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Taiwan Detrital Thermochron (As collision progressed S (Depo ages (H5 (H5…
Taiwan Detrital Thermochron
Taiwan Tectonics
Phillipine sea plate subducts in N direction
Produces Ryuku arc which collides with Taiwan
Taiwan Erosion
Provides 2 % global sediment flux
from 0.024% global land surface !
Highly seasonal as f ( typhoons & monsoons )
Long-term erosion rates
Historic ≈ 5 mm/yr
Long-term (kyr) ≈ 4 - 6 mm/yr
Locally @ Taroko Gorge
≈ 26 mm/yr for many kyr
36Cl data - cosmogenics
Ages
Core of orogen = Young
< 5 Ma
Zircon/Apatite FT
When did high erosion rates start in Taiwan?
Coastal Range
Depo ages
North
Oldest seds @ Shuilien (3.8 - 3.6 Ma)
Mid-Section
Plio-Pleistocene (2.9 - 0.7 Ma)
South
Sanshien & Mawu (1.6 - 0.9 Ma(
What ?
∴ Ocean sediments easily sampled
≈ Basin offshore of Taiwan obducted onto continental margin
Coastal range - has bits of arc thrust atop seds from basn
Collision arc & margin
has progressed from N to S
collision = oblique ∴ sediments young southwards
Shuimuting section
Repeating silt/mudstone cycles
Only sampled siltstone
Yields grains of correct size for thermochron dating
∴ Biased sampling - only bedload sampled
Represents high energy events
(sufficient energy to carry bed material)
Volcanic tuffs
- Mawu River
= Good age markers
Correlate with horizons in sediments
Coastal Ranges Detrital Record
4 km sediment preserved in collisional basin
Sed deposited in Plio-Pleistocene (4 - 0.7 Ma)
Rests uncomformably atop Luzon arc volcanics
Depositional
age dating
Biostratigraphy
index fossils i.e. forams
Magnetostratigraphy
lotsa polarity reversals past 20 Ma
Hypothesis
Taiwan mountain building ended ~ 10 - 4 Ma?
Zircon FT Grain Ages
Shuilien & Shuimuting sections
Unreset - No young ages
No grain ages < 10 Ma
Lotsa old ages (> 100 Ma)
Eroding off cover?
Sanfu-Chi & Hsui-Kulian-Chi
Up until 1.8 Ma
No evidence for fast eroding system
Ages are old
After ~ 1.2 Ma
Most of sample young (< 8 Ma)
i.e. recently exhumed
Interp
: sthing
big
happened around 1.8-2 Ma to change exhumation rates
As collision progressed S
Unroofing occurred
In stepwise manner
∴ Seds with same depo age
--> have different depo age distrib
= Consistent with growing orogen model
[1 Ma from Z --> surface --> basin =
rapid
]
Older sample - took ~ 15 Ma
Younger sample - took ~ 4Ma
Depo ages
From Hsui-Kulian-Chi section
H5 (2.47 Ma) deposited before H3 (0.9 Ma)
H5
H5 ≈ ∆T > 15 Ma
H5 td ≈ 2.5 - 1.8 Ma
H3
H3 ≈ ∆T = 4 - 4.2 Ma
H3 td = 0.9 - 0.77 Ma
= Independently constrained