1. LITHOLOGIC CORRELATIONS ACROSS THE EASTERN NORTH PAMIR SUGGEST A REGIONALLY EXTENSIVE THRUST NAPPE
Daniel B. Imrecke1, Alexander C. Robinson1, Thomas J. Lapen1, Chen Jie2, Wenqiao Li2, Xiaodong Yang2, Zhaode Yuan2
1 Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, USA
T51E - 2639
2 Laboratory of Neotectonics and Geochronology, State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing, 100029, China
1 Abstract Methods 4 Results 5 Discussion
Older, widely used, geologic maps of the eastern North Pamir show a Summary of the Data
collage of Paleozoic lithologies. Recent work has documented that - 6 Samples were separated using Igneous Zircon Results -The hanging Wall and Footwall of the Baoziya/Torbashi Thrust (Figure 4B)
many of these lithologies are Triassic in age and regional similarities conventional techniques (1) AR-9-10-03-4 (2) AR-9-10-03-7 detrital zircon samples (4C) show Triassic maximum depostional ages with
in lithologies and structural relationships suggest a broad continuity -Samples were then mounted in epoxy and A Quartzofeldspathic Orthogneiss Garnet-Biotite Granite probability peaks consistent with the Karakul Mazar Terrane. The hanging
in the tectonic architecture across the region. We compare two polished to expose grain interiors (A). data-point error ellipses are 2
0.039
data-point error ellipses are 2
wall of the Torbashi thrust is intruded by a granite with a Triassic/Jurassic
regional areas: 1) the Baoziya Thrust and Tanymas Thrust are -Samples were analyzed using a Varian 0.10 crystallization age (Figure 4A).
600
exposed north and northwest of the Muztaghata massif, in the quadrupole inductively coupled-mass 0.037
230 -Sample 11 shows Ordovician maximum depositional ages consistent with
hanging wall of the Kongar Shan normal fault. The Baoziya/Tanymas spectrometer (Q-ICP-MS) and Photon 0.09 the age of the South Qiangtang Plutons (Figure 4C yellow star) and the
0.035
thrust hanging-wall consists of upper greenschist to amphibolite facies Machines Exicemer Laser Analite 193 (B) crystallization age of Sample 1 (Figure 4A, red star of Figure 4C).
schists and quartzites with abundant Triassic igneous intrusions. 210
-U/Th ratios were analyzed (not shown) and consistently had low (<10) values.
206Pb/238U
206Pb/238U
500 0.033
0.08
Detrital zircon analyses of the hanging wall yield Permo-Triassic
maximum depositional ages and Late Triassic metamorphic age A B 0.031
We interpret this to indicate the zircons do not contain a metamorphic
signatures (Robinson et al., 2004; 2012). The footwall of the Baoziya 0.07 190 overprint.
Thrust consists of greenschist facies marbles, phyllites, and quartzites 400
0.029
with Triassic maximum depositional ages, whereas the Tanymas 0.06
Other Considerations
thrust footwall consists of Paleozoic sandstones, limestones, and
0.027 170
-Sample 3 and 4 are mapped in the same tectonic unit, yet they display
marls. 2) The Torbashi Thrust is exposed south of the Muztaghata Figure 3A: Epoxy mount of detrital Figure 3B: Analytical equipment 0.05 0.025
significantly different relative age probability distributions. The different
massif as a large folded klippe in the hanging wall of the Shen-ti fault. zircon sample availble at the University of Houston 0.3 0.5 0.7
207Pb/235U
0.9 1.1 0.17 0.19 0.21 0.23
207Pb/235U
0.25 0.27 0.29 age distribution of Sample 4 relative to other hanging wall samples is
Its hanging wall consists of amphibolite facies schists and gneisses interpreted to be due to it representing an interval which only received
Figure 4A: Concordia plots and TuffZirc ages (inset) for zircon samples extracted from igneous rocks from this study (yellow squares
with abundant igneous intrusions. Like the Baoziya/Tanymas on Figure 4B). Black ovals are concordant ages, red ovals are discordant ages. volcanic detritus.
