CONTRACTION AND EXTENSION IN CONVERGENT OROGENS—NORTH
HIMALAYAN GNEISS DOMES: STRUCTURAL, PETROLOGIC, AND
GEOCHRONOLOGIC ANALYSES OF MABJA DOME
PROJECT DESCRIPTION
1. Introduction
The Himalayan orogeny records the continent-continent collision and continued convergence
between India and Asia since Eocene time. This extraordinary geologic event has resulted in the
~70-80 km thick crust and high elevation of the Himalaya and Tibetan Plateau. The timing and
mechanisms of formation of overthickened crust and high elevations are hotly debated topics that
have resulted in a number of theories, including underthrusting of India, lower crustal flow,
distributed or discrete intracontinental shortening, lithospheric delamination, continental extrusion,
and combinations of these (Argand, 1924; Zhao and Morgan, 1985; Dewey and Burke, 1973;
England and Houseman, 1988; Tapponnier et al., 1982; Harrison et al., 1992; and many others).
These models make predictions about the evolution of the Himalaya and Tibetan plateau that can be
tested by examining the geometry, kinematics, and timing of deformational, intrusive, metamorphic,
and exhumation events. Yet, our ability to test these models is limited because our knowledge of the
geologic history of the Tibetan Plateau is still in its infancy.
One of the more prominent features within southern Tibet is the belt of North Himalayan gneiss
domes, 12-15 metamorphic and plutonic culminations that lie south of the Indus-Tsangpo Suture
Zone (ITSZ), north of the Southern Tibetan Detachment System (STDS), and along the axis of the
North Himalayan antiform within the Tethyan Himalaya (Fig. 1). Gneiss domes are found in
orogenic belts worldwide and are typically composed of a core of granitic migmatites or gneisses
structurally overlain by a mantle of high-grade metasedimentary rocks (e.g. Eskola, 1949). The
origin of gneiss domes is a world-class problem and is commonly attributed to three processes:
diapirism (e.g. Ramberg, 1980), crustal shortening (e.g. Brun, 1983; Ramsay, 1967), or crustal
extension (e.g. Brun and Van Den Driessche, 1994; Miller et al., 1992). Each of these processes has
been proposed as the mechanism by which the North Himalayan gneiss domes formed and each has
distinctly different implications for the tectonic evolution of southern Tibet and the High Himalaya.
Le Fort and coworkers (Le Fort, 1986; Le Fort et al., 1987) suggested a diapiric origin for the
North Himalayan domes, whereby thrusting along the Main Central Thrust (MCT) of hot portions
of the Tibetan Slab over weakly metamorphosed sediments resulted in large-scale release of fluids
that rose above the MCT and induced anatexis. Doming was suggested to result from compressivestress induced undulations enhanced by the buoyancy of anatectic melts (Le Fort, 1986; Le Fort et
al., 1987). Harrison et al. (1997), on the basis of numerical simulations, suggested that the anatectic
leucogranites were products of deformational heating along the Himalayan decollement. They
implied that these magmas were hot enough and of low enough viscosity to rise diapirically into the
middle crust.
In contrast, Burg et al. (1984) argued that the Kangmar Dome, one of the North Himalayan
gneiss domes (Fig. 1) formed by a thrust duplex at depth on the basis of reconnaissance
observations of S-verging folds in the upper part of the metasedimentary carapace, the reorientation
of fold axes to a N–S trend in the lower part of the carapace, and increasing shear strain with
structural depth. More recent reconnaissance studies in the Kangmar Dome (Chen et al., 1990)
documented a top-to-the-N mylonitic fabric in the high-grade metasedimentary rocks and
orthogneiss. In addition, Chen et al. (1990) interpreted the contact between the orthogneiss core and
metasedimentary mantle as an extensional detachment fault, similar to those associated with
C-1
C-2
metamorphic core complexes of the western US, and argued that displacement along the Kangmar
detachment was northward, the same as the STDS. They concluded that like the STDS, the
Kangmar Dome is an extensional feature that formed in response to gravitational collapse of the
Himalayan topographic front.
