Pullen A., Kapp, P., DeCelles, P.G., Gehrels, G.E., and Ding, L., 2011



Pullen A., Kapp, P., DeCelles, P.G., Gehrels, G.E., and Ding, L., 2011
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Tectonophysics 501 (2011) 28–40
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j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o
Cenozoic anatexis and exhumation of Tethyan Sequence rocks in the Xiao Gurla
Range, Southwest Tibet
Alex Pullen a,⁎, Paul Kapp a, Peter G. DeCelles a, George E. Gehrels a, Lin Ding b
Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA
Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100029, China
a r t i c l e
i n f o
Article history:
Received 5 March 2010
Received in revised form 30 December 2010
Accepted 5 January 2011
Available online 13 January 2011
Tethyan Himalayan sequence
Gurla Mandhata
India–Asia suture
U–Pb zircon
a b s t r a c t
In order to advance our understanding of the suturing process between continental landmasses, a geologic
and geochronologic investigation was undertaken just south of the India–Asia suture in southwestern Tibet.
The focus of this study, the Xiao Gurla Range, is located near the southeastern terminus of the active, rightlateral strike-slip Karakoram fault in southwestern Tibet. The range exposes metasandstone, phyllite, schist
(locally of sillimanite facies), calc-gneiss and marble, paragneiss (± pyroxene), quartzite, metagranite, and
variably deformed leucogranite. These metamorphic rocks are exposed in the footwall of a domal, top-to-thewest low-angle normal (detachment) fault, structurally beneath Neogene–Quaternary basin fill and
serpentinized ultramafic rocks of the Kiogar-Jungbwa ophiolite. The detachment is interpreted to be the
northeastern continuation of the Gurla Mandhata detachment fault system that bounds metamorphic rocks of
the Gurla Mandhata Range ~ 60 km to the southwest. U–Pb geochronology on five detrital zircon samples of
schist, phyllite, and quartzite yielded maximum depositional ages that range from 644–363 Ma and age
probability distributions that are more similar to Tethyan sequence rocks than Lesser Himalayan sequence
rocks. A felsic gneiss yielded a metamorphic zircon age of 35.3 ± 0.8 Ma with a significant population of early
Paleozoic xenocrystic core ages. The gneiss is interpreted to be the metamorphosed equivalent of the CambroOrdovician gneiss that is exposed near the top of the Greater Himalayan sequence. Leucogranitic bodies
intruding metasedimentary footwall rocks yielded two distinct U–Pb zircon ages of ~ 23 Ma and ~15 Ma.
Locally, rocks exposed in the hanging wall of this fault and of the southward-dipping, northward-verging
Great Counter thrust to the north consist of serpentinite-bearing mélange and conglomerate of inferred
Paleogene age dominated by carbonate clasts. The mélange is intruded by a 44 Ma granite and the
stratigraphically highest conglomerate unit yielded detrital zircon U–Pb ages similar to Tethyan sequence
rocks. We attribute the middle Eocene magmatism south of the suture to break-off of the Neo-Tethyan oceanic
slab. In addition, our observations are consistent with the late Eocene shortening and crustal thickening
within the Tethyan Himalayan sequence, early-middle Miocene leucogranite emplacement being related to
underthrusting and melting of the Greater and possibly Lesser Himalayan sequences, and late Miocene arcparallel extension in the hinterland of the southward propagating Himalayan thrust belt.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
A fundamental issue in tectonics concerns the behavior of the
Earth's lithosphere during intercontinental collisional orogenesis. The
Himalayan orogen is the manifestation of the ongoing continent–
continent collision between India and Asia and provides an ideal
natural laboratory to investigate tectonic processes involved in such
collisions. However, the full understanding and exportability of
concepts learned from this orogen requires knowledge of the
geological evolution of the India–Asia suture zone and deformation,
⁎ Corresponding author. Present address: Department of Earth and Environmental
Sciences, University of Rochester, Rochester, New York 14627, USA. Tel.: + 1 585 275
0040-1951/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
metamorphism, and anatexis of Tethyan Himalayan rocks that
comprise the early evolutionary history of the Himalayan fold-thrust
belt during Eo-Oligocene time.
The Tethyan Himalayan sequence composes the structurally
highest tectonic unit of the Himalayan fold and thrust belt and were
the first Indian-affinity rocks to be deformed immediately after final
subduction of Neo-Tethys oceanic lithosphere. The timing of onset of
the India–Asia collision is commonly taken to be ~55 Ma (Garzanti
et al., 1987; Leech et al., 2005; Searle et al., 1987); however an
uncertainty of ±15 Ma (Aitchison et al., 2007; Yin and Harrison,
2000) highlights the need for improvements in our understanding of
the initial collisional orogenesis between India and Asia.
We conducted a geological investigation of the Tethyan Himalaya
and the India–Asia suture zone in southwestern Tibet. In this paper, we
present a new geologic map and U–Pb ages for igneous, metamorphic,
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A. Pullen et al. / Tectonophysics 501 (2011) 28–40
and detrital zircon samples. The results provide a better understanding
of the history of crustal deformation, metamorphism, and anatexis
within the hinterland of the Himalayan fold-thrust belt during middle
Eocene–late Miocene time.
