... by tectonic and/or surficial processesâ (Ring et al., 1999). ACTIVE VS. âDECAYINGâ LANDSCAPES photo: Michael Strickland photo: Lech Magnuszewski ...
L A N D S C A P E E V O L U T I O N I N T H E H A N G AY D O M E , M O N G O L I A
Kalin McDannell
Lehigh University IUP Geoscience Day 2014
C O N T E X T: H A N G AY D O M E , M O N G O L I A
~2700 KM
GEOLOGIC OVERVIEW • Terrane accretion during the Archean • Archean-Early Proterozoic metamorphic basement rocks • unconformably overlain by Cambrian-Devonian strata • xlline/seds intruded by Permo-Triassic granitoids &
deformed in Late Paleozoic compressional event
• Oligocene-Recent clastics with volcanic basalt flows
C O N T E X T: H A N G AY D O M E , M O N G O L I A
S I B E R I A N C R AT O N
S AYA N
AL
TIEN SHAN
TA
Y
&
KHENTII
G
O
BI
AL
TA
Y
O V E R A L L H A N G AY R E S E A R C H G O A L S *addressed by this work • Questions about lithospheric structure, magmatic processes, and
surface uplift/landscape evolution • lithosphere/sub-lithosphere: structure? evolution? nature/affinity
of mantle flow? surface/at depth deformation consistent? • magmatic processes: composition/state of mantle? mantle
melting? spatio-temporal nature of volcanism? • surface*: history of surface elevations and landscape evolution?
spatio-temporal patterns? paleoelevation? http://www.ees.lehigh.edu/groups/mongolia//
QUESTIONS ABOUT THE H A N G AY D O M E L A N D S C A P E
• Where, when, and how fast was the epeirogenic episode in
Mongolia? • What is the exhumation history of central Mongolia and is it
variable in time/space? • Is the form of the landscape (and apparent Hangay uplift) due
to exhumation by the Selenga River (not regional rock uplift)?
METHOD: A PAT I T E ( U - T H ) / H E T H E R M O C H R O N O L O G Y
~100µm
4He=8238U(eλ238t−1)+7/137.88238U(eλ235t−1)+6232Th(eλ232t−1)
DODSON (1973): CLOSURE T E M P E R AT U R E
T H E R M O C H R O N O L O G Y: C L O S U R E T E M P E R AT U R E CRUSTAL DEPTH
COOL
~40℃ PRZ
WARM
~80℃ Go!
HOT 0
4He
100
LANDSCAPE EVOLUTION
A C T I V E V S . “ D E C AY I N G ” L A N D S C A P E S Exhumation: “the unroofing history of a rock, as caused by tectonic and/or surficial processes” (Ring et al., 1999)
photo: Lech Magnuszewski
photo: Michael Strickland
D E C AY I N G L A N D S C A P E Hangay Dome, Mongolia Exhumation Rate: ~0.03 mm/yr for ~150 Ma Active landscape exhumation rates are ~1-10s mm/yr
170
140
133 105 170 90 120 104 78 115 107 103 112
117 113
meters 500
1000
1500
2000
2500
3000
3500
S E L E N G A B A S I N D E T R I TA L A PAT I T E S
Selenga Detrital Apatite (U-Th-Sm)/He Analyses (n=55) 22
16
11
5
0 0
40
80
120
160
200
240
280
320
360
Orkhon detrital (n=15) 7 6 5 4 3 2 1 0 0
30
60
90
120
150
180
210
240
HeFTy (Ketcham, 2005) Inverse Thermal Modeling
• uses grain age, radius, and kinetic model to solve for
diffusion of a single grain
• user specified thermal constraints (high-T age(s), surface
temp., etc.)
