MINIMAL EROSION IN CENTRAL TIBET SINCE THE EOCENE AND
IMPLICATIONS FOR PLATEAU DEVELOPMENT
by
Alexander Rohrmann
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Minimal erosion in central Tibet since the
Eocene and implications for plateau
development
The growth history of the Tibetan plateau remains elusive, despite its
importance for assessing mechanisms of continental lithosphere deformation
and associated changes in surface elevation and climate system dynamics. A
characteristic of the modern Tibetan plateau interior, which distinguishes it
from the actively growing plateau margins, is low erosion rates. Here, we
present low-temperature thermochronologic (K-feldspar 40Ar/39Ar, apatite
fission-track, and apatite (U-Th)/He) results from samples collected away from
late Cenozoic rifts in the Lhasa and Qiangtang terranes of central Tibet. The
data indicate that in most places, low erosion rates (< ~0.05 mm/yr) were
established by the time India collided with Asia ~50 Myr ago, following earlier
episodes of more rapid exhumation that correspond in time with documented
Cretaceous – Eocene thrust belt activity. Findings of large-magnitude (≥50%)
upper-crustal shortening and substantial exhumation prior to 50 Myr ago,
followed by minimal subsequent denudation, support the establishment of a
proto-plateau in central Tibet prior to the Indo-Asian collision.
The mean elevation of the Tibetan plateau is more than 5000 m from the
Pamir in the west, to ~96°E longitude, east of which it gradually decreases at a slope
of ~0.1°. Distinguishing the Tibetan plateau from its margins are low relief (generally
<500 m) and low erosion rates (3± 1 mm/ka of Lal et al. 2003) in all places except
along late Cenozoic rift systems and externally drained river valleys. Very low relief
erosional surfaces have been identified in eastern Tibet (Clark et al., 2006) as well as
within the higher part of the Tibetan plateau (Shackleton and Chang, 1988; our own
observations). The eastern Tibetan margin is characterized by a high elevation, low
relief landscape that is dissected by narrow river gorges. The surfaces in eastern
Tibet are interpreted to have developed at low elevation prior to river incision since
the Miocene. Given little evidence for Neogene upper-crustal shortening, thickening
of the crust by eastward channel flow has been invoked to explain uplift of the
surfaces to their current high elevation (Clark et al., 2006).
The most direct approach that has been applied to reconstruct paleoelevation
in Tibet is stable isotope paleoaltimetry (DeCelles et al, 2007, Rowley et al., 2006,
Wang et al., 2006 and Cyr et al., 2005). Recent studies indicate that localities in the
southern Lhasa terrane (Oiyug), and along the Bangong suture zone to the north
(Nima and Lunpola), achieved near-modern elevations by mid-Miocene and EoOligocene (but certainly by 26 Myr ago) time, respectively. Although uncertainties
exist regarding the source and composition of ancient meteoric water in northern
Tibet, data from Late Eocene (?) strata near Erdaogou similarly indicate a
paleoelevation of >4 km (DeCelles et al., 2007). Additional evidence supporting the
latter includes the appearance of high-altitude pines in the pollen record at ~38 Myr
ago in a basin rimming the northeastern margin of the plateau (Dupont-Nivet et al.,
2008). The only maximum age constrain on elevation gain is that it must postdate the
deposition of the youngest marine strata (~100 Myr) exposed in Tibet north of the
Indus-Yarlung suture (Kapp et al., 2005). Until paleoaltimetric data are obtained on
older basin deposits, we must rely on more indirect approaches to infer the history of
plateau growth.
Shortening history
Shortening of the upper crust must be compensated in the deeper crust may
be compensated by either homogneous shortening and thickening of the deeper
crust or lateral displacement such that crust is thickened in adjacent regions by
crustal underthrusting. In either case, major upper-crustal shortening events should
result in susbantial crustal thickening. Inferring surface elevation gain from the uppercrustal shortening history by using isostatic argumenets is complicated however,
because of uncertainties of how/where the shortening is accommodated in the
deeper crust and possibilities of dense, eclogitic root development (which would tend
to suppress surface elevation gain) and lithospheric removal (which would tend to
enhance surface elevation gain). Nevertheless, a reasonable inference is that major
upper-crustal shortening must produce some level of crustal thickening and surface
elevation gain.
Whereas almost all popular models attribute Tibetan plateau formation to the
collision between India and Asia since ~50 Myr ago, the bulk of upper-crustal
shortening in much of central Tibet is older (150-50 Ma) and of sufficient magnitude
(≥50%) to have resulted in substantial crustal thickening and elevation gain. This
contractional tectonism is attributed to the collision between the Lhasa and
Qiangtang terranes along the Bangong suture, which began during the Early
Cretaceous, and the development of an Andean-style Gangdese retroarc thrust belt
along the southern margin of Asia during Cretaceous to Eocene northward
subduction of Neo-Tethyan oceanic lithosphere (Fig. 1; Murphy et al., 1997; Kapp et
al., 2007). Intriguingly, initiation of Indo-Asian collision marks the cessation of major
upper-crustal shortening in the Lhasa terrane. During the 50–30 Ma time interval, the
upper crust of the Qiangtang was only moderately shortened (~25%; Kapp et al.,
2005), whereas major (≥50%) shortening was localized to the north in the Hoh-Xil –
Fenghuo Shan – Nanqian thrust belt (HFNTB; Fig. 1; Coward et al., 1988; Spurlin et
al., 2005), and to the south of the Indus-Yarlung suture in the Tethyan Himalaya
thrust belt (THTB; Fig. 1; Ratschbacher et al., 1994). An explanation is that the
gravitational potential energy related to the development of thick crust and high
elevation in the Lhasa and Qiangtang terranes (e.g, a Lhasa-Qiangtang protoplateau) was sufficient to inhibit major upper-crustal shortening and to promote
thrusting at lower elevation regions to the north and south.
Thermochronology: Previous work
Low temperate thermochronolgy is widely used to infer erosion rates in various
tectonic settings. The thermal history of rocks, and therefore the time that has
elapsed since passage through the closure temperature, enables one to calculate a
time-averaged erosion rate . The (U-Th)/He system is especially sensitive to surface
erosion and relief forming processes (Reiners et al., 2003), due to the near-surface
(i.e. shallow) closure temperature isotherm, which can range from 50 to 70 °C
depending on erosion/cooling rate. The sensitivity of the (U-Th)/He system makes it
an ideal tool to identify exhumation events in the upper ~2-3 km of crust.
Multi-diffusion domain modeling of K-feldspar 40Ar/39Ar data from Shiquanhe
(Kapp et al., 2003), Nima (Kapp et al., 2007), and Amdo indicate relatively rapid
cooling to below <150°C at 65 Ma, 105 Ma, and 119 Ma, suggesting <5-6 km of
erosion in these areas at these times—all of which predate the Indo-Asian collision.
Rapid cooling is attributed to documented thrust belt development in these regions in
response to the Cretaceous Lhasa-Qiangtang collision. Samples from Kapp et al.
(2003) from western Tibet show a period of slow cooling during the Early Cretaceous,
followed by a pulse of accelerated cooling coeval with Late Cretaceous shortening,
and consistent with a K-feldspar age of 67 ± 3 Ma from Rutog, 100 km north (Matte et
al., 1996). JV1(NMA) cooled rapidly at ~100 Ma, coincident with Cretaceous thrust
faulting (Kapp et al., 2007a). Two samples from the Amdo basement (PK1A and
PK3A) (Fig. 1) have similar cooling histories, attributed to exhumation as a result of
the Early Cretaceous Lhasa-Qiangtang collision (Guynn et al., 2006).
Fission track data from north-central Tibet (Wang et al., 2008) suggest <4 km
of erosion since 60 Ma, yielding time-averaged erosion rates of ~0.05 km/myr. Very
little post-Eocene – Oligocene erosion is consistent with the preservation of pristine,
flat-lying 45 – 29 Ma volcanic rocks in the Qiangtang terrane (Ding et al., 2007).
Thermochronology: New work
We present the first low-temperature thermochronologic ((U-Th)/He apatite and
apatite fission track) data from samples collected away from the influence of major
late Cenozoic rifts (where Miocene and younger apatite He ages have been
determined; Kapp et al., 2008; Stockli et al., 2006) in the northern Lhasa and
southern Qiangtang terranes in central Tibet.
A total of 17 samples were collected for (U-Th)/He analyses, including: nine
samples from the southern Qiangtang terrane, seven samples from the northern
Lhasa terrane, and one sample from the southern Lhasa terrane near Lhasa (Fig.1).
Each sample consists of 2-4 replicates, except for three samples of poor quality
where only one aliquot was analysed.
The majority of the samples yield Eocene and older ages. Exceptions include
AR1, AR2, PK6 and SH2, and can be explained by local phenomena (Fig. 1). AR1,
AR2 and PK6 are all located north of the Bangong-suture zone and are located in the
hanging wall of a thrust fault of Tertiary age (Kapp et al., 2003). The youngest age of
17 Ma is obtained from sample AR2, which is located the closest to the fault trace.
AR1 and PK6 possess older ages of 19 Ma and 28 Ma, respectively, and were
sampled progressively farther away from the fault trace. These ages may indicate
local thrust reactivation and exhumation during the early Miocene, which perturbed
the thermal field in this area.
Sample SH2 was sampled in a presently externally drained part of the Plateau in
the southern Lhasa region and yields an age of 15 Ma. Previously obtained apatite
fission track ages for granites located to the south in the Lhasa area range from 1844 Ma, and were interpreted to represent the timing of movement along the
Gangdese thrust (Copeland et al., 1995). However, we interpret the youngest apatite
fission track ages (~18 Ma), and our obtained (U-Th)/He age, to mark onset of river
incision in the southern Lhasa region. The obtained (U-Th)/He age stands in sharp
contrast with (U-Th)/He ages just 50 km to the north that are ~50 Ma (Fig. 1). This
suggests the presence of a strong erosional gradient across this region of the
Tibetan Plateau.
The remaining samples in the southern Qiangtang and northern Lhasa terranes
(PU1, PK3A, PK1A, JV4, JV1, PK2, PK3, AR3, GT1, AR5, AR6) yield Eocene and
older apatite He ages (Fig. 1). These ages suggest that all samples were located at
uppermost-crustal levels (~2 km) no later than early Eocene. The oldest obtained (UTh)/He age of 122 Ma emphasizes the nature of persisting minimal erosion in the
Plateau interior. The most northerly located sample, AR4, has an age of 37 Ma,
indicating that this sample cooled through its closer temperature coeval with thrust
belt activity in the northern Qiangtang terrain (Kapp et al., 2007; Murphy et al., 1997).
Apatite fission track (AFT) and K-feldspar 40Ar/39Ar data were obtained from
samples that yielded Eocene and older (U-Th)/He ages in order to resolve continuous
cooling histories (Figs. 1, 2; see Data repository). Six AFT analyses yield ages
between 74 Ma and 53 Ma, consistent with the apatite (U-Th)/He results. They
indicate moderate to slow cooling (provide rates) during the late Cretaceous to early
Eocene . Our AFT results for the Amdo basement differ from Wang et al. (2007),
which may be due to poor quality or compositionally inhomogeneous apatites of the
Wang et al. (2007) study, offsetting their ages by a consistently younger factor.
Continuous cooling histories and erosion rates
The combination of (U-Th)/He, AFT and K-feldspar and biotite 40Ar/39Ar data
enables us to construct four continuous cooling histories (Fig. 2). All cooling histories
are similar and consist of a Cretaceous cooling event recorded by K-feldspar
40
Ar/39Ar, followed by continuous moderate cooling through the AFT- closure
temperature (110°C) (Naser and Faul, 2007), and the finally very slow cooling
through the (U-Th)/He closure temperature. We calculated the (U-Th)/He closure
temperature using the program Closure by Mark Brandon (for details see Datarepository) and obtained very low closure temperatures, between 46°C and 53°C, as
a result of the slow cooling experienced by the samples. If we use a standard
geothermal gradient for Tibet of ~30°C/km (Nansheng, 2002), this means that all
samples were emplaced into the upper-most crustal level (~1.5-2 km) by early
Eocene time. Thermal modeling results using Hefty (Fig. 2; see Data repository for
details) suggest that the samples cooled slowly through the rest of the Cenozoic with
a rate of 1-1.5 °C/myr, and an initially faster cooling event on the order of 3°C/myr
before Eocene time.