2. Geologic setting
The central Himalaya fold-thrust belt has accommodated N600 km
of upper-crustal shortening between India and Asia (DeCelles et al.,
2001, 2002; Murphy and Yin, 2003; Robinson et al., 2006; Srivastava
and Mitra, 1994) (Fig. 1). The Indian-affinity rocks deformed within
the Himalayan fold-thrust belt are exposed in four tectonostratigraphic zones that are bounded by major faults zones (Gansser, 1964;
LeFort, 1986; Upreti, 1996). The structurally lowest unit, the
Subhimalayan unit, consists of Miocene–Pliocene foreland basin
deposits that have been incorporated into several major thrust sheets
in the frontal part of the range (DeCelles et al., 1998; Mugnier et al.,
1993). These rocks are truncated by the Main Boundary thrust, which
carries Paleoproterozoic to Mesoproterozoic metasedimentary, sedimentary, volcanic, and plutonic rocks of the Lesser Himalayan
sequence in its hanging wall. The Lesser Himalayan sequence lies
structurally beneath amphibolite-grade metasedimentary rocks of
Late Proterozoic–Cambrian age intruded by Cambrian–Ordovician
plutons of the Greater Himalayan sequence along the north-dipping
Main Central thrust, which was active during early Miocene time
(DeCelles et al., 2000, 2004; Hodges et al., 1992, 1994; Hubbard and
Harrison, 1989; Pêcher, 1989; Parrish and Hodges, 1996; Vannay and
Hodges, 1996; Figs. 1 and 2). The northern boundary of the Greater
Himalayan sequence is marked by the generally northward dipping
South Tibetan detachment system (Burchfiel et al., 1992; Hodges et
al., 1996). Upper Precambrian–upper Paleozoic marine sandstone,
shale and limestone, Triassic limestone and fine-grained clastic rocks,
Jurassic shale and limestone, and Cretaceous shallow marine
sandstone of the Tethyan Himalayan sequence compose the hanging-wall of the South Tibetan detachment (Brookfield, 1993; Cheng
and Xu, 1987; Gansser, 1964; Garzanti, 1999; Gaetani and Garzanti,
1991 and Heim and Gansser, 1939). The youngest, well-documented
Tethyan Himalayan sequence strata deposited before final consumption of Neo-Tethys oceanic lithosphere in Tibet are marine and
Paleocene to Eocene in age (Willems et al., 1996). The Eocene strata
are interpreted to record the transition from oceanic subduction to
India–Asia continental collision (Ding et al., 2005, Zhu et al., 2005),
although the possibility of an Eocene collision between India and an
intraoceanic arc has also been raised (Aitchison et al., 2007). Motion
on the South Tibetan detachment system is thought to have initiated
prior to 22 Ma and ceased by ~ 19 Ma in most areas (Dézes et al., 1999;
Searle, 1999), however some argue for slip as early as 35 Ma (Lee and
Whitehouse, 2007) and as late as 12 Ma (e.g. Murphy and Copeland,
Metamorphic domes are widespread within the Tethyan Himalayan physiographic zone, spanning most of the N2400 km arc-length
between the eastern and western syntaxes (Fig. 1). The domes are in
places cored by high-grade metamorphic rocks (Burg et al., 1984) and
were exhumed by orogen-perpendicular (Tso Morari, Kangmar, and
Mabja domes) (Berthelsen, 1953; de Sigoyer et al., 2000; Lee et al.,
2004) or orogen-parallel extension (Leo Pargil, Gurla Mandhata, and
Ama Drime; Cottle et al., 2007; Jessup et al., 2008; Murphy et al., 2002;
Thiede et al., 2006).
Rocks exposed to the north of the India–Asia suture are Cretaceous–
Cenozoic granitoids of the Gangdese batholith, Cretaceous to Eocene
marine strata of the Gangdese forearc (and forearc successor) basin,
and Oligo-Miocene nonmarine strata of the Kailas (Gangrinboche)
Formation (Aitchison et al., 2009; Gansser, 1964). All of the latter
rocks are exposed structurally beneath the south-dipping, northvergent Great Counter thrust, which is interpreted to have been
active at 19–13 Ma (Ratschbacher et al., 1994; Yin et al., 1999).
Fig. 1. Regional tectonic map of the Himalayan thrust belt and India–Asia suture zone.
After Hodges, 2000.
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Fig. 2. Simplified geologic map of the Gurla Mandhata area of southwestern Tibet.
After Murphy et al., 2002 and Pan et al., 2004.