• Monte-Carlo inversion to solve for potential thermal
histories
0
Sample 11KWW04-01
20 40
Temperature (°C)
60 80 100 120 140 160 180 200 250 240 230 220 210 200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 Time (Ma)
HeFTy t-T Modeling 00 Temperature Temperature(°C) (°C)
Temperature (°C)
0 100 200 300 250
Ksp Ar/Ar
200
150 100 Time (Ma)
12MN47-3 50 0
300 300 200 200
150 100 150 100 Time (Ma) (Ma) Time
12MN34-1 50 50 00
0 Temperature Temperature (°C)
Temperature (°C)
200 200
250 250
0 100 200 300 250
100 100
200
150 100 Time (Ma)
12MN30-1 50 0
100 12MN47-3
200
300
12MN30-1
~3 km since ~150-125 Ma
12MN34-1
250
200
150 100 Time (Ma)
50
0
PECUBE (BRAUN, 2003) 3 D F I N I T E - E L E M E N T T H E R M O K I N E M AT I C M O D E L
• solves 3D heat transport equation in crustal block
undergoing uplift/surface erosion
• characterized by time-variable, finite surface topography • specified thermal parameters and code solves for model
cooling ages at N-time steps for given geologic scenario
PECUBE MODEL SETUP EXAMPLES McDannell et al., 2014, in prep., G-cubed
Table&2:&Pecube&Numerical&Modeling&Parameters& !
Parameter'
Model'Input'
! Model&duration& Number&of&time&steps& Crustal&block&dimensions&(x,y,z)& DEM&grid&spacing&(x,&y)&
1!
!
40!x!40!x!35!km!
!
0.00027!
! !
1!
Fault&motion&time&intervals&
3!
Normal&fault&dip&angle&
!!
100!start,!80,!40!Ma! ! 75°S! 35!
!
Vertical&node&spacing&
0.8!km!
!
Thermal&conductivity&
3.35!W!m71!K71!
Surface&heat&flow&
80790!mW!m72!
Mantle&heat&flow&
35!mW!m72!
!!
Mantle&density&
3200!kg/m73!
!
Nodal&points&in&z&direction&
Crustal&density& Surface&temperature&z=0&
73
2700!kg/m !
! !
!
0!C! !
Basal&model&temperature&(z&=&35)&
875!C!
Initial&geothermal&gradient&
25!C!
!
0!
!
Isostacy& Atmospheric&lapse&rate&
6!C!km71!
Heat&production&
0!C/Ma!
& !
100!Ma!
Topographic&lification&factor& Timing&of&exhumation&rate&change&
!
!
! ! ! !
PECUBE EXAMPLES
McDannell et al., 2014, in prep., G-cubed 0
50
Temperature (C)
100
150
200
250
300
350
400 100
80
60
40
Time (Ma)
20
0
McDannell et al., 2014, in prep., G-cubed 0 80
BE N5 6-3 Pe cu be
He PRZ
160
2
- 4 2 N BE
240 320
Biotite 40Ar/39Ar
400 480 560 100 90
d.
Granite U/Pb Zircon Ages 80
70
60
50
40
Time (Ma)
30
20
10
0
250-0 Ma 0.03 mm/yr !
long, slow
250-5 Ma 0.01 mm/yr !
long, slow then rapid !
5-0 Ma 0.65 mm/yr
ISOTHERMS AND TOPOGRAPHY Low-temp closure isotherms are affected by topography Topo
Elev.
z=0 2 km
Low Tc
Age
Elev.
15 km
High Tc
Depth
Tc = closure temperature
Age
after Braun, 2002
SLOW COOLING + RELIEF LOWERING Low-temp closure isotherms mimic topography YOUNGER ages at higher elevations
Elev.
original topo
z=0 Low Tc
50 iso-age surfaces (Ma) 40 30 20 10 -1 km depth
Age
CONCLUSIONS !
The Hangay Dome landscape has experienced minimal exhumation since the MidMesozoic (~3 km over 100 Ma) i.e. rocks have been close to/at the surface for ~100 Ma! Selenga basin detrital data supports “old landscape” story from sediment provenance NW of the Hangay No evidence of localized, recent coupled rock uplift & erosion Current data are suggestive of relict landscape with relief lowering through time ! !
THE END
QUESTIONS?
Apollo Butterfly (parnassius apollo), seen in Mongolia, photo by me threatened species