On the basis of new detailed geologic mapping, structural analysis, metamorphic petrology, and
thermochronology in the Kangmar Dome, Lee et al. (1999, in review a, b) suggested that the domal
form resulted from contraction followed by extension, and ending with exhumation by thrust
faulting and erosion. Lee and colleagues interpreted the formation of the extensional fabrics as a
result of vertical thinning and horizontal extension at mid-crustal depths as a consequence of
maintaining a stable wedge geometry. Subsequent doming during the middle to late Miocene was
attributed to thrusting upward and southward over a north-dipping ramp along the GyirongKangmar Thrust fault system (GKT) (Fig. 1) above cold Tethyan sediments. If true, these relations
imply that middle to late Miocene thrusting in the Kangmar Dome region was synchronous with
normal slip along the STDS and thrust motion along the Renbu Zedong thrust fault (RZT) (Fig. 1),
leading Lee et al. (in review a) to hypothesize a kinematic link among these structures and
simultaneous contractional and extensional deformation within southern Tibet.
This exciting new kinematic picture of the evolution of the Himalaya and southern Tibet needs
to be confirmed. To test this hypothesis, we propose to characterize the deformational,
metamorphic, and exhumation history of the Mabja Gneiss Dome, located ~150 km west of the
Kangmar Dome (Figs. 1 & 2), because preliminary studies suggest that it exposes similar
deformational and metamorphic histories as in the Kangmar Dome. However, the Mabja Dome
exposes migmatites, syn- to post-tectonic pegmatites, and syn(?)- to post-tectonic granites (Lee et
al., 1998) suggesting a diapiric origin or diapirism accompanied by a significant component of
regional deformation.
This proposal seeks funding for a two-year period to conduct an integrated field-based
investigation of the Mabja Gneiss Dome in collaboration with Dr. Wang Yu, of the Institute of
Geology, State Seismological Bureau, Beijing, China. Our primary objective is to document and
characterize the geometry, kinematics, magnitude, and timing of brittle and ductile deformation, the
pressure and temperature conditions of high-grade metamorphism during deformation, the timing of
metamorphism and magmatic activity, and the cooling history in the Mabja Dome in order to assess
whether it’s origin is contractional, diapiric, extensional, or some combination of these. In the ideal
end-member setting, each of these postulates make specific and testable predictions:
(1) If the dome formed as a result of contraction, we expect to document:
• a unidirectional stretching lineation and an up-dip sense of shear;
• structural duplication across major faults or shear zones;
• that metamorphism was the product of conductive relaxation of isotherms;
• that magmatism was not necessarily genetically linked to deformation; and
• that cooling ages (exhumation) are asymmetric across the dome and decrease down structural
dip in a direction opposite to the sense of shear.
(2) If, on the other hand, the dome formed as a result of diapirism, we expect to document:
• a radially oriented stretching lineation and down-dip sense of shear;
• no structural duplication or omission;
• spatial and temporal association of magmatism, metamorphism, and deformation; and
• symmetrically distributed cooling rates that decrease from rapid to slow, indicative of cooling
following pluton emplacement.
(3) Finally, if the dome formed as a result of extension, we expect to document:
• a unidirectional stretching lineation and down-dip sense of shear;
C-3
• structural omission across major faults or shear zones;
• that magmatism was not necessarily genetically linked to deformation; and
• that cooling ages (exhumation) are asymmetric across the dome and decrease down structural
dip in the direction of shear.
