448
Chapter 9 Denudation and Mass Transport
talSOST ATIC REBOUND ONL Y
o = D me + D ch
t.H = C j • t.D
I
b PROCESSES
E (=0)
EXHUMATION
(DENUDA TION)
• I •••••••••
ULTIMATE
EQUILIBRIUM
LEVEL
: 0.63 Ho
: (3.15 Ho
: eroded)
2nN4{ynnn//n) /f.lf t.H = SURFACE
1
77 TU = TECTONIC, _:_...J.
UPLIFT
, ,/ .ll.:.:.. UPLIFT :, ,; I
E~1';I,_,/,,"---,,,',.. IR+TU=ROCK
f,_, I~=ISOSTATIC_
UPLIFT
-~ ~,~E.~?~N~,
(RU)
I ."\ -
/ '- /
..... / , - I ....
BASE LEVEL
,/,' 1'"\," ',':. / ~_,
- _ . _ _ . _ - -
/ ' - . ) , ' , ' "/ .... J"" ........ "_ . . . . . _ _ • _ _ . - - -
Fig. 9.17. a Model of the
interplay between total
(meehanieal and ehemieal)
denudation D, isostatie
rebound IR (Airy model),
and lowering. dH of the
landseape in diserete steps;
no teetonie uplift. H, mean
elevation of the landseape
above base level (e.g. sea
level or the level ofultimate isostatie equilibrium).
The amounts of D, IR, and
dH deerease with lowering H (in the time intervals tl-t o ete.). t c is eharaeteristie response time.
b Interplay between denudation or exhumation E,
isostatie rebound IR, teetonie uplift TU, and elevation of the land surfaee H.
(Based on England and
Molnar 1990). c Baekstakking method to determine
TU and t.H for a eertain
time period. (After Brown
1991). See text for further
explanation
, CRUST AL BLOCK
C BACKSTACKING
INITIAL
LAND SURFACE
INITIAL p,T
CONDITIONS
THERMOBAROMETRY,
FISSION TRACKS
r-l--'
I
I
I
I
I
I
I E I
I
I
I
I
I
PRESENT
LAND
SURFACE
TIME :>
TU = E - IR + t.H
t.H = Hp - Hj ± t.SL
where Ci is the eompensation ratio for Airy isostasy:
Ci = (Pa - PYPa (ef. Seet. 8.1).
Then isostatie rebound IR for a time inerement dt is
(m).
(9.7)
Struetural geologists and petrographers often replaee
D t by the term "exhumation" E (see below).
U sing the model parameters of Fig. 8.1 b for the
asthenosphere and erust, Ci is 0.16; for Pa = 3.40 and
Pe = 2.70 the value of Ci is 0.21 (Pazzaglia and
Brandon 1996). For reasons of simplieity, the value