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

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