Documenting and characterizing the processes that created these gneiss domes have important
implications for mechanisms proposed for development of overthickened crust and high elevation in
the Himalaya and Tibetan plateau. For example, if these domes are the result of crustal contraction,
quantifying the magnitude and geometry of contraction involved in their exhumation may yield
better constraints on the degree of deformation partitioning between crustal contraction and lateral
extrusion, both of which accommodate convergence between Asia and India (e.g. Tapponnier et al.,
1982). Furthermore, if we can constrain the timing of this contractional deformation, we can assess
whether slip along the STDS and contractional deformation within the gneiss domes occurred
contemporaneously or sequentially during the Miocene. If, on the other hand, these domes are
diapirs, documenting the geometry and timing of diapirism will lead to a better assessment of
Harrison et al.’s (1997) numerical model for their generation by shear heating along the Himalayan
decollement and subsequent anatexis. In order to test existing mechanisms of gneiss dome
development, or to formulate new mechanisms, we need a much better understanding of the
structure, kinematics, metamorphic conditions, and timing of deformational, intrusive, and
metamorphic events within the core, the mantle of high grade metamorphic rocks, and the
surrounding lower grade rocks. Finally, data from our studies, integrated with results from seismic
reflection investigations in Tibet by other workers (e.g. INDEPTH), will provide an excellent
opportunity to document the space-time evolution of gneiss dome formation within the tectonic
framework of southern Tibet. Distinguishing the process that led to unroofing of these domes has
important implications for the tectonic evolution of the Tibetan Plateau.
2. Geologic Setting
A series of plutonic and metamorphic culminations referred to as the North Himalayan Gneiss
domes are exposed within the Tethys Himalaya, just south of the ITSZ (Fig. 1). This region is
underlain by a miogeoclinal sedimentary sequence deposited upon the passive northern margin of
the India continent. This marine sedimentary sequence is nearly continuous in age from Cambrian
to Eocene (Gansser, 1964; Le Fort, 1975), with the Eocene marine sedimentary rocks probably
marking an upper bound on the timing of the India-Asia continental collision. The zone is
structurally complex, exhibiting Cretaceous to Holocene contractional and extensional structures in
a variety of orientations (e.g. Armijo et al., 1986; Burg and Chen, 1984; Le Fort, 1975; Quidelleur
et al., 1997; Ratschbacher et al., 1994; Searle, 1983). The oldest structures are S-directed thrust and
fold nappes of Paleocene age that are probably related to obduction of an ophiolite (Ratschbacher et
al., 1994; Makovsky et al., 1999). Younger, Eocene to Miocene, structures include shallow-dipping,
S-directed thrusts and steeply dipping N-directed backthrusts (Burg and Chen, 1984; Coward et al.,
1988; Quidelleur et al., 1997; Ratschbacher et al., 1992, 1994, Searle et al., 1988; Yin et al., 1994).
Imbrication of the Tethyan Himalaya is thought to be Miocene (Ratschbacher et al., 1994). Pliocene
to Recent N–S striking grabens (e.g. Armijo et al., 1986) appear to be the result of gravitational
collapse of overthickened crust and high elevations. The E–W direction of extension is attributed to
eastward extrusion along the Pacific margin (e.g. Molnar and Lyon-Caen, 1989; Molnar and
Tapponnier, 1975; Molnar and Tapponnier, 1978; Tapponnier et al., 1982) and has been linked to a
postulated abrupt increase in surface elevation of the plateau as a consequence of possible
convective removal of the lower continental lithosphere (e.g. England and Molnar, 1993).
C-4
3. North Himalayan Gneiss Domes
3.1 Introduction
The North Himalayan Gneiss Domes are cored by an orthogneiss mantled by high-grade
metasedimentary rocks intruded by middle to late Miocene two-mica granites (Burg et al., 1984;
Chen et al., 1990; Lee et al., 1998, 1999, in review a, b; Maluski et al., 1988; Scharer et al., 1986).
Summaries of reconnaissance studies in the most-accessible of these domes, the Kangmar Dome, by
Burg et al. (1984) and Chen et al. (1990), are provided in the Introduction (see section 1). Below,
we present a summary of our previous multidisciplinary investigations within the Kangmar Dome,
the most accessible of the domes, and preliminary studies within the Mabja Dome. The primary
contribution of this research is documentation of the space-time evolution of gneiss dome formation
within an overall convergent setting, and assessment of the implications for the tectonic evolution of
the Tibetan plateau. This research was funded by National Science Foundation grant EAR-9526861,
and was conducted in collaboration with Drs. Wang Yu and Chen Wenji, Institute of Geology, State
Seismological Bureau, Beijing, China.