Cenozoic crustal melts have been widely identified within the
Himalaya and are generally divided into two geographic groups: the
High Himalayan leucogranites which generally consist of dikes, sills,
and laccolithic bodies that were emplaced along the South Tibet
detachment fault (LeFort, 1996; Searle, 1999); and the North
Himalayan granites which are typically exposed in the cores of the
North Himalayan domes of the Tethyan Himalayan physiographic
zone (Lee et al., 2000). These melts are thought to have been the
product of radiogenic heat production and anatexis within thickened
continental crust, shear heating, and/or isothermal decompression
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(England and Thompson, 1984; Harris and Massey, 1994; Harrison
et al., 1998; LeFort, 1975). The majority of U–Pb ages of High
Himalayan leucogranites lie in the 23–15 Ma range (Harrison et al.,
1997b, 1998; Noble and Searle, 1995; Schärer 1984; Schärer et al.,
1986; Searle and Godin, 2003; Searle et al., 1997 and references
therein) whereas North Himalayan granites have ages in the range of
~ 29–9 Ma (Harrison et al., 1997a; Schärer et al., 1986; Zhang et al.,
2004). Recent documentation of ~ 45 Ma leucogranites within the
Tethyan Himalaya physiographic zone (e.g. Aikman et al., 2008; Ding
et al., 2005) raises the possibility that major crustal thickening in
the Himalaya may have already been occurring at this time. The
continuation of early Himalayan crustal thickening (e.g. Godin et al.,
2001; Hodges, 2000) is consistent with the timing of peak metamorphic conditions and migmatite generation (35–32 Ma) within
mid-crustal rocks coring the North Himalayan domes (Lee and
Whitehouse, 2007; Lee et al., 2000, 2004).
The Karakoram right-lateral strike-slip fault can be traced for at
least 800 km along the southwestern margin of Tibet from the Pamirs
in the northwest to the Gurla Mandhata area in the southeast (Fig. 2).
Slip along the Karakoram fault is widely taken to be ~ 120 km in its
central part (Searle et al., 1998) and to have initiated along the central
segment by ~ 16 Ma (Phillips et al., 2004). Southeastward, the
Karakoram fault feeds slip either into the Gurla Mandhata detachment,
a top-to-the west low-angle normal fault system active by 9 Ma (Fig. 2;
Murphy et al., 2000) or farther eastward into the India–Asia suture
zone (Lacassin et al., 2004; Peltzer and Tapponnier, 1988; Valli et al.,
Evidence for arc-parallel extension in southern–western Tibet is
widespread (e.g. Armijo et al., 1986; Molnar and Tapponnier, 1978;
Rothery and Drury, 1984), and includes the Qusum detachment of the
Zada basin (Saylor et al., 2009), the Gurla Mandhata detachment of
the Pulan graben (Murphy et al., 2000), and the Dangardzong fault
of the Thakkhola graben (Colchen, 1999; Colchen et al., 1986; Fort
et al., 1982). The transition from arc-normal shortening to arc-parallel
extension within the Tethyan Himalaya is thought to have occurred at
16–14 Ma (Garzione et al., 2003; Hintersberger et al., 2010; Murphy
and Copeland, 2005; Thiede et al., 2006). This transition is proposed to
have occurred because of the continued foreland propagation of the
Himalayan thrust wedge over the Indian continent driving radial
expansion of the Himalayan arc (Murphy and Copeland, 2005;
Murphy et al., 2009) and/or by collapse of the interior of the Tibetan
Plateau (e.g. Royden and Burchfiel, 1987; Hintersberger et al., 2010;
Hodges et al., 1996).
The footwall of the Gurla Mandhata system is composed of Greater
Himalayan sequence rocks that are intruded by mid-Miocene
leucogranite (Cheng and Xu, 1987; Murphy, 2007, Murphy et al.,
2002). These leucocratic melts represent partial melting of Greater
Himalayan and Lesser Himalayan rocks (Murphy, 2007). The northern
trace of the Gurla Mandhata detachment is buried beneath Neogene–
Quaternary basin deposits. Previous mapping suggests that the fault
continues northward, forming an east–west trending synformal
corrugation between Gurla Mandhata in the south and the Xiao
Gurla Range in the north (Murphy and Copeland, 2005; Murphy et al.,
2009; Fig. 2).
3. Geology of the xiao gurla range and India–Asia suture zone in
Southwest Tibet
Geologic mapping was conducted within the Xiao Gurla Range and
north across the India–Asia suture zone on 1:50,000 scale satellite
images. Metasedimentary rocks within the range were divided into
three units: (1) phyllite with minor metacarbonate, and quartzite
(‘phy’, Fig. 3); (2) garnet-bearing schist, quartzo-feldspathic schist,
paragneiss, and quartzite (‘gs’, Fig. 3); and (3) marble and calc-gneiss
(‘m’, Fig. 3, Fig. 4A and B). Metamorphic minerals commonly present
in schist samples include garnet, sillimanite, tourmaline, staurolite,
chlorite, and muscovite (see supplementary data). The metasedimentary units are intruded, in some areas extensively, by granite bodies,
and two-mica granite and leucogranite (+garnet ± tourmaline) sills
and dikes. Mylonitic augen gneiss (biotite + k-feldspar + quartz +
sillimanite) is exposed on the south side of the Xiao Gurla Range. In
addition, serpentinized ultramafic rocks associated with the KiogarJungbwa ophiolite and low-grade Tethyan sequence strata are
exposed structurally above the metasedimentary units that compose
the core of the range (Fig. 3). Tethyan strata consist of arkosic
sandstone, shale, and limestone and are mapped as undifferentiated
Paleozoic–Mesozoic strata, although we infer that most are upper
Paleozoic. Quaternary terrace deposits nonconformably overlie the
orthogneiss on the south side of the range.