Using the aforementioned results, we calculated erosion rates using the
program AGE2EDOT by Mark Brandon (for details see Data-repository) to be on the
order of 0.03-0.05 km/myr. For comparision, these rates are twice in magnitude lower
than typical late Cenozoic erosion rates in the Himalaya (1-4 km/myr; Thiede et al.,
2005; Jain et al., 2000; Blythe et al., 2007; Wobus et al., 2008). Moreover these
rates suggest that minimal erosion and tectonic denudation occurred in the internally
drained central part of the Tibetan Plateau since early Eocene. The geomorphic
expression of a low eroding landscape is generally associated with low relief
(Montgomery et al., 2002), which is widespread in the central part of the plateau.
Very low erosional exhumation on the Plateau is supported by studies by Horton et
al. (2002) and Spurlin et al. (2005), which noted minimal mass removal in adjacent
areas of local Eocene to Miocene basins in the eastern Qiangtang terrane. However,
the erosion-rate estimates are somewhat lower than late Quaternary rates of 0.3- 4
km/myr determined for the interior of the Tibetan Plateau (Lal et al., 2003), which
may be a result of greater denudation during the most recent glacial cycles.
Discussion
The thermochronolgical results derived from the central Tibetan Plateau region
have important implications for the growth history of the Tibetan Plateau. Ages in the
northern Lhasa and southern Qiangtang terranes are Eocene and older in age.
Towards the north and south of this zone, ages become progressively younger. It is
to note however, that the youngest ages in the north and south are also found at the
lowest elevation (Fig. 3). The observed age distribution and trend relates temporally
and spatially to known ages of major thrust belt activity (Fig. 1; 3). Pre-Eocene and
Eocene ages, which are found along the Bangong-suture, as well as the first order
observation of a north and southward progressive younging of ages, can both be
related to coeval upper-crustal shortening events; in the first case with a propagation
of a fold and thrust belt during Late Cretaceous times (NLTB; Kapp et al., 2003: Kapp
et al., 2007) and in the second with a development of a thrust belt in the northern
Qiangtang terrane during early Oligocene times (HFNTB; Kapp et al., 2007; Murphy
et al., 1997) (Fig. 1). Interestingly, the majority of thermochronologic ages along the
Bangong suture do are all older than the Indo-Asian collision and can be attributed to
the earlier Lhasa-Qiangtang collision. These results suggest that the initial growth
and establishment of the Tibetan Plateau was not initiated by the collision of the
Indian and Asian plates, as invoked in most popular models of plateau formation
(Meyer et al., 1998), but rather from a zone of already thickened crust in the central
part of the Tibetan Plateau. Moreover, the theromochronologic results and temporalspatial distribution of upper-crustal shortening support models those invoke
northward and southward growth of a Tibetan Plateau during Indo-Asian collision
from a zone of thick crust in central Tibet that can be attributed to the collision of the
Lhasa and Qiangtang terranes along the Bangong suture zone in early Cretaceous.
We propose the establishment of a Lhasa-Qiangtang Proto-Plateau that is
characterized by early Eocene and older (U-Th)/He ages, requiring minimal
subsequent erosion and low relief similar to the modern day (Montgomery et al.,
2002). Younger (U-Th)/He ages represent local denudation events and modification
to an overall formed Proto-Plateau in pre Indio-Asian collision times. Later the IndianAsian collision itself reshaped the appearance of the Plateau and thus following
features can be ascribed to the most recent collision, broadening of the Tibetan
Plateau, the Himalayan Mountain range as well as localized, late Cenozoic
exhumation associated with regions of active upper-crustal extension (Kapp et al.,
2008; Stockli et al., 2006).
Studies from along the eastern margin of Tibet reveal the establishment of a
regional landscape that underwent minimal erosion since the end of the Cretaceous,
and disrupted by mid-late Tertiary exhumation in discrete tectonic zones, including
the steep and narrow topographic slope of the Longmen Shan ( Reid et al., 2005;
Arne et al., 1997; Xu and Kamp, 2000; Kirby et al., 2002; Fig. 1). The Longmen
Shan mountain range seem to be a Late Miocene to Pliocene development in
response to plateau expansion (Kirby et al., 2002). In the low-gradient margin of
southeastern Tibet, rocks at high elevation (> 3500 m) yield Cretaceous-early
Tertiary AFT and (U-Th)/He ages, and those at lower elevations within gorges (<
3000 m) yield late Tertiary ages (Clark et al., 2005). The authors interpret this split in
ages to be the result of Miocene river incision in response to surface uplift (Clark et
al., 2006). We note that the (U-Th)/He ages determined by Clark et al. (2005) from
samples at high elevation, AFT ages by Reid et al. (2004), and AFT-results from Lai
et al. (2007), are similar in age to the AFT and (U-Th)/He ages that we obtain from
the internally drained central part of the Plateau (Fig. 1). The obtained Eocene and
older ages at high elevation by Clark et al. (2005) imply similar low erosion rates, as
samples from the high plateau. The only major difference between these two regions
is that we observe major river incision in eastern Tibet. This incision has been
assumed to mark progressive surface uplift. However, our data indicate that the
Lhasa area was incised at the same time as eastern Tibet ~15 Ma, yet most available
evidence suggest that it was uplifted significantly earlier. A recent study by Wu et al.
(2008) noticed major Miocene lake deposits that are spanning from the central part of
the Plateau towards the east. The deposits cross the modern boundary between
internal and external drainage. This has significant implications, since lakes naturally
form in an internally drained area, but today parts of the area are being externally
drained, implying that the internally drained part of the plateau extended further east
in the past. This can be interpreted as headward river migration upstream showing a
non steady state configuration of the drainage network in the Tibetan region.
Collectively, these observations suggest that a Lhasa-Qiangtang Proto-plateau could
have extended much further eastward. Miocene onset of river incision in eastern
Tibet may be in response to a change in climate, such as intensification of the Asian
monsoons as this time, rather than surface uplift. Not only could have a possible
Proto-Plateau extended further towards the east, but a recent study by Van der Beek
et al. (in review) from the western edge of the Tibetan Plateau, found possible
remnants of a further extending westward Tibetan Plateau, that is today incised by
rivers and externally drained.
Conclusion
K-feldspar 40Ar/39Ar, AFT and (U-Th)/He data from the internally drained,
interior part of the Tibetan Plateau indicate that the Tibetan Plateau is not solely a
result of the Indo-Asian continent-continent collision, but parts of the plateau where
established before early Eocene. The Lhasa-Qiangtang Proto-Plateau is
characterized by major upper-crustal shortening during the Cretaceous to early
Eocene and (U-Th)/He ages older than ~50 Myr, suggesting that the plateau
characteristic of low-erosion rates was achieved by this time. Geomorphic and
comparison of low-temperature thermochronology from central Tibet and from the
eastern plateau margin are suggesting that a possible Proto-Plateau could have
extended much further eastwards. Furthermore, Miocene river incision at the Eastern
Plateau margin is coeval with the onset of river incision in the Lhasa area. All
existing low temperature thermochronolgic data are consistent with a model in which
the Tibetan Plateau grew northward and southward from a zone of thickened crust
along the Bangong-suture in response to the Indo-Asian collision.
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105(13): 4987-4992.
Wang, Y., T. Deng, et al. (2006). "Ancient diets indicate significant uplift of southern Tibet
after ca. 7 Ma." Geology 34(4): 309-312.
Wang, Y., X. Zhang, et al. (2007). "Cooling history and tectonic exhumation stages of hte
south-central Tibetan Plateau (China): Constrained by 40Ar/39Ar and apatite fission track
thermochronology." Journal of Asian Earth Sciences 29: 266-282.
Wobus, C., M. Pringle, et al. (2008). "
A Late Miocene acceleration of exhumation in the Himalayan crystalline core." Earth and
planetary science letters 269(1-2): 1-10.
Wu, Z.H., P.J. Barosh, et al.(2008)."Vast early Miocene lakes of the central Tibetan Plateau."
Geological society of america bulletin 120(9-10):1326-1337
Xu, G. and P. J. J. Kamp (2000). "Tectonics and denudation adjacent to the Xianshuihe Fault,
eastern Tibetan Plateau: Constraints from fission track thermochronology." Journal of
Geophysical Research 105(B5): 19231-19251.
Xu, G. Q. and P. J. J. Kamp (2000). "Tectonics and denudation adjacent to the Xianshuihe
Fault, eastern Tibetan Plateau: Constraints from fission track thermochronology." Journal of
Geophysical Research-Solid Earth 105(B8): 19231-19251.
Yuan, W.M., J.Q. Dong, et al., (2006)."Apatite fissiontrack evidence for Neogene uplift in the
eastern Kunlun Mountains, northern Qianghai-Tibet Plateau, China." Journal of asian earth
sciences 27(6): 847-856.
Acknowledgements
A particular thanks goes to S. Nicolescu for his assistance during the (U-Th)/He analysis
procedures, conducting the wet-chemistry analysis and measuring the samples on the ICPMS.
The apatite fission track work was performed at the Department of Geosciences of the
University of Potsdam. 40Ar/39Ar analysis for samples PK3A, JV4, PU1 was carried out by
Matthew Heizler at the New Mexico Geochronological Research Laboratory. Thanks go as
well to fellow graduate students for discussion and comments and all other people that
supported this work.
Figure captions
Figure 1. Monochromatic SRTM-30 base map showing major tectonic elements of the
Tibetan Plateau and locations of low temperature thermochronlogical studies and ages: green
stars- this study; red diamonds- Wang et al., 2008; blue triangles- Copeland et al., 1995; light
blue rectangles- Kirby et al., 2002; purple circles- Lai et al., 2007; dark blue circles- Reid et
al., 2005; pink circles- Clark et al., 2005; yellow pentagons- Jolivet et al., 2001; brown
flattend circles- Yuang et al., 2006; yellow hexagon- Van der Beek et al., in review. (UTh)/He ages are shown in green, AFT ages are shown in orange. Purple lines represent
outlines of terrains: IYS- Indus Yarlung Suture; BNS- Bangong Suture; JS- Jinsha River
suture; Kunlun Suture. Yellow and aquamarine lines represent extend of upper crustal
shortening events. Main tectonic elements and age of activity: QCUL- Qiangtang
Culmination; NLTB- Northern Lhasa Thrust Belt; GRTB- Gangdese Retroarc Thrust Belt;
THTB- Tethyan Himalayan Thrust Belt; Himalayan Thrust Belt; HFNTB- Hoh-Xil-Fenghuo
Shan–Nanqian thrust belt; KQTB- Kunlun-Qaidam Thrust Belt. Red outline represents area of
suggested Lhasa-Qiangtang Proto-Plateau
Figure 2. Continuous cooling histories for samples PK3A, JV4, PU1 and JV1 with a 2-sigma
error for the closure temperature. Area in pink represents HeFTy modeling results for
individual samples cooling paths. No modeling was obtained for JV4.
Figure 3. The upper part shows a longitudinal (N-S) mean topographic swath profile along
93°E. Superimposed are projected sample locations and its elevations into the plane of the
profile. Purple dashed lines indicate locations of suture zones; for abbreviations refer to figure
1. Lower part shows ages of projected samples into the plane of the profile. Grey boxes are
representing times of upper crustal shortening events. Orange circles represent AFT-ages on
the same sample that is represented as a green star (This study).
Figure 2
Figure 3
Data Repository
m Item
Supplementary Geochronologic and Thermochronologic Data
Table DR01 – Location and description of samples
Sample
Lon.
