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Response of river systems to tectonic deformation

Response of river systems to tectonic deformation. Chris Paola* St Anthony Falls Lab University of Minnesota * On behalf of the experimental stratigraphy group, SAFL. Today’s topics. A little about tectonics and uplift Tectonic subsidence and sedimentation

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Response of river systems to tectonic deformation

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  1. Response of river systems to tectonic deformation Chris Paola* St Anthony Falls Lab University of Minnesota * On behalf of the experimental stratigraphy group, SAFL

  2. Today’s topics • A little about tectonics and uplift • Tectonic subsidence and sedimentation • How tectonic subsidence was thought to affect channel stacking in the subsurface • What happened when we tested it experimentally • A simple time scale analysis • Another experimental test • Dramatic conclusion • A word from our sponsors

  3. Examples Mand River, Iran (Zagros) Isacksen Salt Dome, Alaska

  4. Context: tectonic rates • Plate tectonic speeds of the order of several cm/yr • Vertical rates are of the order of 10% of horizontal rates, so mm/yr

  5. Crustal subsidence: the dark side of mountain building

  6. Near the continents, subsidence ~ sedimentation Laske and Masters, 1997

  7. Tectonic subsidence Total sediment thickness = 9.5 km Mt Everest

  8. Long-term storage is an important part of the budget in depositional rivers • “Graded” state is replaced by a condition in which sediment extraction balances subsidence, i.e. • Measured extraction losses in coastal rivers are in the range 30-50% (e.g. Des Walling et al.)

  9. Long-term storage is an important part of the budget in depositional rivers • Major effects: long profile concavity, downstream fining, avulsion

  10. Subsidence + sedimentation + avulsion = preserved subsurface channels avulsion

  11. floodplain channel Effect of lateral tilting on channels Prediction: lateral tilting should attract channels to lateral subsidence maxima (Alexander and Leeder)

  12. The Experimental EarthScape basin (“Jurassic Tank”)

  13. The XES system under construction

  14. 6 m 3 m Run 99-1 Plan view 108 subsidence cells 4 feed points Constant base level

  15. Run 99 Flow + topography 6 m 3 m

  16. Initial condition - 0 hours Fluvial surface Latex “basement”

  17. X 6m 0 0 Y Surface and basement topography 3m Stage isopach map in millimetres End of stage I 40 hours

  18. X 6m 0 0 Y Surface and basement topography 3m Stage isopach map in millimetres End of stage II 70 hours

  19. 0 20 cm 40 2.40 m downstream

  20. Stage I Stage II Lateral distribution of channel fraction

  21. Lateral distribution of channel fraction

  22. What happened? • Lateral maximum in sedimentation rate did not attract channels • Proposed explanation: channel system was “too fast”: time scale for lateral channel migration was < time scale for lateral subsidence variation to influence surface slope • How to quantify this…

  23. Tectonic time scale • Channels are steered by lateral tilting if: • Which suggests the following tectonic time scale: Downstream bed slope Lateral (cross stream) bed slope Lateral differential subsidence Tectonic rotation rate Lateral length scale

  24. Channel time scale • Time scale for surface occupation by fluvial channels: • or: Total dry width Characteristic lateral migration speed Tt >> Tc sediment dominated Tc >> T tectonic dominated

  25. Does this explain the observation? • During XES 99 run • Sx = 0.05 • Ds = 0.2m / 40 hr = 0.005 m/hr • Lf= 1 m • Therefore: • Tt = 10 hr • Measured: • Tc = 10 hr for flow to visit entire surface (conservative!) not subsidence dominated suggests subsidence domination requires a substantially lower Tt/Tc

  26. Design a new experiment • Time scale ratio: • Goal: minimize qs, Sx, Bwet /B • maximize

  27. XES 05-1: flow steering by tectonics • Flow-perpendicular normal fault • Maximum throw 700 mm • Relative uplift by lowering base level • Channel migration time scale << run 99

  28. Programmed subsidence

  29. Eureka! It worked!

  30. Channel pattern

  31. XES 05-1: Relay Ramp Slice at 1250 mm from the right side of the XES wall Slice at 1000 mm from the right side of the XES wall Slice at 760 mm from the right side of the XES wall

  32. Application to field scales • Suppose: • Channel time scale Tc = 5000 yr • Downstream slope Sx = 10-5 • Lateral length scale = 100 km • Then for parity in the time scales we would need: • Lateral subsidence variation  = 0.2 mm/yr A plausible but high value for tectonic subsidence, BUT well within the range of observed values for compaction and fluid-pumping effects

  33. Who we are: institutions University of Minnesota (SAFL) University of California, Berkeley Johns Hopkins University Fond du Lac Tribal and Community College Massachusetts Institute of Technology Princeton University Science Museum of Minnesota University of Colorado University of Illinois You are here!

  34. NCED community participation opportunities: • Visitors program • Sabbatical visits • Workshops • Working groups • Postdocs • Short courses Contact us via: www.nced.umn.edu

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