The metasedimentary rocks and some granitic bodies in the Xiao
Gurla Range show variable degrees and styles of deformation (Fig. 4A
and B). Stretched quartz, mica, and feldspars define a pervasive
stretching lineation for most of the Xiao Gurla Range (Fig. 3). The mean
stretching lineation trends toward 280°, which is indistinguishable
from the slip vector of the Gurla Mandhata detachment fault and mean
stretching direction for rocks in its footwall (Murphy et al., 2002).
Where observed, shear bands and mesoscale S–C fabrics indicate topto-the-west sense of shear (Fig. 4C). Brittle-ductile mesoscale faulting
indicates top-to-the-west with a normal sense of offset (Fig. 4D).
Winged objects, typyically σ-clasts and δ-clasts mantled by feldspar
porphyroclasts indicate top-to-the-west sense-of-shear (Fig. 4E). The
dismemberment of leucogranite dikes along shear planes is pervasive
throughout the range. This deformation coupled with local stretching
dircection data suggests top-to-the west sense-of-shear (Fig. 4F). We
interpret the metasedimentary rocks of the Xiao Gurla Range to be
exposed in the footwall of a low-angle normal (detachment) fault
based on the juxtaposition of lower-grade on higher-grade rocks, east–
west stretching, and the similarity of structural style with Gurla
Mandhata. As with the Gurla Mandhata detachment, most of the fault
zone of the Xiao Gurla range is eroded, projecting above the range top.
In addition, the fault surface is buried along the western flank of the
range beneath Neogene (?)–Quaternary basin fill in buttress unconformity with metamorphic rocks.
Sedimentary rocks exposed within the India–Asia suture zone
consist of serpentinite bearing mélange, conglomerate of inferred
Paleogene age, and Neogene basin fill. The mélange includes blocks of
serpentinized ultramafic rocks, Cretaceous fossiliferous limestone,
turbiditic lithic sandstone, bedded chert, and marble, within a black
shale matrix (‘mlg’, Fig. 3). Unconformably overlying the mélange unit
is a matrix-supported carbonate-clast conglomerate (‘cgl3’, Fig. 3) and
a clast-matrix supported limestone conglomerate (with some
quartzite, black and red shale, lithic sandstone, and chert clasts)
(‘cgl2’, Fig. 3). The two conglomerate units along with the mélange
unit are exposed in the hanging wall of an east–west striking, southdipping fault that is interpreted to be part of the Great Counter thrust
system. Another conglomerate unit (‘cgl1’, Fig. 3) is exposed in the
footwall of this thrust and nonconformably overlies Gangdese granite
of the southern Lhasa terrane. This conglomerate consists of clastmatrix supported granite pebble–cobble conglomerate and is interpreted to be a part of the Oligo-Miocene Kailas Formation. Intrusive
rocks studied here consist of a biotite-bearing granite that intrudes
suture-zone mélange and a biotite-bearing granite exposed north of
the suture (sample 07AT136, Fig. 3).
4. Geochronological results
4.1. Methods
Zircons separated from 11 detrital and 7 igneous or meta-igneous
samples were dated using U–Th–Pb isotopic ratios determined with a
laser-ablation-multicollector-inductively-coupled-plasma-massspectrometer at the Arizona LaserChron Center. Zircon grains were
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A. Pullen et al. / Tectonophysics 501 (2011) 28–40
Fig. 3. Geologic map and schematic north–south and east–west cross-sections of the Xiao Gurla Range and India–Asia suture zone.
separated using conventional techniques, mounted in epoxy, and then
polished to expose grain interiors. Laser ablation was conducted with
a 35 μm beam diameter and at 8 Hz for most detrital and igneous
samples (e.g. Gehrels et al., 2008). Zircons from leucogranites and
meta-igneous samples were examined using cathodoluminescence
microscopy prior to dating. Most grains exhibited zoning patterns
typical of igneous cores with thin (~5–20 μm) high U metamorphic or
recrystallized rims (Corfu et al., 2003) that required a small laser
aperture (10 μm) in order for ablation pits to remain within
metamorphic growth domains. The small spot sizes required isotopic
measurements to be made using total counts of 238U, 207Pb, 206Pb, and
Pb; Pb isotopes where measured on channeltron electron multiplying collectors while U and Th were analyzed using a faraday
collector (Johnston et al., 2008). Isotopic ratios and an outline of
analytical techniques are available in supplementary data (Table A1).
U–Pb ages N1.0 Ga are reported as 206Pb/207Pb ages and ages b1.0 Ga
are reported as 206Pb/238U ages. Uncertainties associated with igneous
and metamorphic ages reported here include random and systematic
errors added quadratically.
