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Applications (Dating topog relief House 1998 (Theory (if canyons recent…
Applications
Horizontal sampling strategy
What?
i.e. Sample @ same Z beneath valley & ridge
Sample @ same elevation across landscape ∴ compare ages horizontally
Expect
Valley sample
older
than ridge
Valley:
= On way out of PRZ
∴ accumulates most He it produces
Ridge:
In middle of PRZ ∴ accumulates some,
but not all
He it produces
Remember
Isotherms = much more compressed beneath valleys vs peaks
Valley vs Ridge Sample Age
Sample below valley
= older vs that beneath ridge
i.e. regional pattern He ages @ equal ages
Dating topog relief
House 1998
Canyon systems incised by rivers
Equal elevation profile across orogen canyons
Expect:
Canyons - old ages
Peaks intervening - young ages
Theory
Could infer how long canyons existed?
if canyons old
expect big disparity btwn valleys vs ridge ages
if canyons recent
expect little disparity
samples hadn't been through PRZ much earlier vs ridges
House Findings
Oldest ages (75-80 Ma) coincide with deep/wide canyons
Interp
Relief existed since Cretaceous
i.e. Sierra Nevada elevated since Late Cretaceous
∴ Topog affects isotherms
w palaeocanyons located in same place vs today
Grand Canyon Debate
Concerning when mass incision
Contrasting proposals when formed
Sedimentology - 5 - 6 Ma
Canyon topog ~ 75 Ma
Solving Grand Canyon Debate
4He/3He thermochron employed
Modelled expected profile for ancient vs young scenarios
Observations support
ancient canyon model
∴ GC formation ~75 Ma
Later technique supports earlier He dates...
Alaskan Coast Mtns
4He/3He thermal models support cooling path
Rapid, recent cooling ~ 1.7 Ma
∴ Relief formation ~ 1.8 Ma (rapid)
Age variation ~6.7 - 1.7 Ma
All samples - denudation accelerated ~ 1.7 Ma
Climate driver?
coincides w Pleistocene onset (warm-cold transition)
∴ increased glacial incision
Controversy Summary
Remember - all techniques make assumptions
Must understand caveats
What techniques can & cannot be used for
Context in which samples have & do operate in
Need to use multiple techniques
Study employing 1 technique in isolation = useless
To infer concrete relief/denudation/exhumation trends