hanging-wall, the Torbashi Thrust hanging-wall yields Permo-Triassic -The maximum depositional age caclulated for sample 10 is slightly higher than
maximum depositional ages and a Late Triassic metamorphic B Tectonic Map expected (265 Ma). Two overlapping zircon ages for the sample have an Model
signature. The northern exposure of the footwall consists of
greenschist facies marbles, quartzites, phyllites, and metavolcanics
C Gehrels et al., 2011
Songpan Ganzi
Detrital Zircon Results age of 234 Ma. It is likely that with additional analysis (i.e. more zircons
with concordant ages) a younger maximum depositional age will be
with Triassic metamorphic age signatures, whereas the southern discovered.
exposure consists of Permian limestones and slates. Based on N -The crystallization age of Sample 1 indicates previous interpretations of the
Karakul-Mazar
structural juxtaposition and lithologic similarities we correlate the
8 southern trace of the Torbashi thrust (Figure 6) appear to be incorrect.
north exposure of the Torbashi Thrust with the Boaziya Thrust, and Statistically
Similar We have reinterpreted its location and sugest greater structural complexity
correlate the southwest exposure of the Torbashi thrust with the
Tanymas thrust. We propose the Torbashi and Baoziya/Tanymas
0 500 1000 1500
(Same color Hexagons)
in this area.
(3) Yang et al. 2010
faults were a continuous structure which formed a regionally extensive 7 lt S
Kon
Kon
n=30 n Fau
5 Max Depositional Age = 222 ± 1.5, Metamorphic Overprint = 220
N
Kon gar S
ha
thrust nappe in the Northern Pamir. The thrust sheet was A A’ A’’
gar
Sh
gar
en Previous Interpretation
subsequently cut by the Muztaghata gneiss dome in the Miocene. 20 km -T
iF
au
of Torbashi Thrust
(4) AR-4-28-00-11a Location
Sha
Sha
lt
n=66
Max Depositional Age = 222 ± 1.5
10 km Torbashi Thrust
t
nE
hrus
nE
Torbashi Thrust
aT
Baoziya Thrust
2 Background
oziy
Hanging Wall
Ba
(5) AR-5-28-00-2
xte
xte
10 km
A n=116 lt
F au
?
Max Depositional Age = 231 ± 15 -Ti
nsio
nsio
en
Sh
Baoziya Thrust Torbashi Thrust Kexi Liqi Fault
?
Kexi Liqi Fault
A B 6 (6) AR-6-17-00-1
-10 km
nal
nal
n=98
Shen-Ti Max Depositional Age = 245 ± 15
Sys
Sys
Figure 6. Geologic cross section of the Baoziya/Torbashi Thrust emphasizing the structural architecture of the
Fault 9 study area. Similar patterns in pink indicate similar tectonic/structural affiliation relative to the Torbashi/Baoziya
tem
tem
(7) AR-8-20-03-2
Thrust
n=143
Max Depositional Age = 242.8 ± 2.6
(8) AR-5-20-00-3
Question C
6 Conclusions
Yin and Harrison (2000)
n=131
Robinson et al. (2004)
12
Muztaghata Max Depositional Age = 220.9 ± 2.6
-Do U-Pb in Zircon isotopic data,
Gneiss Dome
obtained from samples in the (9) AR-4-28-00-7
A’
n=84 Zircon U-Pb isotopic signatures indicate the Torbashi Thrust and Baoziya Thrust
hanging wall and footwall of the
Footwall
Max Depositional Age = 212 ± 3.1
are interpreted to be the same structure and represent a continuous thrust
Torbashi and Baoziya thrust, have
Ke
sheet across the Pamir. This structure was cut by the Miocene Muztag
xi
similar maximum depositional ages (10) AR-4-27-00-9
Li
Gneiss Dome.
qi
n=63
Fa
and age probability signatures? D Schwab et al (2004)
Shen-Ti Max Depositional Age = 265 ± 3.4 (235 ± 1.6 where n=2)
ul
Lacassin et al. (2004)
Results are consistent with recent interpretations for the region: previously
t
Fault interpreted Paleozoic basement rocks in the east Pamir are Triassic
Motivations AR-9-10-03-4
10 (11) AR-6-26-00-7
metasedimentary units.