4.2. Detrital zircon results
U–Pb detrital zircon studies have been previously used to
differentiate the tectonostratigraphy of the Himalayan fold-thrust
belt (DeCelles et al., 2000, 2004; Gehrels et al., 2006b; Martin et al.,
2005; Myrow et al., 2003; Parrish and Hodges, 1996). In order to
determine the affinity of metamorphic rocks in the Xiao Gurla Range,
we conducted U–Pb analyses on detrital zircon samples from the
range. Where studied, the Tethyan Himalayan sequence generally
exhibits greenschist–sub-greenschist facies metamorphism. The moderately deformed nature and higher grade metamorphic facies exhibited
by rocks in the Xiao Gurla Range required further examination to
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Fig. 4. Field photos from the footwall of the Xiao Gurla Range normal fault. (A) Typical parasitic folding of calc-gneiss unit; hinge-lines and axial planes are generally oriented east–
west for this lithology. (B) Weakly deformed two-mica granitic dike cutting across foliation of marble-chert unit; (C) S-C fabric developed in mylonitic granitic sill suggesting top-tothe west sense of shear; (D) mesoscale brittle normal faults in phyllitic metasandstone indicating top-to-the west extension; (E) σ-clasts in quartz-biotite schist indicating top-tothe west sense of shear; and (F) deformed leucogranite dike intruded into phyllite and quartz vein.
substantiate identification as Tethyan Himalayan sequence rocks.
Detrital zircon samples of quartz-schist, quartzite, calc-silicate gneiss,
and quartz-phyllite footwall rocks (07AT76, 07AT31, 07AT48, 07AT84,
and 07AT86; Fig. 5) yielded a broad distribution of detrital zircon U–Pb
ages in the range from ~1000–750 Ma with more tightly defined peaks
in the ranges of 1000–920 Ma and 580–500 Ma. These distributions of
ages are similar to those that have been determined for Tethyan
Himalayan strata in the previous work (‘Tethyan’, Fig. 5) as well as
Tethyan Himalayan strata in the Annapurna region of Nepal to the
southeast of the field area (DeCelles et al., 2000; Martin et al., 2005;
‘Thini Chu1’, Fig. 5). These metasedimentary samples include lithologies
and metamorphic mineral assemblages similar to much of the Greater
Himalayan sequence. However the units reported here yield much
younger maximum depositional ages and are interpreted to be of
Tethyan Himalayan affinity. Sample 07AT143 taken from structurally
beneath ultramafic rocks associated with the Kiogar-Jungbwa ophiolite
and in the hanging-wall of the Gurla Mandhata detachment system also
yielded age distributions similar to known Tethyan Himalayan strata
(Fig. 5).
Detrital zircon samples were also collected from rocks in the India–
Asia suture zone in the hanging-wall of the Great Counter thrust to
determine maximum depositional ages and provenance of these units.
Sample 6-11-07-1DZ, a coarse-grained sandstone with abundent mica
clasts, was exposed in depositional contact atop the mélange
unit (‘mlg’, Fig. 3). This sample yielded a single cluster of ages in the
96–56 Ma range, with a youngest age population (n ≥ 3 overlapping
ages) at 75 Ma providing a maximum depositional age. Three samples
(07AT174, 07AT180, and 07AT181) were taken from the limestone
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Fig. 5. U–Pb age probability plots for detrital zircon samples from the Xiao Gurla Range and India–Asia suture zone. Composite age probability plots for the Tethyan, Greater
Himalayan, and Lesser Himalayan sequences were compiled from previous work (DeCelles et al., 2000; Gehrels et al., 2006a,b; Martin et al., 2005).
conglomerate (‘cgl2’, Fig. 3) unit deposited above serpentinized
mélange. They yielded age probability peaks similar to those of Tethyan
Himalayan strata and minimum age populations of 513–502 Ma (Fig. 5).
This conglomeratic unit is interpreted to have been derived from
Tethyan Himalayan strata and to be early Cenozoic in age based on
depositional and structural relationships. This interpretation is consistent with Paleogene shortening in the Tethyan fold-thrust belt and
sedimentation within the Himalayan foreland basin (Burg et al., 1984;
DeCelles et al., 2004; Najman et al., 2005; Searle, 1986).
4.3. Igneous and metamorphic zircon results
Zircons from leucogranite and gneiss exposed in the footwall of the
Xiao Gurla normal fault system were analyzed to better understand
the timing of metamorphism and crustal anatexis. The TuffZirc age
extractor (Isoplot 3.0, Ludwig 2003) was applied to these samples
because of complicated 206Pb/238U ages from inheritance. This
algorithm assigns an age to the median of the largest cluster of ages
and is reliable in the presence of xenocryst cores and Pb loss (Ludwig
and Mundil, 2002). Uncertainties reported here include random
and systematic errors. Sample 07AT169 from the mylonitic augen
gneiss yielded a TuffZirc age of 35.2 ± 1.6 Ma (98.4% confidence, from
coherent group of 10) and a significant cluster of ages around
~461 Ma (n = 9) (Fig. 6). This age was generated from 45 spot
analyses including 15 analyses on the rims of the crystals. Analyses
from the rims of zircon crystals yielded ~206Pb/238U ages in the range
of 31–40 Ma, whereas the remaining analyses, from the cores of
crystals, yielded inheirited early Palezoic and older ages. Analyses
from 07AT169 that yielded ~ 35 Ma ages where from high-U
(~N2000 ppm) overgrowths around inherited cores exhibiting typical
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Fig. 6. U–Pb concordia and age probability plots for igneous and meta-igneous rocks from the Xiao Gurla Range and India–Asia suture zone.