Lat.
Description
Crystallization
Age
AFT pooled
age (Ma)
Source
PK1A (PK-97-6-4-1A)
31.881 91.699 Amdo orthogneiss
Cambrian
72.9 ± 3.0 Kspar - Guynn et al., 2006; AFT - this study
PK1B (PK-97-6-4-1B)
31.881 91.699 Amdo orthogneiss
Cambrian
72.7 ± 4.4 AFT - this study
PK2 (PK-97-6-4-2)
32.026 91.705 Amdo orthogneiss
Precambrian
74.4 ± 3.0 Kspar - Guynn et al., 2006; AFT - this study
PK3A (PK-97-6-4-3A)
32.124 91.711 Amdo orthogneiss
Precambrian
71.9 ± 3.1 Kspar - Guynn et al., 2006; AFT - this study
PU4 (AP070504-A)
33.250 92.007 Tanggula granitoid
Late Cretaceous
53.6 ± 2.1 U-Pb, Kspar and AFT - this study
JV1 (JV61504-1)
31.335 89.875 Bange granite
Early Cretaceous
61.7 ± 2.9 AFT - this study
JV4 (JV61504-4)
31.365 89.892 Bange granite
Early Cretaceous
53.7 ± 2.6 AFT and Kspar - this study
SQH (97-7-3-3bpk)
32.496 80.328 Longzi La granite
Early Cretaceous
Kspar - Kapp et al., 2003a
JG1 (JG082103-4)
31.515 85.087 Xiagangjiang granitoid Early Cretaceous
Kspar - this study
NMA (7-19-98-2)
32.220 87.220 Xiabie granite
Early Cretaceous
Kspar - Kapp et al., 2007
Lon. - Longitude; Lat. - Latitude; AFT - Apatite Fission Track; Kspar - K-Feldspar Argon/Argon. Sample numbers in parentheses are original
sample numbers and/or from previous sources.
GEOCHRONOLOGIC & THERMOCHRONOLOGIC METHODS
U-Pb GEOCHRONOLOGIC METHODS for PU-4 (AP070504-A)
All separation and analysis was carried out at the University of Arizona. Zircons were
separated from a ~2 kg sample of fresh rock using standard crushing and separation techniques,
including a water table, magnetic separator and heavy liquids. Individual zircon crystals were
then hand-picked under a binocular microscope to select large, inclusion free grains.
Approximately 50 grains were then mounted in epoxy and polished to approximately 2/3 of the
crystal thickness. The grains were ablated with a New Wave DUV193 Excimer laser, operating
at a wavelength of 193 nm and using a spot diameter of 35 microns. The ablated material is then
carried in argon gas into the plasma source of a Micromass Isoprobe multicollector ICP-MS and
ionized. The Micromass Isoprobe is equipped with a flight tube of sufficient width that U, Th,
and Pb isotopes are measured simultaneously using nine Faraday collectors, an axial Daly
detector and four ion-counting channels. All measurements were made in static mode, using
Faraday detectors for
238
U, 232Th, and
208-206
Pb and an ion-counting channel for
204
Pb.
235
U is
determined from the 238U measurement assuming 238U/235U = 137.88. Ion yields are ~1 mV per
ppm. Each analysis consists of one background run with 20-second integration on peaks and the
laser off, 20 1-second integrations with the laser firing, and a 30-second delay to purge for the
next analysis. The laser ablates at ~1 micron/second, resulting in an ablation pit ~20 microns in
depth. A common lead correction was performed by using the measured
204
Pb and assuming an
initial Pb composition from Stacey and Kramers (1975) (with uncertainties of 1.0, 0.3, and 2.0
for
206
Pb/204Pb,
207
Pb/204Pb, and
by the presence of
208
Pb/204Pb, respectively). Measurement of
204
Pb is unaffected
204
Hg because backgrounds are measured on peaks (thereby subtracting any
background 204Hg and 204Pb) and because very little Hg is present in the argon gas.
The errors in determining
206
Pb/238U and
206
Pb/204Pb result in a measurement uncertainty of
several percent (at 2- level) in the 206Pb*/238U age. The low concentrations of 207Pb in younger
samples (approximately < 1000 Ma), due to the low concentration of
in a substantially larger measurement uncertainty for
235
206
Pb/207Pb.
206
U relative to 238U, result
The
207
Pb*/207Pb* ages for younger grains accordingly have larger uncertainties.
Pb*/235U and
Inter-element
fractionation of Pb/U is generally <20%, whereas isotopic fractionation of Pb is generally <5%.
These fractionations were corrected by analysis of a standard, with a known, concordant IDTIMS age, between every three unknown analyses. The zircon standards are fragments of a
large zircon crystal from a Sri Lankan pegmatite (e.g. Dickinson and Gehrels, 2003) with an age
of 564 ± 4 Ma (2 ). The uncertainty resulting from the calibration correction is generally ~3%
(2 ) for both
207
Pb/206Pb and
206
Pb/238U ages. U and Th concentrations were determined by
analyzing a piece of NIST 610 glass with ~500 ppm U and Th and using the resulting measured
intensities to calibrate the sample measured U and Th intensities.
The crystallization age reported in this paper (69.2 ± 1.4 Ma; Fig. DR01b) is a weighted
average of individual, concordant spot analyses using the 206Pb*/238U ages, since the 207Pb*/235U
and 207Pb/206Pb* ages are less precise for these younger granitoids. The stated uncertainties (2 )
on the assigned crystallization ages are absolute values and include contributions from all known
random and systematic errors. Random errors are included in the data tables. Systematic errors
(2 ) is 1.05%. Analyses which have very low 206P/204Pb ratios, high 204Pb intensities or very low
U concentrations are discarded due to the impractically high uncertainty.
Fig. DR01a is a concordia plot of all the acceptable spot analyses (24 total); Fig. DR01b
shows the weighted mean of the
206
Pb*/238U ages.
All U-Pb plots and weighted average
calculations were made using Isoplot 3.00 (Ludwig, 2003).
U-Pb References
Dickinson, W.R. and Gehrels, G.E., 2003. U-Pb ages of detrital zircons from Permian and
Jurassic eolian sandstones of the Colorado Plateau, USA: paleogeographic implications.
Sedimentary Geology, 163(1-2): 29-66.
Ludwig, K.R., 2003. Berkeley Geochronology Center Special Publication No. 4.
Stacey, J.S. and Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a
two-stage model. Earth and Planetary Science Letters, 26(2): 207-221.
Table DR02 – U-Pb data for PU4 (AP070504-A)
Isotopic ratios
Sample
U
spot
(ppm)
206
Pb
204
U/Th
Pb
207
Pb*
235
± (%)
U
206
Pb*
238
Apparent ages (Ma)
± (%)
U
error
corr
206
Pb*
238
± (Ma)
U
207
Pb*
235
± (Ma)
U
206
Pb*
207
Pb*
± (Ma)
1C
809
2839
0.8
0.07890
16.11
0.01077
2.22
0.138
69
1.5
77
12.0
334
364
2C
270
912
1.2
0.05700
55.63
0.01209
4.80
0.086
77
3.7
56
30.5
-773
1675
5C
445
914
0.7
0.10230
23.91
0.01227
2.56
0.107
79
2.0
99
22.5
621
520
7C
2591
3901
1.8
0.07470
6.04
0.01091
1.32
0.219
70
0.9
73
4.3
179
138
8C
2107
5641
0.9
0.06650
7.20
0.01033
1.72
0.239
66
1.1
65
4.6
34
168
11C
259
128
2.5
0.11662
38.49
0.01170
7.55
0.196
75
5.6
112
40.8
994
795
13C
1049
1827
3.4
0.11449
47.74
0.01085
1.13
0.024
70
0.8
110
49.8
1110
1014
14C
399
870
0.9
0.07962
24.63
0.01021
4.53
0.184
65
3.0
78
18.4
475
543
15C
583
991
1.0
0.07610
27.48
0.01097
2.83
0.103
70
2.0
74
19.7
210
644
16T
352
946
1.6
0.09090
32.96
0.01074
4.67
0.142
69
3.2
88
27.9
653
718
17T
546
666
1.0
0.10227
50.75
0.01216
7.99
0.157
78
6.2
99
47.8
639
1150
18T
407
2506
1.5
0.11120
22.94
0.01169
3.72
0.162
75
2.8
107
23.3
899
473
19T
506
2224
1.7
0.07856
26.30
0.01092
3.78
0.144
70
2.6
77
19.5
293
603
20T
546
535
1.3
0.07819
12.70
0.01017
1.46
0.115
65
0.9
76
9.3
444
281
21T
412
1822
1.5
0.09208
17.61
0.01110
1.96
0.111
71
1.4
89
15.1
608
381
22T
493
2172
1.9
0.09422
21.04
0.01127
2.23
0.106
72
1.6
91
18.4
626
456
23T
550
2224
2.2
0.07497
27.00
0.01188
3.42
0.127
76
2.6
73
19.1
-15
658
24T
1092
5173
1.6
0.07843
11.34
0.01071
1.46
0.128
69
1.0
77
8.4
333
256
25T
601
2990
1.4
0.08500
18.98
0.01037
2.02
0.106
66
1.3
83
15.1
584
413
26T
438
756
1.7
0.07692
32.45
0.01119
4.25
0.131
72
3.0
75
23.5
188
766
27T
604
3555
1.5
0.08199
18.52
0.01061
2.80
0.151
68
1.9
80
14.2
454
409
28T
360
783
1.5
0.08921
49.20
0.01124
3.88
0.079
72
2.8
87
40.9
513
1145
29T
384
1766
1.7
0.09893
25.90
0.01128
4.68
0.181
72
3.4
96
23.7
728
548
30T
756
1510
1.7
0.11421
16.06
0.01058
2.22
0.138
68
1.5
110
16.7
1154
318
0.020
a)
110
0.016
0.012
70
206
Pb*/
238
U
90
50
0.008
30
0.004
10
0.000
-0.05
0.05
0.15
207
U
b)
85
75
206
Pb*/
238
U Age (Ma)
95
235
Pb*/
0.25
65
Mean = 69.2 ± 1.2 Ma
Average = 69.2 ± 1.4 Ma
MSWD = 3.6
55
Figure DR01. U-Pb plots for sample PU4 (AP070504-A). All uncertainties are at the 2-σ level. a)
Condordia plot of all analyzed zircons. b) Weighted average of 206Pb/238U ages. Green bar represents
the mean. The "Mean" age only includes random uncertainties; the "Average" age includes random and
systematic uncertainties and is the reported age for this sample.
40Ar/39Ar
THERMOCHONOLOGIC METHODS
40
A r /39A r M E T H ODS A ND A G E C A L C UL A T I ONS
A ll isotopic data were measured with a MA P-215-50 mass spectrometer equipped
either a B alzers SE V 217 or Johnston electron multiplier. T he multipliers are operated at
about 1.3 kV or 2.2 kV , respectively and yield a gain above the Faraday collector of
about 5000 to 20,000 depending upon source sensitivity. R esolution at 5% peak-height at
mass 40 was 450 to 600. A dditional information about the New Mexico Geochronology
R esearch L aboratory can be found within New Mexico B ureau of Geology and Mineral
R esources open file report OF-A R -1 at
http://geoinfo.nmt.edu/publications/openfile/argon/home.html.
Furnace Step-Heating:
T he K -feldspar and biotite were step-heated in a double vacuum Mo resistance
furnace. Heating times for the K -feldspars were highly variable and are designed to
maximize recovery of the diffusion coefficients and to resolve excess argon
contamination by performing several isothermal replicate-heating steps (T able DR03).
