 (1/s). The line numbers correspond to the properties: 1 – PRL, wet quartzite (Kronenberg, Tullis, 1984), 2 – PRS, gabbro/amphibolite (Zhou et al., 2012), 3 – AR, dry granite (Ranalli, 1995), 4 – PRLh, anhydrous granulite (Leloup et al., 1999), 5 – mc, mafic crust (Perchuk et al., 2018).

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15. Fig. 14. Model structure of the crust of the Meyer tectonic zone and the geometry of the computational domain along the AB section line (Fig. 1). PRL – Ladoga series; PRS, – Sortavala series; AR – Archean complex; PRLh – Lahdenpohja series; mc – mafic crust beneath the Svecofennides. V – allochthon movement velocity (Southern domain) at a fixed autochthon (Northern domain). Other boundary conditions are given in the text. The position of the metavolcanics of the Sortavala PRS series is shown for two model variants: dotted lines – for the R2.3-geom2 models, solid lines – for the others in Table 4.

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16. Fig. 15. Schemes of experiments on deformation of a sheet-like structure in a box under compression (a) and under tension (b) with a moving right wall. The formulation of the problem and the properties of the material correspond to the laboratory and numerical experiments presented in (Buiter et al., 2006).

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17. Fig. 16. Comparison of the results of an analog experiment on compression of a sand layer and the results of mathematical modeling. (a) – thrust formation patterns were obtained in the experiment (Schreurs et al., 2006); (b, c, d) – in mathematical modeling using the I2ELVIS (Gerya, Yuen, 2003), LAPEX-2D (Babeyko et al., 2002) and MSC.Marc (this work) packages. The thrust formation patterns are presented; the color from blue to red shows the rate of plastic deformations.

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18. Fig. 17. Comparison of the results of analog experiments and mathematical modeling of tension. (a, b) – deformed configurations obtained in experiments (Schreurs et al., 2006); (c, d) – results of modeling using the I2ELVIS (Gerya, Yuen, 2003), LAPEX2D (Babeyko et al., 2002) packages; (d, e) – modeling of tension performed using the MSC.Marc package (this work) with the Drucker-Prager (d) and Huber-Mises (e) plasticity models. The intensity of plastic deformations is shown in shades from blue to red.

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19. Fig. 18. Results of thermomechanical modeling (model R2-geom2, Table 4) of tectonic interaction of the allochthonous and autochthonous blocks of the Meyer tectonic zone at the time of 1.67 million years (the magnitude of horizontal compression of the crust is 11.5%). (a) – total pressure (the color shows an increase in pressure from red to blue, 1 GPa = 10 kbar); (b) – distribution of the shear component of stresses (MPa); (c) – intensity of plastic deformations; (d) – strain rate (1.e-13/s), the dotted line shows the position of the fault.

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20. Fig. 19. The distribution pattern of pressure (a) and plastic deformations (b) in the model (R3-geom2m1m2-m4) with identical rheological properties of the upper crust blocks of the Southern and Northern domains (allochthon and autochthon) and different properties of the lower crust blocks. The patterns are shown for the moment of the end of the block convergence (the horizontal compression of the crust is 7%). The anomalous region of superlithostatic pressure in the upper crust is absent compared to the R2-geom2 model (Fig. 18). The pressure range at the block contact is indicated. The dotted line shows the position of the fault.

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21. Fig. 20. Duration of existence of non-lithostatic pressure in the near-contact region in the center of the model (see insert). The undisturbed pressure at this depth is 4 kbar (points a, b, c) and 6 kbar (point g).

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