From Continental Subduction to Uppercrustal Nappes Stacking A - - PowerPoint PPT Presentation

From Continental Subduction to Uppercrustal Nappes Stacking A Numerical Analysis

CARRY Nicolas, Frédéric GUEYDAN, Jean Pierre BRUN, Denis GAPAIS

Géosciences Rennes, UMR 6118, Université de Rennes 1

Didier MARQUER

Géosciences, , EA2642, Université de Franche-Comté

Introduction This study : understand the mechanisms responsible for a detachment of a crustal piece Methods : simple thermal model and force balance analysis Continental subduction : between oceanic closure and mountain belt surrection Continental margin : first continental part subducted in

moho Lithosphere

Stacking Natural examples

Strength??

STACKING OCCURS IF MATERIAL STRENGTH < ACTING STRESS

Force balance analysis

Acting stress Material strength

Acting stress: decrease in overlying lithosphere increase in overlying asthenosphere

Ts: Tectonic stress Bs: Buoyancy stress µO: Overlying weigth stress Sum: Acting stress

Overlying

BURIAL

Stacking Natural examples 100 km 300 km

2D finite elements Conductive Transitory No crustal heterogenity

Numerical code SARPP after

• Y. Leroy & F. Gueydan 2003

2D thermal and strength evolution in a subducted continental margin

Acting stress Material strength

Stacking Natural examples

Thermal and strength evolution

Temperature Strength 2 My (36,4 km) 4 My (77,34 km) 6 My (100,9 km) Strength Strength Temperature Temperature

Strength (MPa)

Brittle law Ductile law Acting stress Material strength

Stacking Natural examples

Crustal strength evolution

2 My

BRITTLE

Acting stress Material strength

Stacking Natural examples

Crustal strength evolution

3 My

BRITTLE

Acting stress Material strength

Stacking Natural examples

Crustal strength evolution

4 My

BRITTLE DUCTILE

Acting stress Material strength

Stacking Natural examples

Crustal strength evolution

6 My

DUCTILE

Acting stress Material strength

Stacking Natural examples

Material strength vs Acting stress

Acting stress Material strength Difference between Acting stress and Material strength

Stacking BRITTLE DUCTILE

Acting stress Material strength

2 My 4 My 6 My BURIAL

Stacking Natural examples

Stacking evolution

2 My (36,4km) 4 My (77,34 km) 6 My (100,9 km) Difference: Stress - Strength

Able stacking area

Acting stress Material strength

Detached unit

Large difference Weak material Easy stack

=

Burial depth 45 km

Stacking Natural examples

Parametric study: crustal units thickness

30° - 10 cm.y-1 20° - 5 cm.y-1 10° - 2,5 cm.y-1 5° - 2,5 cm.y-1 Dip angle and velocity Length Thickness Acting stress Material strength 30kb

BURIAL

Depth (km)

Pressure

15kb

45kb

0 kb

Stacking Natural examples

Parametric study: crustal units length

30° - 10 cm.y-1 20° - 5 cm.y-1 10° - 2,5 cm.y-1 5° - 2,5 cm.y-1 Dip angle and velocity Length Acting stress Material strength 30kb

BURIAL

Depth (km)

Pressure

15kb

45kb

0 kb

Stacking Natural examples

Validation : The Lepontine case

Lv Si Ad Ta Su

N

25 km

Tectonics maps of Central Alps

Gothard

Legende Lv: Leventina Ta: Tambo Si: Simano Su: Suretta Ad: Adula

Acting stress Material strength

Cross section of the Central Alps

(modified after Schmidt 2004)

Lv Si Ad Ta Su

Aar

Lepontine crustal units Thin and long units Only upper-crustal rocks

Stacking Natural examples

Validation : The Lepontine case

Simano

N

25 km

Tectonics maps of Central Alps

• Pic pressure (D1): 12kbar (R. Rütti, 2003)
• Length: around 35 km
• Thickness: around 2,5 km

Acting stress Material strength

Cross section of the Central Alps

(modified after Schmidt 2004)

Simano Example : Simano crustal unit

Stacking Natural examples

Validation : The Lepontine case

30° - 10 cm.y-1 20° - 5 cm.y-1 10° - 2,5 cm.y-1 5° - 2,5 cm.y-1 Su Ta Si Ad Lv Ad Lv Si Ta Su Slab angle :

• Around 20°
• In agreement with PT angle

estimations (for example, Dale & al., 2003) Dip angle and velocity

Lv: Leventina Si: Simano Ad: Adula Ta: Tambo Su: Suretta

Acting stress Material strength

Burial depth (km)

Conclusions

Perspective: estimating the velocity and dip of subduction slabs from P-T datas

The stacking of upper-crustal units in the subduction zone : (1) is a direct consequence of heating from above (2) does not require crust heterogeneity (3) involves thin & long units

The modelling results are consistent with the Lepontine nappes example