Heating time for the biotite was 5 minutes for each step. T he samples were gettered
during heating using a SA E S GP-50 getter operated at ~450¡ C . Following heating, gas
was expanded to a second-stage of the extraction line where it was reacted with 2 GP-50
getters (one at 20¡ C , one at ~450¡ C ) and a W filament operated at about 2000¡ C . T he K feldspar gettering time in the second stage was 1-2 minutes, whereas biotite was gettered
for 3 minutes. T he furnace thermocouple was calibrated by melting copper foil and the
recorded temperature underestimated the sample temperature by 15 to 50¡ C . For the K feldspar data, correction of thermocouple temperature to the Cu foil melting point was
done and the reported temperature in the data table is calibrated to the foil melting.
E stimated accuracy of the heating temperature is ±15¡ C for any given step.
B lanks and backgrounds for the K -feldspar was determined during a 15-minute,
800¡ C blank run. For the long and higher temperature heating steps this method undercorrects the true blank and therefore the reported radiogenic yield for these steps contains
DR 1-1
atmospheric argon that is not solely derived from the samples. However, the blank
contribution to the long heating steps is still less than 5% of the total 40A r signal. B lanks
for the biotite were run before, during and after the step heating and typically yielded
values of 2x10-15, 4.7x10-18, 6x10-18, 1.5x10-17, and 4.5x10-18 moles for masses 40, 39, 38,
37 and 36, respectively.
I rradiation, Flux monitoring and A ge Calculations:
T wo irradiations were preformed. Samples from NM-173 were irradiated at the
Hamilton, Canada McMaster reactor (position 5C). I t was 75 MW hours with samples
being irradiated in air while enclosed within an A l vessel. NM-194 samples were
irradiated within the central thimble at the USGS Denver T riga reactor for 15 hours. Here
samples we vacuum enclosed within a quartz vessel prior to irradiation.
Fluence gradients were monitored with Fish Canyon (FC-2) sanidine with an
assigned age of 28.02 Ma (R enne et al., 1998). Fusing 4-10 individual crystals from each
location monitored 4 to 6 locations within individual sample trays. A plane was fit to the
weighted mean value of each location and J-factors were determined for the unknowns
based on their geometry and the calculated curve. J-factor errors range from 0.1 to 0.17%
(1! ). Correction factors for interfering reactions were measured with K -glass and CaF 2.
T ypically 4 to 5 grains of each were fused with the CO 2 laser to obtain a weighted mean
value for each correction factor.
A plateau age is determined from the inverse variance weighted mean of the
chosen steps with the error magnified by the square root of the MSWD for MSWDÕs
greater than 1 (T able DR03). T otal gas ages and errors were calculated by quadradically
summing isotopic values from each heating step.
K -feldspar Multiple Diffusion Domain (MDD) thermochronology:
T he determination of a thermal history using the MDD method (L overa et al.,
1989) requires many steps that are outlined below. T his is an abbreviated description of
the method and more details can be obtained at the New Mexico B ureau of Geology open
file report #26 by Sanders and Heizler (2005) or data repository from Sanders et al.
DR 1-2
(2006). T he step-heating schedule is designed to resolve the A rrhenius parameters (e.g.
activation energy, diffusion coefficients) of 39Ar transport, a high-resolution age spectrum
and to correct of excess argon (T ables DR03 and DR04). T o correct the age spectrum for a
characteristic behavior of excess argon release we compare the relationship between
apparent age and release of 38A r derived from chlorine (Harrison et al., 1993; 1994). I n
this study, JG082103-4 K -feldspar reveals significant excess argon that displays a
characteristic oscillation in apparent age for isothermal heating steps that shows an old
age for the first isothermal step followed by a younger age for the second step (Figure DR02;
T able DR03). T his age variation also correlates with a change in amount of 38A r derived from
chlorine that is interpreted to be a systematic decrepitation of fluid inclusions that contain
both excess 40A r and chlorine. T able DR04 shows the relationship between age ( 40A r*/39A rK )
and chlorine ( 38A rCl/ 39ArK ) and Figure DR03 plots the difference between these values for the
first 5 isothermal duplicate steps. T he slope of a best-fit line with y-intercept of zero
yields the relationship between excess 40A r and chlorine ( 40A rE /38A rCl) that is then used to
produce a Cl-corrected age spectrum for MDD thermal history determination. T he
chlorine corrected ages are given in T able DR04. For sample 5July-4 K -feldspar there is very
minimal excess argon and no correction was required (Figure DR04). For JV 61504 K -feldspar
(Figure DR05) the spectrum has oscillatory age behavior for the first few percent39of
Ar
released that was corrected in a fashion outlined by Sanders et al. (2006). A more
problematic part of the spectrum is the intermediate age hump (cf. L overa et al., 2002)
between about 10 and 20% of 39A r released. Here the measured ages are corrected in a
somewhat ad hoc fashion prior to MDD modeling by setting the ages to 102 Ma with the
large uncertainty shown by the green spectrum in Figure DR03. T his is done to prevent the
automated modeling process from trying to fit the complex part of the spectrum, but also
assumes that the age gradient from about 100 to 110 Ma is accurate.
T he diffusion coefficients for 39A r are calculated for a plane-sheet geometry using
the fraction of
39
A r released and time of each heating step. T hese data are used to
construct the A rrhenius and log(r/ro) plots (Figures DR02,03,04; b & c). T he activation
energy (E ) and initial diffusion coefficient is given by the slope of the low temperature
diffusion coefficients on the A rrhenius plot and this A rrhenius law is referred to as ro.
Using these parameters, the entire release of 39A r for steps at or below 1100¡ C are used to
DR 1-3
determine the diffusion domain distribution. T he log(r/ro) is simply a different way of
viewing the A rrhenius relationship and links the percent of
39
A r to individual diffusion
coefficients (L overa et al., 1991).
Once the domain distribution is determined, the measured age spectrum is
forward modeled by inputting thermal histories until a good match is produced between
the measured and modeled age spectrum. For this paper we the automated routines
(Quidelleur et al., 1997) to deliver a family of thermal histories (Figures DR01,03,04; d).
DR 1-4
R eferences cited
Harrison, T .M., Heizler, M.T ., L overa, O.M., 1993, I n vacuo crushing experiments and
K -feldspar thermochronometry. E arth. Planet. Sci. L ett., 117, 169-180.
Harrison, T .M., Heizler, M.T ., L overa O.M. and Wenji, C., 1994, A chlorine disinfectant
for excess argon. E arth Planet. Sci. L ett., 123, 95-104.
L overa, O.M., Grove, M., and Harrison, T .M., 2002, Systematic analysis of K -feldspar
40
A r/39A r step-heating results: I I . R elevance of laboratory argon diffusion properties
to nature, Geochim. Cosmochim. A cta, v. 66, no. 7, p. 1237-1255.
L overa, O.M., Harrison, T .M., R ichter, F.M., 1991, Diffusion domains determined by
39
A r released during step heating, Journal of Geophysical R esearch, v. 96, p. 20572069.
L overa, O.M., R ichter, F.M., Harrison, T .M., 1989, 40A r/39A r thermochronology for
slowly cooled samples having a distribution of domain sizes: Journal of Geophysical
R esearch, v. 94, p. 17,917-17,935.
Quidelleur, X ., Grove, M., L overa, O.M., Harrison, T .M., Y in, A ., 1997, T hermal
evolution and slip history of the R enbu Zedong T hrust, southeastern T ibet: Journal of
Geophysical R esearch, v. 102, n. B 2, p. 2659-2679.
R enne, P. R ., Swisher, C. C., Deino, A. L ., K arner, D. B., Owens, T . L ., and DePaolo, D.
J., 1998, I ntercalibration of standards, absolute ages and uncertainties in 40A r/39Ar
dating: Chemical Geology, v. 145, no. 1-2, p. 117-152.
Sanders, R .E . and Heizler, M.T ., 2005, E xtraction of MDD thermal histories from
40
A r/39A r K -feldspar step heating data: NMB GMR Open File R eport OF-A R 26.
Sanders, R .E ., Heizler, M.T . and Goodwin, L .B., 2006, 40A r39A r thermochronology
constraints on the timing of Proterozoic basement exhumation and fault ancestry,
southern Sangre de Cristo R ange, New Mexico, Geol. Soc. A m. B ull., v. 118, no.
11/12, p. 1489-1506.
Steiger, R . H., and JŠger, E ., 1977, Subcommission on geochronology: Convention on
the use of decay constants in geo- and cosmochronology: E arth and Planetary
Science L etters, v. 36, p. 359-362.
T aylor, J.R ., 1982. A n I ntroduction to E rror A nalysis: T he Study of Uncertainties in
Physical Measurements,. Univ. Sci. B ooks, Mill V alley, Calif., 270 p.
DR 1-5
Table DR03. Argon isotopic data and age assignments.
ID
Temp
Ar
Age
(%)
(Ma)
(Ma)
52.9
68.36
69.15
68.53
68.38
68.91
69.00
69.10
68.67
67.80
68.74
2.3
0.61
0.14
0.14
0.28
0.13
0.12
0.13
0.14
0.69
0.14
99.4
68.87
0.13
JV61504-4, Biotite, 4.96 mg, J=0.003657±0.17%, D=1.002±0.001, NM-194E, Lab#=56146-01
A
640
14.54
0.0641
26.62
6.19
8.0
45.9
1.4
B
740
19.17
-0.0003
1.887
48.2
97.1
12.0
C
840
19.02
0.0014
0.7691
120.6
362.3
98.8
38.5
D
910
19.45
0.0090
0.8261
46.6
56.5
98.8
48.7
E
990
19.82
0.0171
0.8646
57.4
29.8
98.7
61.4
F
1065
18.98
0.0134
0.2577
112.9
38.0
99.6
86.2
G
1100
18.93
0.0239
0.3831
28.46
21.4
99.4
92.4
H
1170
18.89
0.1218
0.0037
24.27
4.2
100.1
97.8
I
1200
18.87
0.1466
1.399
6.68
3.5
97.9
99.3
J
1240
20.07
0.0777
7.934
3.38
6.6
88.3 100.0
Integrated age ± 1!
n=10
454.8
27.8
K2O=9.63%
43.49
118.72
119.85
122.42
124.65
120.56
120.02
120.53
117.88
113.3
119.73
0.90
0.29
0.23
0.31
0.23
0.19
0.24
0.25
0.69
1.1
0.25
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
-3
(¡C)
(x 10 )
39
(x 10
ArK
-15
K/Ca
x
x
x
x
x
x
x
x
x
x
steps B-J
Plateau ± 1! no plateau
n=9
n=0
MSWD=2.80
MSWD=0.00
461.4
0.000
DR 1-6
Ar*
(%)
mol)
5July-04, Biotite, 5.21 mg, J=0.0036038±0.13%, D=1.003±0.001, NM-194B, Lab#=56112-01
x A
640
39.52
0.1349
105.8
2.76
3.8
B
740
17.25
0.0172
22.09
17.9
29.7
C
840
12.07
0.0053
4.132
86.5
96.7
D
910
11.11
0.0032
1.212
91.2
158.5
E
990
11.13
0.0095
1.360
31.2
53.7
F
1065
11.27
0.0262
1.566
44.9
19.5
G
1100
11.50
0.0300
2.282
49.8
17.0
H
1170
11.66
0.0666
2.771
88.5
7.7
I
1200
11.47
0.0470
2.370
46.6
10.9
J
1240
11.56
0.0256
3.129
4.83
19.9
Integrated age ± 1!
n=10
464.1
18.8
Plateau ± 1!
40
39
20.9
0.6
62.2
4.5
89.9
23.1
96.8
42.7
96.4
49.5
95.9
59.1
94.2
69.9
93.0
88.9
93.9
99.0
92.0 100.0
K2O=9.50%
60.7 ±50.6
0.000±0.000
0.0
0.00
±1!
0.000
Time
(min)
Table DR03 cont'd. Argon isotopic data and age assignments.