igneous oscillatory zoning and convoluted zoning (Corfu et al., 2003)
(Fig. 7A). The ~ 35 Ma overgrowths have high U/Th ratios (N10)
suggesting that they crystallized during metamorphism (e.g. Rubatto,
2002). These results are interpreted to indicate Late Eocene
metamorphism of Cambro-Ordovician gneisses that have been
documented near the top of the Greater Himalayan sequence (Gehrels
et al., 2003; Stöcklin, 1980; Stöcklin and Bhattarai, 1977). The rocks in
the footwall of the Xiao Gurla fault, interpreted as Tethyan-affinity,
are not observered in fault contact with the underlying Greater
Himalayan rocks. We speculate that the Gurla Mandhata detachment
(which exposes Greater Himalayan sequence rocks in its footwall in
the Gurla Mandhata Range) cuts up-section to the north of the Xiao
Gurla Range based on the prescence of this augen gneiss and
structural similarities between the two ranges. This implies that the
tectonostratigraphy may continue uninterrupted from the Gurla
Mandhata Range to the east and that the gneiss exposed on the
south-side of the Xiao Gurla Range is metamorphosed equivalent
to the Cambro-Ordovician gneisses near the top of the Greater
Himalayan sequence.
Sample 07AT30, a two-mica granite sill intruding a garnet-biotite
schist was sampled from the Xiao Gurla Range in the footwall of the
low-angle normal fault. This sampled yielded a TuffZirc 206Pb/238U age
of 19.5 ± 1.5 Ma (93.8% confidence, from a coherent group of 7). This
age is determined from analysis of 47 spots within individual zircon
crystals, including 17 analyses from high-U rims and 30 analyses from
inherited cores (Fig. 7B). Analyses were conducted on three additional
granitoid samples following a similar protocol where laser ablation
analyses were conducted on both rim overgrowths and zircon cores.
All three of these sample yielded 206Pb/238U ages of ~ 18.6–15.5 Ma as
well as significant populations of Late Proterozoic–Paleozoic xenocryst
core ages. Sample 07AT216 (garnet + tourmaline leucogranite) was
taken from a leucogranite dike intruded into a mylonitic augen gneiss
in the Gurla Mandhata Range (Fig. 2). This sample yielded a TuffZirc
Pb/238U age of 18.6 ± 0.9 (98.4% confidence, from a coherent group
of 7). Sample 07AT80 (two-mica granite) intruded into a pyroxene
bearing marble and chert unit in the footwall of the Xiao Gurla Range.
This sample yielded a TuffZirc 206Pb/238U age of 17.2 ± 1.3 (96.9%
confidence, from a coherent group of 6). The dike is weakly deformed
compared to the host rock and cuts arcoss foliation in the marble and
deformation fabric of folded chert beds (Figs. 4B, 7C). Sample 07AT51 is
from a garnet-tourmaline bearing pegmatitic leaucogranite sill that
intruded phllytic metasedimentary rocks. This sample yielded a TuffZirc
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Fig. 7. Representative cathodoluminescence images for some igneous and meta-igneous zircon samples. Laser ablation pits are highlighted with white circles. (A) mylonitic augen
gneiss sample 07AT169; (B) two-mica granite sample 07AT30; (C) two-mica granite sample 07A80; (D) leucogranite sample 07A51; and (F) granite pluton sample 07AT136.
Pb/238U age of 15.5± 0.7 (97.9% confidence, from a coherent group
of 10). The sill body is weakly deformed showing some boudinage
structures but lacks the foliation of the host rock (Figs. 6, 7D). The 18–
15 Ma growth domains of these two-mica- and leaucogranite samples
yielded high U/Th (typically N 10), however we suggest that domains of
this age closely date the crystallization of these bodies and were from a
source zone that may have undergone monazite fractionation (e.g. the
Great Himalayan sequence) rather than dating the metamorphism of
these leucogranite bodies.
Two additional samples from the India–Asia suture zone were
dated to provide age control on a cross-cutting relationship and to
better define the age of magmatism within the Gangdese batholith.
Sample 07AT136, a granite pluton (quartz + k-feldspar phenocrysts +
biotite + chlorite) intruded into the serpentinized mélange unit
exposed in the hanging-wall of the Great Counter thrust, yielded a
TuffZirc age of 43.9 ± 0.9 Ma and a weighted mean 206Pb/238U age of
43.9 ± 0.9 Ma (Fig. 3). Zircon crystals from this sample generally show
igneous oscillatory zoning (Fig. 7F). This shows that the mélange unit
formed prior to middle Eocene time. Sample 6-4-05-1, a granite
(quartz + k-feldspar + biotite), is intruded into the Gandgese batholith. Zircon crystals yielded a weighted mean 206Pb/238U age of 21.8 ±
2.9 Ma (93% confidence, from a coherent group of 8) and typically
have U/Th ratios b10. This age is considerably younger than the
youngest ages previously reported for the Gangdese batholith
(Harrison et al., 2000; Honegger et al., 1982; Xu, 1990), however is
similar to the ages of Miocene potassic volcanic rocks in the region
(e.g., Miller et al., 1999).
5. Discussion
The geology of the Gurla Mandhata–Xiao Gurla region of southwestern Tibet reveals a multi-stage history of metamorphism,
deformation, and crustal anatexis in the hinterland of the Himalayan
fold-thrust belt. Based on our findings and previous work, we present
a lithosphere-scale tectonic model for the early evolution of the
Himalayan fold-thrust belt (Fig. 8).
