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  1. Continental collision mountain belts: the Arabia-Eurasia system Paolo Ballato, 11-02-2009

  2. Today's class contents 1) Continental collision: a brief outlook (definition, causes, implications…..) 2) How is tectonics deformation accommodated within the Arabia-Eurasia collision zone? 3) A case study from the Alborz mountains, an intracontinental mountain belt linked to Arabia-Eurasia collision (the record from foreland basin deposits) a) When did the deformation related to continental collision start in the Alborz mountains? b) How did deformation evolve? c) What can we learn from foreland basin deposits (i.e. climate vs tectonic) ?

  3. PART 1 1) Continental collision: a brief outlook (definition, causes, implications…..)

  4. Pre-collisional setting Lower plate Upper plate Prior to a continental collision, the landmasses are separated by oceanic crust, formed during an earlier episode of sea-floor spreading As the continental blocks converge, the intervening sea floor (lower plate) is subducted beneath the upper plate The descending oceanic slab generates a volcanic arc Upper plate deformation is limited (tectonics stress is not transferred far away from trench) and shortening is mainly accommodated along plates interface (accretionary wedge) India-Asia convergence rate decreased from 160 to 50 mm/yr in the last 70Ma www2.bc.edu/~kafka/ge180.f03/PT_4.ppt

  5. Collisional setting As the continental lithosphere of the lower plate approaches the upper plate subduction terminates, suturing occurs, and continents are amalgamated Tectonics stress is progressively transferred to the upper plate, where deformation is accommodated across a broad region (thousands of km from the suture zone). Possibly reactivation of structures forming old orogenic belts A fold and thrust belt develops in the lower plate Mountain range/ranges are formed and several km of crust can be exhumed www2.bc.edu/~kafka/ge180.f03/PT_4.ppt

  6. Why does continental collision occur? • Continental lithosphere (3.1-3.2 g/cm3 ) • crust (density ca. 2.7 g/cm3) • mantle(density ca. 3.3 g/cm3) • Oceanic lithosphere (3.3-3.2 g/cm3) • crust (density ca. 2.9 g/cm3) • mantle(density ca. 3.3 g/cm3) Cloos, 1993 Oceanic lithosphere is denser than continental lithosphere, so it tends to sink (subduction) into the asthenosphere when convergence takes place When the continental crust reach the subduction zone the buoyancy forces oppose resistance to the slab pull forces; subduction ends and the pulling slab will break off sinking into the asthenosphere

  7. How is it tectonics deformation absorbed in the upper plate? 1) Crustal thickening (exhumation) 2) Extrusion tectonics (lateral transport of crustal blocks) Tapponnier et al., 1982, 1986

  8. 3)Large scalefolding (lithospheric buckling) Burg et al, 1999 Difficult to demonstrate….is it an efficient mountain building process? 4) “Intra-collision zone subduction” (subduction of denser microplates located in the collision zone) Matte et al, 1997

  9. PART 1…..Summarizing 1) Continental collision occurs when plate convergence cannot absorbed anymore via subduction process 2) Continental collision takes place because buoyancy forces do not allow large amount of continental subduction 3) During continental collision tectonics deformation is not anymore localized along the plate margin (accretionary wedge), but affects a large area in the upper plate and propagate cratonward in the lower plate (fold and thrust belt) • 4) Deformation in the upper plate is absorbed via : • Crustal thickening • Lateral extrusion of rigid blocks • Possibly via lithospheric buckling • “Intra-collision zone subduction” 5) Intracontinental deformation is generally localized along crustal weakness (i.e. old orogenic belts)

  10. PART 2 2) How is tectonics deformation accommodated within the Arabia-Eurasia collision zone?

  11. The Arabia-Eurasia collision zone Caucasus Black Sea Eurasia Aspheron Anatolia Casp T-I plateau Kopeh Dagh Alborz Aegean Central Iran Cyprus Hellenic Zagros Lut Helmand Nubia Makran Red Sea Arabia Owen FZ Gulf of Aden India Eastern African Riften Somalia

  12. Arabia-Eurasia system: from oceanic subduction to continental collision Opening of the Gulf of Aden McQuarrie et al., 2006

  13. Active tectonics of the Arabia-Eurasia collision zone: seismicity Reilinger et al., 2006

  14. Active tectonics of the Arabia-Eurasia collision zone: quantifying present-day deformation with GPS data Subduction of a denser microplate (Southern Caspian Basin) Westward extrusion of Anatolia (escape tectonics) Crustal thickening Reilinger et al., 2006

  15. Active deformation in North Iran Alborz Intra-collision zone subduction South Caspian Basin crust is thinner and denser than adjacent regions Brunet et al., 2003 Guest et al., 2007

  16. The Arabia-Eurasia collision zone: kinematics model GPS based Black numbers: 3 strike (3) dip slip White numbers: plate velocities Reilinger et al., 2006

  17. Active deformation takes place along crustal heterogeneity (i.e. old suture zone and orogenic belts) Horton et al., 2008

  18. PART 2……..Summarizing 1) Deformation is accommodated along seismic belts (mountain chains and large intracontinental strike-slip faults) bounding aseismic blocks • 2) Deformation in the upper plate is absorbed via : • Crustal thickening (Zagros, Alborz, Caucasus, etc.) • Lateral extrusion of rigid blocks (Anatolia and smaller crust blocks) • Possibly via lithospheric buckling (Alborz-South Caspian basin system?) • “Intra-collision zone subduction” (South Caspian basin) 3) Intracontinental deformation is localized along crustal weakness like inherited structures (paleosutures and old orogenic belts)

  19. PART 3 3) A case study from the Alborz mountains, an intracontinental mountain belt linked to Arabia-Eurasia collision (the record from foreland basin deposits) a) When did the deformation related to continental collision start in the Alborz mountains? b) How did deformation evolve? c) What can we learn from foreland basin deposits (i.e. climate vs tectonic) ?