ID
Temp
(¡C)
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
-3
(x 10 )
39
(x 10
ArK
-15
K/Ca
DR 1-7
Ar*
(%)
mol)
5July04, K-Feldspar, 15.72 mg, J=0.0036097±0.14%, D=1.002±0.001, NM-194B,
B
450 557.2
0.1496
1834.4
1.39
C
450 162.5
0.1835
505.1
0.532
D
500
61.75
0.0929
173.6
1.02
E
500
30.88
0.0758
71.35
1.47
F
550
26.39
0.0556
55.25
2.92
G
550
15.03
0.0515
16.47
3.97
H
600
22.58
0.0657
41.01
9.39
I
600
12.14
0.0591
5.951
8.44
J
650
15.12
0.0500
15.56
13.7
K
650
11.53
0.0408
3.615
13.6
L
700
12.52
0.0373
6.323
14.4
M
700
11.00
0.0308
1.571
19.8
N
750
12.21
0.0302
5.611
26.0
O
750
11.09
0.0230
1.765
26.5
P
800
11.88
0.0198
4.500
32.4
Q
800
11.22
0.0172
1.964
23.9
R
850
12.18
0.0161
5.090
24.6
S
850
11.19
0.0130
2.095
27.2
T
900
13.12
0.0124
8.289
30.6
U
900
12.06
0.0120
4.989
27.0
V
950
15.20
0.0116
15.26
37.9
W
950
13.77
0.0086
10.22
30.6
X
1000
15.99
0.0105
17.62
58.7
Y
1000
14.38
0.0086
12.28
61.9
Z
1050
14.96
0.0085
14.08
110.6
AA
1050
13.28
0.0094
8.637
66.2
AB
1100
13.60
0.0119
9.580
101.9
AC
1100
13.03
0.0119
7.631
85.1
AD
1100
12.77
0.0089
6.813
100.4
AE
1100
12.73
0.0071
6.655
99.1
AF
1200
12.78
0.0056
6.479
71.1
AG
1250
13.54
0.0067
9.249
62.9
AH
1350
13.52
0.0107
8.730
38.2
AI
1690
13.28
0.0143
8.012
60.3
Integrated age ± 1!
n=34
1293.7
40
Lab#=56111-01
3.4
2.8
5.5
6.7
9.2
9.9
7.8
8.6
10.2
12.5
13.7
16.6
16.9
22.2
25.8
29.7
31.8
39.2
41.0
42.5
44.1
59.2
48.7
59.3
60.4
54.0
43.0
43.0
57.3
71.5
91.2
75.7
47.5
35.7
37.5
Ar
Age
(%)
(Ma)
(Ma)
96.0
84.5
66.8
62.7
64.3
64.99
66.86
66.34
67.21
66.85
68.01
67.28
67.43
67.52
67.40
67.95
68.16
67.49
68.13
67.62
68.24
68.60
68.85
68.61
68.91
68.49
68.72
68.79
68.67
68.69
69.34
68.98
69.83
69.64
68.57
15.5
7.9
3.4
2.1
1.2
0.69
0.59
0.32
0.35
0.22
0.23
0.17
0.18
0.15
0.16
0.16
0.19
0.15
0.19
0.17
0.24
0.21
0.23
0.19
0.19
0.16
0.16
0.15
0.14
0.14
0.15
0.17
0.18
0.16
0.18
39
2.7
0.1
8.2
0.1
16.9
0.2
31.7
0.3
38.1
0.6
67.6
0.9
46.3
1.6
85.5
2.3
69.6
3.3
90.8
4.4
85.1
5.5
95.8
7.0
86.4
9.0
95.3
11.1
88.8
13.6
94.8
15.4
87.6
17.3
94.5
19.4
81.3
21.8
87.8
23.9
70.3
26.8
78.0
29.2
67.4
33.7
74.8
38.5
72.2
47.0
80.8
52.1
79.2
60.0
82.7
66.6
84.2
74.4
84.5
82.0
85.0
87.5
79.8
92.4
80.9
95.3
82.2 100.0
K2O=8.76%
±1!
Time
(min)
10.7
20.9
9.9
20.4
9.1
20.0
12.1
22.1
12.0
22.1
12.0
22.2
11.8
22.0
12.5
22.6
12.4
22.5
12.8
22.9
12.7
22.8
12.8
22.8
12.8
22.8
12.8
22.8
56.8
116.8
6.7
6.6
7.0
3.0
Table DR03 cont'd. Argon isotopic data and age assignments.
ID
Temp
(¡C)
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
-3
(x 10 )
39
(x 10
ArK
-15
mol)
K/Ca
40
Ar*
(%)
Ar
Age
(%)
(Ma)
(Ma)
(min)
682.8
129.0
365.0
95.5
200.36
88.09
121.72
91.18
101.09
97.00
101.22
102.26
105.16
105.89
106.59
106.86
106.50
106.54
105.48
103.90
103.01
102.87
102.31
102.71
103.28
103.82
105.51
106.85
107.99
108.81
109.78
111.60
111.82
113.40
110.11
3.8
4.1
1.5
1.1
0.61
0.52
0.28
0.26
0.23
0.21
0.20
0.24
0.20
0.21
0.22
0.23
0.23
0.22
0.24
0.21
0.23
0.21
0.20
0.19
0.19
0.17
0.17
0.17
0.15
0.14
0.14
0.15
0.15
0.15
0.18
10.9
20.9
9.8
20.5
9.1
19.9
12.0
22.3
12.0
22.1
12.1
22.3
11.8
22.1
12.4
22.6
12.4
22.5
12.8
22.9
12.7
22.8
12.8
22.9
12.8
22.8
12.8
22.8
56.9
116.8
6.6
6.6
6.9
3.0
39
JV61504-4, K-Feldspar, 15.56 mg, J=0.0036288±0.12%, D=1.002±0.001, NM-194B, Lab#=56119-01
B
450 254.4
0.0264
431.8
2.37
19.3
49.8
0.2
C
450
68.88
0.0860
164.0
0.729
5.9
29.7
0.2
D
500
74.81
0.0420
44.02
2.44
12.2
82.6
0.4
E
500
19.65
0.0511
15.80
2.34
10.0
76.3
0.5
F
550
37.91
0.0233
18.71
6.51
21.9
85.4
0.9
G
550
14.73
0.0186
3.149
4.92
27.4
93.7
1.2
H
600
21.07
0.0131
6.195
15.7
38.9
91.3
2.2
I
600
14.68
0.0156
1.299
11.8
32.6
97.4
3.0
J
650
16.82
0.0130
3.149
18.5
39.3
94.5
4.1
K
650
15.43
0.0105
0.6780
15.9
48.4
98.7
5.1
L
700
16.43
0.0097
1.751
20.7
52.3
96.9
6.4
M
700
16.22
0.0093
0.4825
18.5
54.6
99.1
7.6
N
750
16.97
0.0093
1.424
22.4
54.8
97.5
9.0
O
750
16.80
0.0110
0.4621
15.3
46.2
99.2
10.0
P
800
17.10
0.0116
1.093
21.7
43.8
98.1
11.4
Q
800
16.97
0.0121
0.4764
17.7
42.3
99.2
12.5
R
850
17.19
0.0136
1.447
17.5
37.5
97.5
13.6
S
850
16.99
0.0092
0.7238
15.7
55.7
98.7
14.6
T
900
17.30
0.0150
2.355
14.2
34.1
96.0
15.5
U
900
16.72
0.0110
1.266
17.6
46.3
97.8
16.6
V
950
17.09
0.0110
3.021
20.8
46.2
94.8
17.9
W
950
16.92
0.0089
2.515
21.7
57.2
95.6
19.3
X
1000
17.51
0.0086
4.799
27.6
59.6
91.9
21.0
Y
1000
17.43
0.0086
4.328
35.9
59.6
92.7
23.3
Z
1050
17.91
0.0132
5.620
51.3
38.5
90.7
26.5
AA
1050
17.74
0.0121
4.756
56.5
42.1
92.1
30.1
AB
1100
17.94
0.0153
4.502
76.3
33.3
92.6
34.9
AC
1100
17.92
0.0132
3.717
69.9
38.7
93.9
39.4
AD
1100
17.83
0.0083
2.767
97.6
61.3
95.4
45.5
AE
1100
17.84
0.0048
2.357
119.1
105.2
96.1
53.1
AF
1200
17.95
0.0025
2.202
153.3
204.4
96.4
62.8
AG
1250
18.13
0.0012
1.807
338.8
442.5
97.1
84.2
AH
1350
18.16
0.0023
1.795
152.5
224.9
97.1
93.9
AI
1690
18.40
0.0052
1.738
96.9
97.3
97.2 100.0
Integrated age ± 1!
n=34
1580.3
74.5
K2O=10.75%
DR 1-8
±1!
Time
Table DR03 cont'd. Argon isotopic data and age assignments.
ID
Temp
(¡C)
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
-3
(x 10 )
39
(x 10
ArK
-15
mol)
K/Ca
40
Ar*
(%)
Ar
Age
(%)
(Ma)
39
JG082103-4, K-feldspar, 11.63 mg, J=0.007862±0.10%, D=1.0035±0.0005, NM-173L, Lab#=54573-01
B
450 123.0
0.0149
89.09
8.50
34.2
78.6
0.3 1019.5
C
450
12.17
-0.0263
13.37
3.78
67.4
0.5
112.6
D
500
17.04
0.0100
5.952
13.3
51.2
89.7
1.0
204.38
E
500
3.179
0.0093
2.112
8.49
54.6
80.2
1.3
35.49
F
550
12.96
0.0232
3.464
40.3
22.0
92.1
2.9
161.51
G
550
2.815
0.1266
1.089
13.7
4.0
88.8
3.4
34.77
H
600
9.479
0.0992
2.722
63.2
5.1
91.6
5.8
118.78
I
600
2.866
0.1342
0.7153
18.2
3.8
92.9
6.6
37.02
J
650
3.571
0.0572
0.9085
29.6
8.9
92.6
7.7
45.92
K
650
2.443
0.0150
0.3018
21.5
34.0
96.4
8.5
32.70
L
700
2.663
0.0310
0.3619
26.1
16.5
96.0
9.5
35.53
M
700
2.412
0.0150
0.2579
26.9
34.1
96.9
10.6
32.45
N
750
2.560
0.0120
0.3234
34.4
42.4
96.3
11.9
34.24
O
750
2.451
0.0045
0.2249
32.5
112.6
97.3
13.2
33.10
P
800
2.639
0.0067
0.2804
38.6
76.2
96.8
14.7
35.503
Q
800
2.570
0.0066
0.2255
34.8
77.7
97.4
16.0
34.772
R
850
2.715
0.0099
0.2935
39.6
51.7
96.8
17.5
36.510
S
850
2.781
0.0046
0.2951
36.0
110.7
96.8
18.9
37.416
T
900
3.106
0.0119
0.5596
37.7
42.9
94.7
20.4
40.84
U
900
3.071
0.0014
0.4297
41.8
373.8
95.8
22.0
40.877
V
950
3.495
0.0140
0.6780
49.1
36.6
94.3
23.9
45.75
W
950
3.558
0.0080
0.6253
59.1
64.0
94.8
26.2
46.828
X
1000
4.049
0.0135
0.7095
89.0
37.9
94.8
29.6
53.275
Y
1000
4.079
0.0055
0.6283
107.8
92.0
95.4
33.8
53.999
Z
1050
4.274
0.0078
0.6073
160.5
65.2
95.8
40.0
56.782
AA
1050
4.285
0.0060
0.4769
123.6
84.7
96.7
44.8
57.457
AB
1100
4.552
0.0065
0.5896
131.6
78.2
96.2
49.9
60.667
AC
1100
4.648
0.0056
0.5575
94.6
91.1
96.5
53.5
62.111
AD
1100
4.880
0.0048
0.5619
142.4
106.8
96.6
59.0
65.267
AE
1100
5.159
0.0030
0.6348
179.3
170.7
96.4
66.0
68.79
AF
1100
5.397
0.0019
0.7662
145.7
267.3
95.8
71.6
71.501
AG
1200
5.481
0.0024
0.7336
278.1
214.5
96.0
82.3
72.773
AH
1250
5.725
0.0009
0.6432
342.6
545.4
96.7
95.6
76.463
AI
1350
5.863
0.0007
0.8824
40.0
692.6
95.5
97.1
77.38
AJ
1700
5.932
0.0017
1.168
74.1
299.7
94.2 100.0
77.16
Integrated age ± 1!
n=35
2586.3
50.9
K2O=10.86%
68.786
DR 1-9
±1!