3.2 Kangmar Dome
The Kangmar Dome lies in the hangingwall of the GKT (Fig. 1) (Burg and Chen, 1984; Lee et al, in
review a; Ratschbacher et al., 1994; Wu et al., 1998) and is cored by a Cambrian orthogneiss (Lee et
al, in review a, b; Scharer et al., 1986) that is mantled by medium- to high-temperature/moderate
pressure metapelites; the grade of metamorphism decreases upsection. Two dominant deformational
events are preserved within the Kangmar Dome. The older event, D1, is best exposed at high
structural levels within unmetamorphosed to low-grade metamorphic rocks on the northern and
southern flanks of the dome. This deformational event resulted in ENE-WSW trending, tight to
isoclinal, S-vergent F1 folds of S0 with an associated moderately to steeply NNW-dipping axial
planar foliation, S1.
The structural fabrics associated with the second deformational event, D2, are manifested at
higher structural levels as a series of open, recumbent, NS-trending folds of bedding and the S1
foliation. With increasing structural depth, bedding and the S1 foliation are first crenulated by a
spaced, subhorizontal S2 foliation and finally transposed parallel to a mylonitic S2 foliation.
Associated with the high strain foliation is a N–S stretching lineation. The S2 foliation dips
moderately north on the north flank and moderately south on the south flank, defining the domal
form. Shear sense within the orthogneiss and high grade metasedimentary rocks during formation of
S2 varies from top-to-the-S on the south dipping flank to top-to-the-N on the north dipping flank of
the Kangmar Dome; the central part of the dome exhibits opposing shear sense indicators or
symmetric fabrics.
The contact between the orthogneiss and the overlying metapelites ranges from a knife-sharp,
concordant boundary with no mesoscopic evidence of brittle deformation, to a locally discordant
contact with an up to 1 m wide zone of fault gouge and breccia developed within crushed and
sheared schist; there is no evidence of brittle deformation within the underlying orthogneiss. We
attribute this local deformation to the rheological contrast between the schist and orthogneiss
because there is no evidence for structural omission across this contact. Furthermore, there is no
discontinuity in mica 40Ar/39Ar ages across this contact (see below), supporting our field
interpretation. Elsewhere in the dome faults are scarce.
Pelites within the metasedimentary carapace record prograde metamorphism, with successive
chloritoid-in, chloritoid-out/garnet-in, staurolite-in, and kyanite-in isograds toward the orthogneiss.
The isograds cut across mapped units and structures, defining a thermal high northeast of the center
of the dome; this requires that the orthogneiss exposed in the core of the dome was not the heat
C-5
source responsible for metamorphism. Microtextures, including crenulated quartz and opaque
inclusion trails within garnet porphyroblasts that we interpret as preserved S1 foliation crenulated
by S2, indicate that these minerals grew after the D1 deformation and prior to or during the early
stages of D2 deformation. Peak conditions reveal an increase in temperature from ~445°C in garnetzone rocks to ~624°C in staurolite+kyanite zone rocks. Significantly, peak pressures increase
northward across the dome from ~660 MPa to 850-930 MPa, with the highest pressures preserved
on the north flank of the dome. In addition, these data indicate an apparent gradient in pressure of
~125 MPa/km, well in excess of the expected gradient of 27 MPa/km for supracrustal rocks with an
average density of 2700 kg/m3. This vertically shortened pressure gradient requires a factor-of-five
subhorizontal stretching after the pressure gradient was “frozen in”.