S. Pamir
-Structural relationships are similar South Qiangtang PLUTONS n=66
Max Depositional Age = 451 ± 19 Samples 1 and 11 can be correlated with the samples from the Qiangtang terrane
between the footwall and hanging 3
Torbashi Thrust in Tibet, whereas the other samples in this study are best correlated with
wall of the Baoziya Thrust (A) and 2 4
the Songpan-Ganzi terrane of Tibet.
the northern exposure of the (12) AR-8-31-03-1
Previously unidentified structural complexity in the southern portion of the study
C. Pamir
E n=118
Torbashi Footwall (B)
Burtman and Molnar (1993)
Robinson et al. (2009) Max Depositional Age = 458 ± 6.5
Robinson et al. (2012)
1 area has been identified. The geometry of the Torbashi thrust hanging wall
-Older maps of the Pamir
0 500 1000 1500 2000 2500 3000 has been reinterpreted to reflect the new data.
indicate much of the Gehrels et al., 2011 Age (Ma)
?
lithologic units are paleozoic to South Qiangtang
proterozoic basement. Recent
References
Gehrels et al., 2011
-1 All uncertainties are reported at the 2s level.
North Qiangtang
-Age cumulative probability plots were created using Isoplot
research suggests a Triassic age 11 ?
4.0 [Ludwig, 2003].
-Maximum depositional age is based on the weighted mean
Tibet equivalent terranes: No direct Tibet
signature for some units. equivalent terranes: 40 km
age of the youngest of a minimum of three overlapping ages.
-Igneous ages were determined from 206Pb/238U ages using Bershaw, J., Garzione, C. N., Schoenbohm, L., Gehrels, G., & Tao, L. (2011). Cenozoic evolution of the Pamir plateau based on stratigraphy, zircon provenance, and
Kunlun Terranes the Isoplot TuffZirc age extractor. stable isotopes of foreland basin sediments at Oytag (Wuyitake) in the Tarim Basin (west China). Journal of Asian Earth Sciences. doi:10.1016/j.jseaes.2011.04.020
-Terrane Correlation between the Songpan-Ganzi Central Pamir A’’
0 500 1000 1500
Robinson, A. C., Ducea, M. N., & Lapen, T. J. (2012). Detrital Zircon and Isotopic Constraints on the Crustal Architecture and Tectonic Evolution of the Northeastern
Pamir. Tectonics, 31(TC2016), doi:10.1029/2011TC003013.
Pamir and Tibet (Figures C-E) is not Qiangtang
Lhasa
Kohistan-Ladakh
Figure 4C: Relative age probability plots of samples from this study (yellow circles on Figure 4B) as well as
Yang, W., Liu, L., Cao, Y., Wang, C., He, S., Li, R., & Zhu, X. (2010). Geochronological evidence of Indosinian (high-pressure) metamorphic event and its tectonic
significance in Taxkorgan area of the Western Kunlun Mountains, NW China. Science China Earth Sciences, 53(10), 1445–1459. doi:10.1007/s11430-010-4081-1
fully understood. Modified from Robinson et al (2012
Figure 4B: Tectonic map of the study area. Colors represent regions of significantly different tectonic
affiliation. Modified from Robinson et al. (2012)
Previous studies (red circles). Samples with similar color hexagons indicated statistical similarity Zhang, C., Lu, S., Yu, H., & Ye, H. (2007). Tectonic evolution of the Western Kunlun orogenic belt in northern Qinghai-Tibet Plateau: Evidence from zircon SHRIMP
and LA-ICP-MS U-Pb geochronology. Science in China Series D: Earth Sciences, 50(6), 825–835. doi:10.1007/s11430-007-2051-z
according to K-S test results.