(Ma)
2.2
1.3
0.55
0.48
0.22
0.25
0.16
0.17
0.14
0.14
0.15
0.11
0.10
0.11
0.092
0.090
0.099
0.094
0.12
0.084
0.11
0.086
0.085
0.084
0.076
0.069
0.079
0.085
0.076
0.34
0.091
0.069
0.086
0.13
0.11
0.086
Time
(min)
10.9
20.9
9.8
20.5
9.1
19.9
12.0
22.3
12.0
22.1
12.1
22.3
11.8
22.1
12.4
22.6
12.4
22.5
12.8
22.9
12.7
22.8
12.8
22.9
12.8
22.8
12.8
22.8
56.9
116.8
6.6
6.6
6.9
3.0
6.6
Table DR03 cont'd. Argon isotopic data and age assignments.
ID
Temp
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
-3
(¡C)
(x 10 )
39
(x 10
ArK
-15
K/Ca
mol)
40
Ar*
(%)
Ar
Age
(%)
(Ma)
39
Notes:
Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interfering reactions.
Errors quoted for individual analyses include analytical error only, without interfering reaction or J uncertainties.
Integrated age calculated by summing isotopic measurements of all steps.
Integrated age error calculated by quadratically combining errors of isotopic measurements of all steps.
Plateau age is inverse-variance-weighted mean of selected steps.
Plateau age error is inverse-variance-weighted mean error (Taylor, 1982) times root MSWD where MSWD>1.
Plateau error is weighted error of Taylor (1982).
Decay constants and isotopic abundances after Steiger and JŠger (1977).
x symbol preceding sample ID denotes analyses excluded from plateau age calculations.
Weight percent K2O calculated from 39Ar signal, sample weight, and instrument sensitivity.
Ages calculated relative to FC-2 Fish Canyon Tuff sanidine interlaboratory standard at 28.02 Ma
Decay Constant (LambdaK (total)) = 5.543e-10/a
D = Mass discrimination. 1 AMU in favor of light isotopes.
Correction factors: NM-194
(39Ar/37Ar)Ca = 0.000676 ± 5e-06
NM-173
(39Ar/37Ar)Ca = 0.00079 ± 2e-05
36
37
(36Ar/37Ar)Ca = 0.000283 ± 5e-06
38
39
(38Ar/39Ar)K = 0.0124
40
39
(40Ar/39Ar)K = 0.02895 ± 0.00059
( Ar/ Ar)Ca = 0.000276 ± 2e-06
( Ar/ Ar)K = 0.0132
( Ar/ Ar)K = 0.01 ± 0.002
DR 1-10
±1!
(Ma)
Time
(min)
(a)
-2
(b)
-3
5J uly-4 K -felds par
-1
70
-4
E=45.7 kcal/mol
-5
Do/ro2=4.67 /sec
65
K -felds par meas ured
K -felds par model
B iotite
60
-6
-7
Domain #
D/r2
1
2
3
4
5
6
7
7.48
6.08
5.89
3.51
3.51
2.48
2.27
-8
-9
0
20
40
Cumulative %
80
-10
100
6
7
8
9
Volume
fraction
0.016
0.058
0.030
0.384
0.138
0.260
0.115
10 11 12 13 14
10000/T(K )
39
Ar released
400
(c)
350
meas ured
model
o
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
60
Temperature ( C)
55
log(r/r o)
meas ured
model
2
log(D/r ) s
Apparent Age (Ma)
75
(d)
300
250
200
150
0
20
40
Cumulative %
60
80
100
100
65
66
67
68
69
70
71
Age (Ma)
39
Ar released
Figure DR02. K-feldspar MDD and biotite results. a) age spectra for biotite and K-feldsar. b) Arrhenius plot,
c) Log (r/ro) plot, and d) calculated thermal history assuming only cooling from an initially
high temperature.
DR 1-11
Table DR04. Cl correlated excess argon corrected ages.
Run ID
Temp (¡C)
40Ar*/39ArK 38ArCl/39ArK delta 38ArCl/39ArK
delta 40Ar*/39ArK
Cl-corrected
Age (Ma)
JG082103-4 K-feldspar, J = 0.007862
54573-01B
54573-01C
54573-01D
54573-01E
54573-01F
54573-01G
54573-01H
54573-01I
54573-01J
54573-01K
54573-01L
54573-01M
54573-01N
54573-01O
54573-01P
54573-01Q
54573-01R
54573-01S
54573-01T
54573-01U
54573-01V
54573-01W
54573-01X
54573-01Y
54573-01Z
54573-01AA
54573-01AB
54573-01AC
54573-01AD
54573-01AE
54573-01AF
54573-01AG
54573-01AH
54573-01AI
54573-01AJ
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
JG082103-4
96.62073
8.188618
15.25754
2.527133
11.91237
2.474954
8.656024
2.637329
3.279039
2.326341
2.529505
2.308615
2.437001
2.355471
2.527867
2.475363
2.6003
2.665521
2.912405
2.914855
3.266949
3.344773
3.812066
3.864688
4.066986
4.116149
4.350008
4.455302
4.685828
4.943357
5.142288
5.23563
5.506794
5.574234
5.558093
0.1463815
0.0219944
0.0296253
0.0139111
0.0241373
0.0128939
0.0206098
0.0134755
0.0139477
0.0124905
0.0126832
0.0126375
0.0126018
0.0122403
0.0124864
0.0123355
0.0123535
0.0125965
0.01296
0.013019
0.0138169
0.0139743
0.0147565
0.01434
0.0148541
0.0143678
0.014706
0.014606
0.0144316
0.0147413
0.0150184
0.0155263
0.0151683
0.0157619
0.0155096
0.1101893
88.432112
0.014995
12.730407
0.0107982
9.437416
0.0067582
6.018695
0.0013438
0.952698
0.0000263
0.22089
0.0003431
0.08153
0.0001406
0.0177526
-0.0002426
-0.0189828
-0.0000833
0.1138853
-0.0001673
-0.028819
0.0004013
-0.0084887
0.0004619
-0.0368537
9.4E-05
0.0227495
Figure DR03. Excess argon - Chlorine plot
100
y = 804.352x
90
R2 = 1.000
Delta 40Ar*/39ArK
80
70
60
50
40
30
20
10
0
0
0.02
0.04
0.06
0.08
Delta 38ArCl/39ArK
DR 1-12
0.1
0.12
32.2
35.0
32.3
23.0
42.0
31.5
34.6
26.5
30.5
32.3
33.1
30.3
32.7
-2
J G 082103-4
(b)
-3
-1
(a)
K -felds par meas ured
K -felds par C l-corrected
K -felds par model
-4
E=43.66 kcal/mol
-5
Do/ro2=5.20/sec
-6
-7
Domain #
D/r2
1
2
3
4
5
6
7
7.77
7.40
6.33
5.86
3.24
1.96
1.60
-8
-9
0
20
40
Cumulative %
60
80
-10
100
6
7
8
9
Volume
fraction
0.030
0.029
0.004
0.046
0.348
0.278
0.265
10 11 12 13 14
10000/T(K )
39
Ar released
2.0
400
(c)
350
o
Temperature ( C)
1.5
log(r/r o)
meas ured
model
2
log(D/r ) s
Apparent Age (Ma)
100
90
80
70
60
50
40
30
20
10
0
1.0
meas ured
model
0.5
(d)
300
250
200
150
0.0
0
20
40
Cumulative %
60
80
100
100
20
30
40
50
60
70
80
Age (Ma)
39
Ar released
Figure DR04. K-feldspar MDD results. a) age spectra measured K-feldsar as well as Cl-corrected data.
b) Arrhenius plot, c) Log (r/ro) plot, and d) calculated thermal history assuming only cooling
from an initially high temperature.
DR 1-13
-2
(a)
120
-1
J V 61504
100
K -felds par meas ured
K -felds par corrected
K -felds par model
B iotite
90
0
20
40
Cumulative %
-4
E=44.11 kcal/mol
-5
Do/ro2=5.54/sec
60
80
-6
-7
Domain #
D/r2
1
2
3
4
5
6
7.13
5.97
5.65
3.03
1.83
1.55
-8
-9
-10
100
6
7
8
9
Volume
fraction
0.029
0.047
0.019
0.271
0.109
0.525
10 11 12 13 14
10000/T(K )
39
Ar released
2.0
400
(c)
350
o
Temperature ( C)
meas ured
model
1.5
log(r/r o)
meas ured
model
2
110
80
(b)
-3
log(D/r ) s
Apparent Age (Ma)
130
1.0
0.5
(d)
300
250
200
150
0.0
0
20
40
Cumulative %
60
80
100
100
80
90
100
110
120
Age (Ma)
39
Ar released
Figure DR05. K-feldspar MDD and biotite results. a) age spectra; green boxes represent a spectrum that
removes the intermediate age hump from the measured spectrum. b) Arrhenius plot, c) Log (r/ro)
plot, and d) calculated thermal history assuming only cooling from an initially high temperature.
DR 1-14
APATITE FISSION TRACK THERMOCHRONOLOGIC METHODS
Apatite Fission Track (AFT) thermochronology provides information on the timing
and rates of cooling occurring at temperature (T) between ca. 60-120°C, defined as the
Partial Annealing Zone (PAZ). The exact T of the upper (hotter) boundary depends on
the kinetic characteristics of the apatites and the cooling rate; the former can be
quantified by measuring the diameter of track etch pits, known as Dpar (Gallagher et al.,
1998; Donelick et al., 1999; Ketcham et al., 1999). In general, smaller Dpar are typical of
flourine-rich apatites and are characterized by lower temperatures of the upper boundary.
Fission track-lengths provide information on the proportion of the cooling history that the
sample experienced within the PAZ, and hence how quickly the apatite passed through
the PAZ. Therefore, in order to interpret the AFT data in terms of a T-t path an integrated
analysis of fission-track age, track length distribution, and kinetic characteristics of the
apatite grains (Dpar) is necessary. Samples were prepared and analyzed following the
procedure described by (Sobel and Strecker, 2003).
An average of twenty grains for each sample was analyzed for seven samples
(Table DR05). Confined track-lengths, angle between the confined track and the Ccrystallographic axis (C-axis projected data), and Dpar were measured. Use of the angular
data mitigates track-measurement bias (Barbarabd et al., 2003) and improves annealing
model results, as confined tracks anneal anisotropically as a function of orientation
(Donelick et al., 1999; Ketcham et al., 1999). All samples pass the chi2 test (Green, 1981;
Galbraith and Green, 1990); therefore, pooled ages, calculated using the Trackkey
program (Dunkl, 2002), are reported in Table DR05.
AFT Thermal Modelling
Thermal modeling was conducted using HeFTy program of (Ketcham, 2005). For
all samples the model was initiated at a time (t) corresponding to at least double the
pooled age of the considered sample. A T between 5-20°C was considered as present-day
surface T; no extra additional t-T constrains were initially used. However, for most
samples the best results (i.e. highest number of good fits) were obtained by applying an
additional lower T constraint. This is because of the consistent presence in all samples of
few lengths at ca. 14 m, and longer, together with lengths between ca. 8-12 m (>60%).
In order for the model to reproduce the data, the samples need to reside in the partial
annealing zone, after the main cooling phase (e.g. crystallization age), for a considerable
proportion of the sample thermal history and then experience a late stage cooling.
Therefore, an extra T constraint of 80-5°C was set from 30 Ma to Present; if a good fit
was not obtained at these conditions, the upper limit of this constraint was reduced by 5
My steps until a good fit was obtained.