40
Ar/39Ar dating of mica and K-feldspar from the orthogneiss and metasedimentary rocks along a
transect parallel to the N–S trending stretching lineation shed considerable light on the cooling
history. Muscovite generally yielded plateaus or slightly disturbed spectra, with ages of 12.2 Ma in
schists on the northern flank of the dome that increase to ~15 Ma across the central and southern
part of orthogneiss and overlying schist. Biotite yielded plateaus or slightly more disturbed spectra,
with ages that are slightly younger than muscovite in the schist, but older than muscovite within the
orthogneiss. Six K-feldspars from the orthogneiss yielded complex spectra with old apparent ages at
the lowest temperature steps, followed by ages that climb gradually from ~10 Ma to ~11 Ma over
the first 45-65% of 39Ar released. Over the last 40-60% of the 39Ar released, all six samples exhibit
complex age spectra characterized by ages that climb steeply and erratically indicating
incorporation of excess argon; ages as old as 45-70 Ma occur at the high temperature steps.
Diffusion modeling (Lovera et al., 1989) of these data indicates relatively rapid cooling (~1030°C/Ma) from as high as ~350°C to as low as ~250°C between 11.5 Ma and 10.0 Ma
synchronously across the dome. Six apatite separates, from the same rock samples that provided the
potassium feldspar separates, yield fission track ages ranging from 4.1±1.9 to 7.9±3.0 Ma (±1 σ).
These ages are indistinguishable, indicating that the dome continued to cool symmetrically through
approximately 120°C at 5.5 Ma, the mean age for all samples. The cooling rate between muscovite
closure and apatite retention was rapid, 25-30°C/Ma, consistent with the calculated K-feldspar
cooling histories.
There are five notable observations that fall from the thermochronology data (Lee et al., in
review a). First, mica ages increase down section, compatible with cooling from below due to
underthrusting of colder rock. Second, mica ages young northward within a given structural
horizon, suggesting that the north flank of the dome resided at slightly deeper structural levels,
compatible with the northward increase in peak metamorphic pressures and temperatures within a
given structural horizon. Accordingly, if we assume subhorizontal isotherms, this relation implies
that exhumation at the northern end of the dome through the 370°C to 335°C isotherms occurred 23 m.y. after the southern flank of the dome. Implicit in this interpretation is that the dome was tilted
northward and exhumed southward. Third, there is no discontinuity in mica ages across the contact
between the orthogneiss and overlying schist, supporting our field interpretation that there is no
structural omission across, and little brittle motion along, this contact. We can not, however, rule
out the possibility that the orthogneiss and metasedimentary rocks had different cooling histories
following peak metamorphism but similar histories (i.e. cooling below ~370-335°C) following
juxtaposition due to faulting (e.g. Chen et al., 1990). Fourth, the uniform cooling histories derived
from the K-feldspar and apatite data suggest that the dome was symmetrically exhumed between
approximately 11 Ma and 5.5 Ma. Fifth, the cooling rate of about 25-30°C/Ma, from the closure
temperature for muscovite to the annealing temperature for apatite, appears to have been relatively
constant across the dome and reflects both refrigeration and exhumation.
C-6
The structural history and PT conditions we have documented in the Kangmar Dome lead to the
following tectonic history. The miogeoclinal section exposed within the Kangmar Dome was
thickened and buried by distributed folding during D1 deformation, such that staurolite-kyanite
zone rocks, the bottom of the section, were buried to ~30 km depth. We attribute metamorphism to
the conductive relaxation of isotherms, because microtextural evidence indicates peak
metamorphism occurred after D1 and prior to or during the early stages of D2, and there is no field
evidence for a magmatic heat source. Following peak metamorphism, the rocks were stretched
subhorizontally by a factor of about five, collapsing the apparent isobars to ~20% of their original
thickness. The dome was subsequently tilted southward ~3°; the northern end of the dome was
lifted about 1 km relative to the southern end. Finally, the core was domed 2-3 km upward relative
to the outside to create the domal form. The S2 foliation, isobars, isotherms, and mica 40Ar/39Ar
isochrons are domed, but K-feldspar 40Ar/39Ar isochrons are not, implying that doming occurred at
temperatures of ~335-300°C at about 11 Ma. In summary, our work has documented a first phase of
deformation characterized by N-S contraction leading to thickening in the Kangmar Dome region,
followed by thermal re-equilibration and peak metamorphism, which in turn was followed by a
second phase of deformation characterized by horizontal extension, vertical thinning, and ending
with a third phase of deformation characterized by doming and exhumation.