5.1. Paleocene–Middle Eocene
A marked decrease in the motion of the Indian plate is proposed to
have occurred at ~ 50 Ma (Copley et al., 2010; Molnar and Stock,
2009), which correlates closely with the age of eclogitization within
the Tso Morari massif (Ladakh, India) in the Himalayan belt northwest
of our field area (de Sigoyer et al., 2000; Leech et al., 2005). This
slowing in convergence rate is interpreted as the tectonic response to
buoyant, possibly thinned Indian continental material entering the
subduction zone. Exhumation of the Tso Morari rocks from N60 km to
mid-crustal depth occurred at ~ 47 Ma (de Sigoyer et al., 1997, 2000;
Leech et al., 2005; Schlup et al., 2003), shortly before emplacement of
the 44 Ma granite intruding ophiolitic mélange within the India–Asia
suture zone. Thickening of the Tethyan Himalayan sequence in
southern Tibet is also thought to have initiated by this time (Aikman
et al., 2008, Ding et al., 2005; Murphy and Yin, 2003). Detritus derived
from the India–Asia suture zone and the Tethyan Himalaya first
appears in middle Eocene strata of the Himalayan foreland basin
(DeCelles et al., 2004). Collectively, these results are consistent with
buoyant continental material having entered the subduction zone
beneath the active forearc of southern Asia by ~50 Ma. Resistance to
subduction by buoyant material resulted in slab break-off and
foundering of the Neo-Tethys oceanic slab (Bird, 1978; Davies and
von Blanckenburg, 1995; DeCelles et al., 2002; Kohn and Parkinson,
2002). Slab break-off prior to 44 Ma and the thermal perturbation
produced by replacement of subducting crust and mantle lithosphere
with asthenosphere resulted in partial melting of mantle lithosphere
or lower crust of the overriding Asian plate and emplacement of midEocene granites in the Tethyan Himalayan (Fig. 8A). The northward
drift of India with respect to the detached oceanic slab is consistent
with magmatism south of the Gangdese batholith (e.g. Kohn and
Parkinson, 2002).
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Fig. 8. Model for the Cenozoic deformation, metamorphism, and magmatism within the the Tethyan Himalayan physiographic zone. GCT— Great Counter thrust, GT— Gangdese
thrust, MCT— Main Central thrust, RT— Ramgarh thrust, and STDS— South Tibet detachment system. A: From 50 to 38 Ma, break-off of Neo-Tethyan oceanic lithosphere resulted in
magmatism south of the India–Asia suture. B: ~ 35 Ma shortening of the Tethyan Himalayan sequence through crustal duplexing, Tethyan Himalayan topographic high shedding
detritus to the north towards the India–Asia suture, and possible underthrusting of the Great Himalayan sequence beneath Tethyan sequence strata. C: From 23 to 15 Ma,
underthrusting of the Lesser Himalayan sequence beneath the Greater Himalayan sequence along the Main Central thrust with thickening leading to crustal anatexis. D: From 15 to
9 Ma, orogen parallel extension in the hinterland of the Himalayan thrust belt tectonically exhumed high grade metamorphic Greater and Tethyan Himalayan rocks in the footwall of
the top-to-the west Gurla Mandhata detachment system as the Himalayan thrust belt propagated toward the foreland with the initiation of the Ramgarh thrust.
5.2. Late Eocene
We suggest that during Late Eocene time the lower crust of India,
now detached from dense Neo-Tethys oceanic lithosphere, began
underthrusting Asia (e.g. DeCelles et al., 2002; Ni and Barazangi, 1984;
Powell and Conaghan, 1973). In response, the Tethyan Himalayan
sequence continued to shorten (e.g. Wiesmayr and Grasemann, 2002)
and thicken, producing a topographic high within the Tethyan
Himalayan region and Tethyan Himalayan sequence strata shedding
detritus southward into the peripheral foreland basin (DeCelles et al.,
2004; Najman et al., 2005) and possibly northward towards the India–
Asia suture (i.e. samples 07AT174, 07AT180, and 07AT181 of interpreted Paleogene age) (e.g. Gansser, 1964) (Fig. 8B). We speculate
that a tectonic burial event generated the 35 Ma phase of metamorphic
zircon crystallization documented in the gneiss unit of the Xiao Gurla
Range (i.e. sample 07AT169) and could be explained by burial beneath a
south vergent package of Tethyan Himalayan sequence rocks along the
South Tibetan detachment in a thrust sense (e.g. Webb et al., 2007; Yin,
2006). In addition, migmatization within the Tethyan Himalayan zone at
~35 Ma has been documented along-strike to the east in southern Tibet
(Lee and Whitehouse, 2007). Although the metamorphic and shortening history of the Xiao Gurla Range is not well understood, the possibility
of a Eocene Tethyan topographic high, the presence of high-grade
metamorphism in Tethyan Himalayan sequence strata, augen gneiss
with 35 Ma metamorphic zircon ages, and late Eocene crustal anatexis
elsewhere (e.g. Lee and Whitehouse, 2007) suggest shortening during
this time in the Tethyan Himalaya. This shortening may have been
accommodated by crustal duplexing (e.g. Burg et al., 1984; Makovsky
et al., 1996; Wiesmayr and Grasemann, 2002). In this scenario, Tethyan
Himalayan strata that typically exhibit low grade metamorphism may
represent the upper portion of a crustal duplex system, whereas higher
grade Tethyan Himalayan strata typically exhumed in the footwalls of
the North Himalayan gneiss domes (e.g. Aoya et al., 2005; Lee and
Whitehouse, 2007; Lee et al., 2000; Watts et al., 2010) and in this study
in the footwall of the Gurla Mandhata detachment system may
represent the lower levels of this duplex (Fig. 8B).
