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 Total sediment thickness = 9.5 km Mt Everest

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.)

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 Experimental EarthScape basin (“Jurassic Tank”)

The XES system under construction

3 m 6 m Run 99-1 Plan 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 0 06m X Y Stage isopach map in millimetres Surface and basement topography

End of stage II 70 hours 3m 0 06m X Y Stage isopach map in millimetres Surface and basement topography

cm 2.40 m downstream

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

Channel time scale Time scale for surface occupation by fluvial channels: or: Characteristic lateral migration speed Total dry width T t >> T c sediment dominated T c >> T tectonic dominated

Does this explain the observation? During XES 99 run S x = 0.05  = 0.2m / 40 hr = m/hr L f = 1 m Therefore: T t = 10 hr Measured: T c = 10 hr for flow to visit entire surface (conservative!) not subsidence dominated suggests subsidence domination requires a substantially lower T t /T c

Design a new experiment Time scale ratio: Goal: minimize q s, S x, B wet /B maximize 

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

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 scales Suppose: Channel time scale T c = 5000 yr Downstream slope S x = 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

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