Comparison of our results with Wang et al., (2007)
The 40Ar/39Ar results of Guynn et al. (2006) for the Amdo basement match well with
those of Wang et al. (2007), but the AFT results presented here do not. Their pooled
AFT age for sample NT-7 is ~40 Ma compared to ~73 Ma for PK1A and PK1B, located
near NT-7. Their NT-9 is near PK2, yet the former is ~105 Ma compared to ~74 Ma for
the latter. Given that 1) NT-7 and NT-9 do not pass the
2
test (i.e. P( 2)<5%), which
makes a pooled age not meaningful, 2) ages for NT-7 and NT-9 are very different though
only ~12 km apart, and 3) all four samples from the Amdo basement presented here pass
2
and have consistent ages, we feel that our results are more robust. NT-10 of Wang et
al. (2007) is located at the very northern edge of the Amdo basement and shows moderate
cooling in the mid-Tertiary. This sample is farther north than the PK samples, closer to
the northern limit of the suture zone where there may have been Tertiary reactivation of
the thrust belt (Kapp et al., 2007) or Tertiary strike-slip and/or normal faulting may that
have resulted in younger exhumation (Kidd and Molnar, 1988).
Sample PU4 is close (~25 km north) to NT-12 from Wang et al. (2007). AFT
modeling of NT-12 by Wang et al. (2007) shows rapid cooling at ~160 Ma, while our
AFT modeling of PU4 shows a period of slow cooling following crystallization and rapid
cooling in the Late Cretaceous. The difference between the two may simply reflect a
later crystallization age of PU4. However, there is also some inconsistency in the AFT
modeling procedures adopted by Wang et al. (2007) including: NT-12 was modeled
despite failing the
2
test; there is a lack of compositional data (Dpar); and the assumption,
possibly incorrect, that the apatites of NT-12 are compositionally similar to Durango
apatites. For these reasons we consider our modeling to be more robust.
References
Barbarabd, J., Hurford, A.J., and Carter, A., 2003, Variation in apatite fission-track
length measurement: implications for thermal history modeling: Chemical
geology, v. 198, p. 77-106.
Donelick, R.A., Ketchman, R.A., and Carlson, W.D., 1999, Variability of apatite fission
track annealing kinetics: II. Crystallographic orientation effects: American
Mineralogist, v. 84, p. 1224-1234.
Dunkl, I., 2002, Trackkey.
Galbraith, R.F., and Green, P.F., 1990, Estimating the component ages in a finite mixture:
Nuclear tracks and radiation measurements, v. 17, p. 197-206.
Gallagher, K., Brown, R., and Johnson, C., 1998, Fission track analysis and its
applications to geological problems: Annual Review of Earth and Planetary
Sciences, v. 26, p. 519-572.
Green, P.F., 1981, A new look at statistics in fission-track dating: Nuclear tracks and
radiation measurements, v. 5, p. 77-86.
Guynn, J.H., Kapp, P., Pullen, A., Heizler, M., Gehrels, G., and Ding, L., 2006, Tibetan
basement rocks near Amdo reveal "missing" Mesozoic tectonism along the
Bangong suture, central Tibet: Geology, v. 34, p. 505-508.
Kapp, P., DeCelles, P.G., Gehrels, G.E., Heizler, M., and Ding, L., 2007, Geological
records of the Lhasa-Qiangtang and Indo-Asian collisions in the Nima area of
central Tibet: Geological Society of America Bulletin, v. 119, p. 917-933.
Ketcham,
R.A.,
2005,
Forward
and
inverse
modeling
of
low-temperature
thermochronometry data: Mineral. Soc. Am. Rev. Mineral. Geochem, v. 58, p.
275-314.
Ketcham, R.A., Donelick, R.A., and Carlson, W.D., 1999, Variability of apatite fissiontrack annealing kinetics: III. Extrapolation to geological time scales: American
Mineralogist, v. 84, p. 1235-1255.
Kidd, W.S.F., and Molnar, P., 1988, Quaternary and Active Faulting Observed on the
1985 Academia-Sinica Royal-Society Geotraverse of Tibet: Philosophical
Transactions of the Royal Society of London Series A-Mathematical Physical and
Engineering Sciences, v. 327, p. 337-363.
Sobel, E., and Strecker, M.R., 2003, Uplift, exhumation and precipitation: tectonic and
climatic control of Late Cenozoic landscape evolution in the northern Sierras
Pampeanas, Argentina: Basin Research, v. 15, p. 431-451.
Wang, Y., Zhang, X., Sun, L., and Wan, J., 2007, Cooling history and tectonic
exhumation stages of the south-central Tibetan Plateau (China): Constrained by
40
Ar/39Ar and apatite fission track thermochronology: Journal of Asian Earth
Sciences, v. 29, p. 266-282.
Table DR05. AFT Analytical Data
Sample
location
code
No
Xls
Rho-S
a
NSc
Rho-I
NIc
P( )2
b
Rho-D
NDf
e
(e5)
(e5)
Age
error
(Ma)
U
ML
Error
(ppm)
( m)
( m)
SD
Dpar
SD
( m)
(N)
PK1A
Amdo
20
13.535
1086
34.211
2745
36.18
1.0321
4009
72.9
3.0
42.86
/
Amdo
20
20.83
1320
52.422
3322
44.08
1.0245
4009
72.7
4.4
64.1
12.7
/
2.5
0.4
0.88
1.3
2.0
0.3
1.05
1.5
2.0
0.2
1.04
1.4
1.9
0.1
1.01
1.7
2.2
0.1
1.5
2.0
0.1
0.1
1.7
0.1
(PK-97-6-4-1A)
*PK1B
N(119)
(PK-97-6-4-1B)
*PK2
Amdo
20
11.757
1176
28.692
2870
51.13
1.0169
4009
74.4
3.0
34.75
*PK3A
Amdo
20
8.969
982
21.501
2354
54.53
1.0018
4009
71.9
3.1
27.47
(PK-97-6-4-3A)
*JV1
Bangge
20
12.876
703
43.446
2372
60.49
1.1488
4773
61.7
2.9
53.81
(5July04 pullen)
12.2
N(77)
Bangge
20
4.496
682
17.91
2717
66.93
1.152
4773
53.7
2.6
19.83
(JV61504-4)
*PU04
13.1
N(70)
(JV61504-1)
JV4
12.9
N(69)
PK-97-6-4-2)
13.1
N(35)
Tangulla
20
9.681
1227
31.837
4035
88.51
0.9715
4009
53.6
2.1
41.34
12.6
1.08
N(106)
Samples analyzed with a Leica DMRM microscope with drawing tube located above a digitizing tablet and a Kinetek computer-controlled stage driven by the
FTStage program (Dumitru, 1993).
Analysis performed with reflected and transmitted light at 1250x magnification. Samples were irradiated at Oregon State University. Samples where etched in
5.5 molar nitric acid at 21oC for 20 seconds. Following irradiation, the mica external detectors were etched with 21oC in 40% hydrofluoric acid for 45 minutes.
The pooled age is reported for all samples as they pass the 2 test, suggesting that they represent a single population. Error is one , calculated using the zeta
calibration method (Hurford and Green, 1983) with zeta of 358.97 ± 4.42 for apatite [unpublished data, 2006, B. Carrapa].
a
No Xls is the number of individual crystals dated.
b
Rho-S and Rho-I are the spontaneous and induced track density measured, respectively (tracks/cm2).
c
NS and NI are the number of spontaneous and induced tracks counted, respectively.
( )2 (%) is the chi-square probability (Galbraith and Green, 1990; Green, 1981). Values greater than 5% are considered to pass this test and represent a
single population of ages.
e
Rho-D is the induced track density in external detector adjacent to CN5 dosimetry glass (tracks/cm2).
f
ND is the number of tracks counted in determining Rho-D.
Dpar: fission track etch pit measurements; SD is the related standard deviation. ML: mean track length; SD: standard deviation. N(x): number of length
measurements. *are the modeled samples.
/: no data.
Time-Temperature History
0
a) PK1A
20
AFT: Track Length Distribution
2E
0.40
0.35
60
80
100
Frequency
Temperature (°C)
40
2E
120
0.20
0.15
0.05
0.00
180
200
20
0.25
0.10
140
160
0
0.30
140 120
100
80 60
Time (Ma)
40
20
0
2
4
6
8 10 12 14 16 18 20
Length (µm)
0
2
4
6
8 10 12 14 16 18 20
Length (µm)
0
2
4
6
8 10 12 14 16 18 20
Length (µm)
0
b) PK2
0.30
60
0.25
80
2E
Frequency
Temperature (°C)
40
100
120
140
0.20
0.15
0.10
160
0.05
180
0.00
200
140
120
100
80
60
Time (Ma)
40
20
0
c) PK3A
0.35
0.30
Frequency
0.25
2E
0.20
0.15
0.10
0.05
0.00
120
100
80
60
40
20
0
Time (Ma)
Figure DR06. AFT thermal modeling results and track densities for a) PK1A (PK-97-6-4-1A),
b) PK2 (PK-97-6-4-2), and c) PK3A (PK-97-6-4-3A).
Time-Temperature History
0
20
AFT: Track Length Distribution
d) PU4
0.40
2E
Temperature (°C)
40
60
100
0.20
140
0.15
160
120
100
80
60
40
Time (Ma)
20
0.10
0.05
0
0
4
8 12 16
Length (µm)
20
0.00
0.40
0.30
0.35
0.25
0.30
0.20
120
0.25
0.15
140
0.20
0.10
0.15
0.05
60
80
100
160
180
200
0
5
10
15
Length (µm)
20
e) JV1
40
Temperature (°C)
0.15
0.10
180
20
0.20
0.25
2E
120
0
0.25
0.30
80
200
0.30
0.35
0.10
120
100
80
60
40
Time (Ma)
20
0
0
4
8 12 16
Length (µm)
20
0.00
0
5
10
15
Length (µm)
Figure DR06 continued. AFT thermal modeling results and track densities for d) PU4
(AP070504-A) and e) JV1 (JV61504-1).
20
UP77-1_PK1A.APA
Tibet
PK1A-UP77-1
Apatite
Cryst.:
20
Ns:
1086
Ni:
2745
Pooled:
Mean:
Central:
25 Jul. 06 --- 24 Feb. 07
--- TRACKKEY ver. 4.1 --Amdo granite
granite
Area:
50
823
RhoS:
13.535
RhoS
RhoI:
34.211
0.396 72.9 ± 3.0
0.406 74.9 ± 2.4
0.398 73.4 ± 3.3
BC
UP
Chi-sq.: 20.56 P (%): 36.18
Dispersion: 0.08
a: 0.894
b: 0.379 r: 0.9
Irr.: UP77
Glass: CN-5
Nd: 4009
RhoD: 10.321
RhoI
Zeta: 358.97 ± 4.42 U.: 42.86
100
2
5
3
12
0
20144 1
6
11
2 15
8 10
90
9
80
16
19
70
18 13
-1
Dp
(± 38 %)
Central value: 74.9 Ma
1
100
3
17
-2
60
7
60 40 30
20
15
Rel. error [%]
Grain accumulation
0
72.88
1
20
Poisson (1x)
St. dev. (1x)
Zero tracks
Chi pass/fail (5%)
Figure DR07. AFT plots and statistical parameters for PK1A.
50
100
UP77-2_PK1B.APA
Tibet
PK1B
Apatite
Cryst.:
20
Ns:
1320
Ni:
3322
Pooled:
Mean:
Central:
28 Jun 2006 --- 24 Feb. 07
--- TRACKKEY ver. 4.1 --Amdo
granite
Area:
50
650
RhoS:
20.83
RhoS
RhoI:
52.422
0.397 72.7 ± 2.8
0.418 76.4 ± 2.7
0.398 72.7 ± 2.8
BC
UP
Chi-sq.: 19.25 P (%): 44.08
Dispersion: 0.02
a: 5.596
b: 0.302 r: 0.86
Irr.: UP77
Glass: CN-5
Nd: 4009
RhoD: 10.245
RhoI
Zeta: 358.97 ± 4.42 U.: 64.1
2
100
Dp
(± 28 %)
Central value: 76.4 Ma
90
3
20
1216
1
8
13
18
7
0
5
1
80
2
14
10 4
15
-1
6
9
70
2
19
11
17
-2
50
30
20
15
Rel. error [%]
Grain accumulation
0
72.65
1
20
Poisson (1x)
St. dev. (1x)
Zero tracks
Chi pass/fail (5%)
Figure DR08. AFT plots and statistical parameters for PK1B.