5.3. Oligocene–Early Miocene
Footwall and hanging-wall cutoffs for the Main Central thrust are
not exposed, and therefore the amount of displacement along this
thrust is poorly known. Early motion along the Main Central thrust
occurred at 23–20 Ma (e.g. Hodges et al., 1992, 1996; Johnson et al.,
2001; Murphy and Harrison, 1999; Walker et al., 1999). Leucogranites
exposed in Gurla Mandhata were derived from Greater Himalayan
and Lesser Himalayan sequence rocks (Murphy, 2007). This implies
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A. Pullen et al. / Tectonophysics 501 (2011) 28–40
that the Lesser Himalayan sequence was buried to sufficient depth
(Godin et al., 2001) to initiate crustal anatexis and leocogranite
generation by early Miocene time (Fig. 8C). We suggest that the
leucogranites exposed in the Xiao Gurla area, like the leucogranites in
the footwall of the Gurla Mandhata detachment system, were also
likely derived from anatexis of Greater Himalayan and Lesser
Himalayan sequence rocks.
5.4. Middle–Late Miocene
Exhumation of the Gurla Mandhata–Xiao Gurla metamorphic
rocks to upper-crustal levels is attributed to arc-parallel extension
(e.g. Murphy and Copeland, 2005; Murphy et al., 2009). This
extension has been attributed to radial outward growth of the
Himalayan orogenic wedge and/or over thickening of the Tibet crust
have been argued a reasons for extension in the Tethyan physiographic zone. This extension was accomplished through normal and
strike-slip faulting (Fig. 8D; Klootwijk et al., 1985; Molnar and LyonCaen, 1988; Ratschbacher et al., 1994). Slip along the central
Karakoram fault was active by 16–14 Ma (Phillips et al., 2004) as
were the structures that led to exhumation of the Leo Pargil dome to
the northwest (Fig. 1; Thiede et al., 2006). However, if the Xiao Gurla
and Gurla Mandhata Ranges share the same footwall to the Gurla
Mandhata detachment fault, as suggested here and by previous work
(i.e. Murphy and Copeland, 2005; Murphy et al., 2009), then the
timing of exhumation of the Xiao Gurla and Gurla Mandhata Ranges is
predicted to be similar. The Gurla Mandhata detachment system is
thought to have been active beginning ~ 9 Ma (Murphy et al., 2002).
This is also consistent with the timing of initial subsidence in the Zada
basin to the northwest along strike (Saylor et al., 2009), although
orogen-parallel extension may have initiated regionally by ~ 15 Ma
(e.g. Thiede et al., 2006) and the ~17–15 Ma leucogranite bodies
intruding the Xiao Gurla Range show variable degrees of deformation
and may have been intruded synextensionally at this time (Fig. 4).
6. Conclusions
This investigation of the Xiao Gurla Range along the India–Asia suture
zone shows that metasedimentary and meta-igneous in the core of the
range are exposed in the footwall of a top-to-the west low angle normal
(detachment) fault. This fault is interpreted to be a northeastward
continuation of the Gurla Mandhata detachment system. U–Pb geochronology on zircon samples from these metamorphic rocks show that
the rocks that core the range are of Tethyan Himalayan sequence affinity.
In addition, leucogranite bodies intruding the footwall rocks yield zircon
ages clustering at ~23 Ma and ~15 Ma. Paleogene conglomerates along
the India–Asia suture zone, in the hanging-wall of the Great Counter
thrust, contain detrital zircons with U–Pb ages that indicate derivation
from Tethyan Himalayan sequence rocks to the south.
In general, this work reiterates the possibility of break-off of the
northward subducting Neo-Tethys oceanic lithosphere prior to 44 Ma
(e.g. Chemenda et al., 2000; DeCelles et al., 2002; Kohn and Parkinson,
2002) which in this study is thought to have contributed to middle
Eocene magmatism south of the India–Asia suture. In addition, the
high grade metamorphism of Tethyan Himalayan sequence rocks seen
in the Xiao Gurla Range indicates that significant tectonic thickening
took place within the Tethyan Himalaya during Eocene–Oligocene
shortening. High-grade metamorphic Tethyan Himalayan and Greater
Himalayan sequence rocks were exhumed in the footwall of the Gurla
Mandhata detachment system during orogen parallel extension
beginning at ~ 15 Ma in the hinterland of the Himalayan thrust belt.
This research was supported by grants from the U.S. National Science
Foundation (EAR-0438120, EAR-0438115, EAR-0908778, EAR-0732436).
We thank Rasmus Thiede, Shuguang Song, and an anonymous reviewer
for their careful reviews, and Ross Waldrip and John Volkmer for their
comments and suggestions.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
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