  20. Foreland basin anatomy and sedimentary facies distribution Grain-size decrease Tectonic load (crustal shortening and thickening; exhumation of crustal section) Plate deflection (flexural subsidence) DeCelles and Giles, 1996 Coarse-grained facies are generally confined in proximity of the fold and thrust belt front. However in some cases they can prograde into the foreland for tens of km……Why?

  21. Lateral and vertical sedimentary facies evolution in a foreland basin system: syn-thrusting progradation of coarse-grained facies Stable thrust front Progradation of gravel facies during a major thrusting phase Distance from the thrust front (km) Burbank et al., 1988

  22. Lateral and vertical sedimentary facies evolution in a foreland basin system: post-thrusting progradation of coarse-grained facies Time (Ma) Flemings and Jordan 1990

  23. Lateral and vertical sedimentary facies evolution in a foreland basin system: climatic forcing Zhang et al., 2001

  24. Simplified tectonostratigraphy of the Alborz Mountains 36 Ma (end of magmatism) The Alborz range is characterized by a complex crustal fabric, with inherited structures related to both compression and extension since Paleozoic time Guest et al., 2006

  25. Central Alborz Mountains ca. 4 mm/yr of left-lateral shearing ca. 6 mm/yr of shortening Modified after Geological maps of Tehran, Semnan, Saveh, Sari, Qazvin and Amol 1: 250 000, Geological Society of Iran, and Guest et al., 2006

  26. 5 km Eyvanekey stratigraphic section ASTER satellite image, bands 731-RGB

  27. N S 5m 70m 4m 4m N Unit 1 Unit 1C: braided river dep. system N S Unit 1B: distal river dep. system Unit 1A: playa lake dep. system N S

  28. Unit 2A: playa lake dep. system N S Unit 2 Unit 2B: braided river dep. system S N

  29. Unit 3 5m Unit 3B: braided river dep. system N S Unit 3A: playa lake dep. system N S N S Unit 3C: alluvial fan dep. system 3C 3B

  30. Stratal geometric relationship ASTER satellite image, bands 321-RGB

  31. Magnetostratigraphy Main prerequisites: Normal Polarity Fine-grained lithologies Continuous sedimentation Reverse Polarity Independent age constrains Reference MPTS

  32. Magnetostratigraphy In 75% of samples a Characteristic Remanent Magnetization (ChRM) was isolated Ballato et al., 2008

  33. Magnetostratigraphic Correlation Ballato et al., 2008

  34. Sediment accumulation rates Coarse-grained sed. Fine-grained sed. Coarse-grained sed. Fine-grained sed. Coarse-grained sed. Fine-grained sed. Ballato et al., 2008

  35. 6.2 Ma PART 3…..Concluding Sed.acc.rate = 0.65 mm/yr 7.5 Ma a) When did deformation related to the Arabia-Eurasia continental collision start in the Alborz mountains? Sed.acc.rate = 0.58 mm/yr At ca. 17.5 Ma the basin records a sharp increase in sedimentation rate (0.04 to 0.58 mm/yr). This increase reflect onset of flexural subsidence related to crustal shortening and thickening 17.5 Ma Sed.acc.rate = 0.04 mm/yr 36 Ma

  36. Tectonic vs climate: retrogradation of coarse-grained facies Increase in slip rate +100% = increase in subsidence and sed. flux Decrease in precipitation -50% = decrease in sed. flux Post-perturbation Post-perturbation Pre-perturbation Pre-perturbation Pre-perturbation Post-perturbation Pre-perturbation Pre-perturbation Facies retrogradation Facies retrogradation Sediment flux Time (Myr) Sediment flux Time (Myr) Time (Myr) Distance from fault (Km) Time (Myr) Distance from fault (Km) In both cases retrogradation of sedimentary facies is recorded in the basin. However, when precipitation decrease thesedimentation rate does not change since there is no perturbation in subsidence Densmore et al., 2007

  37. Tectonic vs climate: progradation of coarse-grained facies Decrease in slip rate -50% = decrease in subsidence and sed. flux Increase in precipitation +50% = increase in sed. flux Post-perturbation Post-perturbation Pre-perturbation Pre-perturbation Pre-perturbation Post-perturbation Pre-perturbation Post-perturbation Facies progradation Facies progradation Time (Myr) Sediment flux Sediment flux Time (Myr) Time (Myr) Distance from fault (Km) Time (Myr) Distance from fault (Km) In both cases progradation of sedimentary facies is recorded in the basin. However, when precipitation increase thesedimentation rate does not change since there is no perturbation in subsidence Densmore et al., 2007

  38. Unit 1

  39. Stratal geometric relationship ASTER satellite image, bands 321-RGB

  40. ca. 5 km ca. 25 km Unit 2

  41. ca. 5 km ca. 25 km Unit 3

  42. PART 3…..Concluding a) How did deformation evolve? The locus of deformation moved forth and back, without a predictable pattern on a time scale ranging from 2 to 0.6 Ma b) What can we learn from foreland basin deposits (i.e. climate vs tectonic) ? In a medial-distal part of a foreland basin high sediment accumulation rates coincide with fine-grained sediments and reflect an increase in subsidence due to tectonic loading Low sediment accumulation rates coincide with coarse-grained sediments and reflect decrease in subsidence related to intraforeland uplift Progradation of coarse grained sediments during a moderate to high subsidence rate seems be related to an increase in sediment flux possibly triggered by enhanced precipitation

  43. Thank you With the contribution of Angela Landgraf, Manfred Strecker, Cornelius Uba, Norbert Nowaczyzk, Anke Friedrich, and many others…