50
100
UP77-3_PK2.APA
Tibet
PK2
Apatite
Cryst.:
20
Ns:
1176
Ni:
2870
Pooled:
Mean:
Central:
23 Jun. 06 --- 24 Feb. 07
--- TRACKKEY ver. 4.1 --Amdo
granite
Area:
20
1026
RhoS:
11.757
RhoS
RhoI:
28.692
74.4 ± 3.0
0.41
0.416 75.5 ± 2.5
74.4 ± 3.0
0.41
BC
UP
Chi-sq.: 18.17 P (%): 51.13
Dispersion: 0.03
a: 4.129
b: 0.262 r: 0.81
Irr.: UP77
Glass: CN-5
Nd: 4009
RhoD: 10.169
RhoI
Zeta: 358.97 ± 4.42 U.: 34.75
2
50
Dp
(± 29 %)
Central value: 75.5 Ma
4
7
135
6
1
90
14
15
819
80
11
0
3 10
9
-1
20
12
1
18
17
16
70
3
2
-2
50
30
20
15
Rel. error [%]
Grain accumulation
0
74.36
1
20
Poisson (1x)
St. dev. (1x)
Zero tracks
Chi pass/fail (5%)
Figure DR09. AFT plots and statistical parameters for PK2.
50
100
UP77-5_PK3A.APA
Amdo-Tibet
UP77-5
Apatite
Cryst.:
20
Ns:
982
Ni:
2354
Pooled:
Mean:
Central:
31 Oct. 05 --- 24 Feb. 07
--- TRACKKEY ver. 4.1 --PK3A
granite
Area:
20
1123
RhoS:
8.969
RhoS
RhoI:
21.501
0.417 71.9 ± 3.1
0.423 72.9 ± 2.9
0.418 72.0 ± 3.2
BC
Potsdam
Chi-sq.: 17.66 P (%): 54.53
Dispersion: 0.04
a: 1.11
b: 0.372 r: 0.92
Irr.: UP77
Glass: CN-5
Nd: 4009
RhoD: 10.018
RhoI
Zeta: 346.06 ± 4.82 U.: 27.47
18
Central value: 72.9 Ma
14
7
13
9
20
19 1615
-1
12
70
3
6
4
60 40 30
80
11
17
0
90
10
5
83
Dp
(± 44 %)
2
1
50
20
1
15
Rel. error [%]
Grain accumulation
0
71.91
1
20
Poisson (1x)
St. dev. (1x)
Zero tracks
Chi pass/fail (5%)
Figure DR10. AFT plots and statistical parameters for PK3A.
100
200
UP77-9_PU4.apa
Tibet
PU4
Apatite
Cryst.:
20
Ns:
1227
Ni:
4035
Pooled:
Mean:
Central:
20 Jan. 06 --- 24 Feb. 07
--- TRACKKEY ver. 4.1 --Tangulla Shan
granite
Area:
50
1300
RhoS:
9.681
RhoS
RhoI:
31.837
0.304 53.6 ± 2.1
0.307 54.1 ± 1.3
0.304 53.6 ± 2.1
BC
Potsdam
Chi-sq.: 12.01 P (%): 88.51
Dispersion: 0.03
a: 0.588
b: 0.284 r: 0.97
Irr.: UP77
Glass: CN-5
Nd: 4009
RhoD: 9.715
RhoI
Zeta: 364.06 ± 4.82 U.: 41.34
2
100
Dp
(± 54 %)
Central value: 54.1 Ma
14
1
3
8
7
9
2
11
4
0
12
60
1719
16
10
20
-1
13
18
5
15
6
4
50
1
-2
50 30
20
15
10
Rel. error [%]
Grain accumulation
0
53.55
1
20
Poisson (1x)
St. dev. (1x)
Zero tracks
Chi pass/fail (5%)
Figure DR11. AFT plots and statistical parameters for PU4.
50
100
eJV1.APA
Tibet
eJV1
Apatite
Cryst.:
22
Ns:
703
Ni:
2372
Pooled:
Mean:
Central:
30 Aug. 05 --- 24 Feb. 07
--- TRACKKEY ver. 4.1 ---
BC
Uni Potsdam
Granite
Area:
50
560
RhoS:
12.876
RhoS
RhoI:
43.446
0.296 61.7 ± 2.9
0.299 62.2 ± 2.5
0.296 61.7 ± 2.9
Chi-sq.: 18.69 P (%): 60.49
Dispersion: 0.03
a: 2.225
b: 0.245 r: 0.9
Irr.: UP75
Glass: CN-5
Nd: 4773
RhoD: 11.488
RhoI
Zeta: 364.06 ± 4.82 U.: 53.81
Central value: 62.2 Ma
Dp
(± 46 %)
18
3
200
80
5
16
4
1
8
2
13
19
20
21
11
7
0
17
10
15
-1
6
14
22
40
30
60
3
12
9
60
70
1
50
20
Rel. error [%]
Grain accumulation
0
61.68
1
22
Poisson (1x)
St. dev. (1x)
Zero tracks
Chi pass/fail (5%)
Figure DR12. AFT plots and statistical parameters for JV1.
50
100
UP75-10_JV4-1.APA
Tibet
JV4-1
Apatite
Cryst.:
20
Ns:
682
Ni:
2717
Pooled:
Mean:
Central:
31 Aug. 05 --- 24 Feb. 07
--- TRACKKEY ver. 4.1 --Bangge granite
BC
Uni Potsdam
Area:
20
1556
RhoS:
4.496
RhoS
RhoI:
17.91
0.251 53.7 ± 2.6
0.261 55.9 ± 2.3
0.251 53.7 ± 2.6
Chi-sq.: 15.82 P (%): 66.93
Dispersion: 0.02
a: -0.538
b: 0.293 r: 0.88
Irr.: UP75
Glass: CN-5
Nd: 4773
RhoD: 11.52
Zeta: 372.8 ± 5.8
RhoI
U.: 19.83
50
(± 37 %)
80
Central value: 55.9 Ma
12
2
70
7
1
1
0
9
1116 14
17
131810
6
15 5
60
3
8
2019
-1
4
50
3
45
-2
2
60 40 30
20
15
Rel. error [%]
Grain accumulation
0
53.68
1
20
Poisson (1x)
St. dev. (1x)
Zero tracks
Chi pass/fail (5%)
Figure DR13. AFT plots and statistical parameters for JV4.
50
100
(U-Th)/ He analyses
Sample preperation
All sample preparation and analyses were performed at the University of Arizona.
Apatite seperates from 17 samples were extracted by standard separation
techniques which include, crushing, sieving, water table, magnetic seperator and the
use of heavy liquids (Lithium-Metatungstate and Methylen Iodide). Individual grains
were handpicked under a Leica MZ16 stereozoom microscope and checked for
inclusion in plane-polarized darkfield. Afterwards inclusion free apatites grains
dimensions were measured to apply alpha-ejection corrections. For apatite we
assume pinacoidal terminations (Farley et al., 1996; Farley, 2002) at the tips of the
crystal. Grains are wrapped in metallic foil for laser heating (House et al., 2002).
Apatite is placed in Nb tubing and the ends are pinched closed. We used the
approach of single aliquot analysis in our study; usually 2-4 replicates per sample.
He extraction and measurement
Crystal-bearing foil packets are placed in a Cu or SS planchet, under a KBr coverslip,
inside a ~7-cm laser cell pumped to <10-9 torr. Aliquots are heated for 3 minutes by a
focused beam of a 1-2 W (Nd:YAG) up ~900-1000 °C. If necessary reheating (reextraction) of single aliquots are preformed to confirm that all 4He has been extracted
at least below blank level. Gas released from heated samples is then spiked with 0.10.2 pmol 3He, and condensed onto activated charcoal the cold head of a cryogenic
trap at 16 K. Helium is then released from the cold head at 37 K into a small volume
(~50 cc) with an activated Zr-Ti alloy getter and the source of a Balzers quadrupole
mass spectrometer (QMS) with a Channeltron electron multiplier. Peak-centered
masses at approximately m/z of 1, 3, 4, and 5.2 are measured. Mass 5.2 establishes
background, and mass 1 is used to correct mass 3 for HD and H3+. Corrected ratios
of masses 4 to 3 are regressed through ten measurement cycles over ~15 s to derive
an intercept value, which has an uncertainty of 0.05-0.5% over a 4/3 range of ~103,
and compared with the mean corrected ratio to check for significant anomalous
changes in the ratio during analysis. Helium contents of unknown samples are
calculated by first subtracting the average mass-1-corrected 4/3 measured on
multiple procedural blanks analyzed by the same method (a “hotblank“), from the
mass-1-corrected 4/3 measured on the unknown. This is then ratioed to the the
mass-1-corrected 4/3 measured on a shot of an online reference 4He standard
analyzed with the same procedure [minus the mass-1-corrected 4/3 measured on a
3
He-only spike shot analyzed using the same procedure as the reference 4He
standard (a ”lineblank“)]. The resulting ratio of measured 4/3's is then multiplied by
the moles of 4He delivered in the reference shot. This procedure assumes linearity
between measured 4/3 and 4He pressure, which has been confirmed over the the
vast majority of the range of 4He contents we analyze by performing multiple replicate
analyses of known-age standards with masses and therefore 4He yields ranging over
three orders of magnitude. This procedure also relies on the accuracy of the 4He
delivery from the reference standard and the precision of its measurement with the
3
He spiking procedure. The delivery and its depletion with time are calibrated by
multiple capacitance manometry measurements of the volumes of the reference tank
and pipette, and the final filling pressure of the tank. Between ~2-5 (depending on the
number of unknowns) 4/3 measurements of spiked 4 He reference standards are
made each measurement day. Average measured 4/3 of lineblanks ( 3He spike only)
are nearly indistinguishable from that predicted by the purity of the 3He spike (99.75%
3
He). Hotblanks, or procedural blanks measured by lasing/heating empty Nb foil
packets are typically 0.05-0.1 fmol 4He.
Following degassing apatite grains are dissolved directly in the Nb foil by addition of
20% nitric acid in order to measure the U, Th and Sm of the sample (House et
al.,2000). Nb foils with dissolved apatite crystals were then spiked with two different
spike solutions, each in 5% HNO3 solution. The first is 25 or 50 μl of a nominally pure
233
U-229Th spike with total U and Th concentrations of 7.55 ± 0.10 ng/ml and 12.3 ±
0.10 ng/ml respectively. The second is 25 or 50 μl of an enriched (97%) 147Sm spike
with a total Sm concentration of 10.8 ± 0.10 ng/ml. Following spiking, 200 μl of
concentrated SeaStar Baseline HNO3 is then added to each sample, and the mixture
is heated at about 90 °C for two hours. After cooling, the solutions are diluted with 2.5
ml of double-distilled 18 MΩ H2O, for final spike isotope concentrations of ~0.1-0.2
ppb.
Each samples, including blanks and standards, 229Th, 232Th, 233U, 235U, 238U, 147Sm
and 152Sm content were measured on a Thermo Finningan Element2. Apatites
samples were run with a method using Escan peak jumping with the magnet parked
at mass 229.031, sample time 2 ms, 100 samples per peak, mass and averaging
windows of 5%, and counting mode, 5 runs and 400 passes for a total of 2000
isotope ratio measurements. Apatite (U-Th)/He ages typically have approximately 1-3%
(1σ) error. For more information see also Reiners et al., 2004.