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Evidence Subglacial Sed Def (Ice Stream B (Whillans) (1. Borehole…
Evidence
Subglacial
Sed Def
Direct obvs
Boulton & Hindmarsh 1987
Displacement of deformable rods in upper till
Rods moved sideways over time
Skepticism
No photographic evidence
Breidamurjokull, Iceland
What is clear
Structures seen within tills in deglaciated envmts
evince their ability to deform
Borehole & Seismics
Siple Coast
Why AIS ice streams flow so quickly ?
up to 600m/yr (similar to some GrIS outlets)
Ice Stream B (Whillans)
1.
Borehole investigation
(Alley 1987)
Borehole inclinometer
showed
only a few m/yr internal def
Drilled borehole ~ 300m upstream from g.l.
∴ Excess surface velocity must result from basal motion
(basal sliding or soft sed def?)
Surface velocity there ~440 m/a
Velocities ~ 50 - 850 m / yr ! (up / downstream respect)
Seismic study tills
(Blankenship 1987)
Seismics showed ~ 7m thick till layer
Probs responsible for 17-30% Whillans motion
What?
Explosives set off @ Z in snowpack
Array of geophones receive signal
Nature of returns informs bed material properties
Sampled till
(Kamb 1991)
Till only support
VERY low basal stresses
Recovered till samples from base of boreholes
∴ Subglacial sediments deform readily
(much > overlying ice)
Borehole evidence elsewhere
Subglacial sed def
Trapridge Glacier, Yukon
Storglaciaren Sweden
Borehole photog
(Engelhardt 1987)
Blue Glacier, Washington
Observed till layers deforming
Direct obvs
(Boulton, 1986)
Deformation structures observed in glacially-overriden sediments
e.g. faults
Deforming sediment structure
Yield stress ↑ with effective pressure (N), ∴ N ↑ with Z
∴ Critical depth exists
beneath which sediments do not fail
Thus
often see:
A) Upper till - deforming
B) Lower till = stable
i.e. doesn't deform below critical Z
Depth @ which deforms = f (rheology & hydrology)
Modelling sed def requires:
Flow law for till
Model for till hydrology
Both = very difficult to derive
Flow law
for till
Various studies tried to find relation
→ btwn driving stress + strain rate for seds
Complicated
by water flow effects in till
Key issue
whether till response is
viscous or plastic
Depth @ which till experiences def f (...
Rheology
Permeability
Meltwater availability