Response of river systems to tectonic deformation Chris Paola* St Anthony Falls Lab University of...
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Transcript of Response of river systems to tectonic deformation Chris Paola* St Anthony Falls Lab University of...
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• 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
Examples
Mand River, Iran (Zagros)
Isacksen Salt Dome, Alaska
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
Crustal subsidence: the dark side of mountain building
Near the continents, subsidence ~ sedimentation
Laske and Masters, 1997
Tectonic subsidence
Tot
al s
edim
ent
thic
knes
s =
9.5
km
Mt
Eve
rest
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.)
x
qs
Long-term storage is an important part of the budget in depositional
rivers
• Major effects: long profile concavity, downstream fining, avulsion
Subsidence + sedimentation + avulsion = preserved subsurface channels
avulsion
Effect of lateral tilting on channels
Prediction: lateral tilting should attract channels to lateral subsidence maxima (Alexander and Leeder)
floodplain
channel
The ExperimentalEarthScape basin(“Jurassic Tank”)
pressurizedwater reservoir
to water supply
solenoidvalve
stainless steelcone
to gravel recycling
transport surface
gravel basement
rubber membrane
experimental deposit
The XES system under construction
3 m
6 m
Run 99-1Plan view
108 subsidence cells
4 feed points
Constant base level
Run 99 Flow + topography
6 m
3 m
Initial condition - 0 hours
Latex “basement”
Fluvial surface
End of stage I
40 hours
3m
006m X
Y
Stage isopach mapin millimetres
Surface and basement topography
End of stage II
70 hours
3m
006m X
Y
Stage isopach mapin millimetres
Surface and basement topography
0
20
40
cm
2.40 m downstream
Stage I
Stage II
Lateral distribution of channel fraction
Lateral distribution of channel fraction
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…
Tectonic time scaleChannels are steered by lateral tilting if:
Which suggests the following tectonic time scale:
1y
xS
S
Tt SxLf
Lateral differential subsidence
Lateral length scale
Tectonic rotation rate
Downstream bed slope
Lateral (cross stream) bed slope
Channel time scaleTime scale for surface occupation by fluvial
channels:
or:
Tc B Bwet um
Tc (B Bwet )qsh
Characteristic lateral migration speed
Total dry width
Tt >> Tc sediment dominatedTc >> T tectonic dominated
Does this explain the observation?During XES 99 run
Sx = 0.05 = 0.2m / 40 hr = 0.005 m/hrLf = 1 m
Therefore:Tt = 10 hr
Measured: Tc = 10 hr for flow to visit entire surface (conservative!)
not subsidence dominatedsuggests subsidence domination requires a substantially
lower Tt/Tc
Design a new experiment
Time scale ratio:
Goal: minimize qs, Sx, Bwet /B
maximize
fsx
wet
LqS
hBBt
XES 05-1: flow steering by tectonicsFlow-perpendicular normal fault
Maximum throw 700 mmRelative uplift by lowering base levelChannel migration time scale << run 99
Programmed subsidence
Eureka! It worked!
Channel pattern
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
Application to field scalesSuppose:
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
NCED: Towards an integrated, predictive science of Earth-surface Dynamics
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
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University of Illinois
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re h
ere!
NCED community participation opportunities:NCED community participation opportunities:
• Visitors program• Sabbatical visits• Workshops• Working groups• Postdocs• Short courses
Contact us via: www.nced.umn.edu