In addition to the data we have collected from the Kangmar Dome, there are two important
regional relations that bear on its development and its role in the tectonic evolution of southern
Tibet. First, INDEPTH seismic reflections have been interpreted to show a ~35 km high antiformal
duplex in the hangingwall of a crustal ramp along the Main Himalayan Thrust (MHT) beneath the
core of the Kangmar Dome (Hauck et al., 1998). Second, surface geologic mapping indicates that
the Kangmar Dome lies in the hangingwall of the N-dipping GKT (Burg and Chen, 1984; Lee and
Dinklage, unpubl. mapping; Ratschbacher et al., 1994; Wu et al., 1998), a fault not imaged by the
INDEPTH seismic data.
Our data, along with these regional relations, rule out a simple metamorphic core complex,
diapir, or thrust-duplex origin for the Kangmar gneiss dome for three salient reasons: D1 fabrics
indicate contractional deformation, D2 fabrics indicate extensional deformation, and the mica
cooling ages suggest underthrusting of a cold slab. One way to explain alternating contraction and
extension is to consider the hanging wall of the MHT as a southward-tapering orogenic wedge (e.g.
Platt, 1986). Lee et al. (in review a) suggest that D2 extensional fabrics formed as a consequence of
increased wedge thickness and decreased rock strength. Underplating of large amounts of material
into the core of the antiform that overlies the N-dipping crustal ramp along the MHT would have
caused an increase in wedge thickness. The rheologically weakened middle crust could then have
thinned vertically and stretched horizontally, resulting in the development of the subhorizontal D2
fabrics, and decreasing wedge thickness. Continued underplating at depth and extension at midcrustal levels could have transported Kangmar rocks to shallower crustal levels. We interpret rapid
cooling (25-70°C) at 15 to 11 Ma as refrigeration by underthrusting of cold Tethyan sediments.
Because mica cooling ages young northward within a single structural horizon, this implies that
Kangmar rocks were captured in the hangingwall of a north-dipping thrust fault, the GKT, and
uplifted southward during the middle Miocene. Subsequent movement of these rocks up and over a
N-dipping ramp along the GKT at ~11 Ma and temperatures of 335-300°C resulted in doming of the
S2 mylonitic foliation, the metamorphic isobars and isotherms, and mica 40Ar/39Ar isochrons.
Symmetric cooling of the dome from approximately 300°C to 120°C between approximately 11 and
5.5 Ma implies that rapid exhumation (~20-40°C/Ma) due to erosion followed thrust faulting.
Because the timing of thrusting along the Gangdese and RZT thrust fault systems to the north is
estimated to be late Oligocene to early Miocene and early to late Miocene, respectively (Yin et al.,
C-7
1994; Ratschbacher et al., 1994; Quidelleur et al., 1997), and initial slip along the MCT is estimated
to be late Oligocene to early Miocene (Hubbard and Harrison, 1989; Harrison et al., 1995; Coleman
and Parrish, 1995), these relations suggest that vertical thinning and horizontal extension in the
Kangmar region may have been concurrent with contractional deformation to the north and south. In
addition, geochronologic studies reveal that slip along the STDS in the Wagye La and Khula Kangri
areas (Fig. 1), south and southeast, respectively, of the Kangmar Dome, occurred at about 12 Ma
(Edwards and Harrison, 1997; Wu et al., 1998). The 15–11 Ma cooling history we have documented
in the Kangmar Dome is synchronous with this, implying that normal slip along the STDS was
accompanied by contraction in the hangingwall of the STDS. This raises the possibility that normal
slip along the STDS was accompanied by thrust faulting in the Kangmar region and along the RZT,
suggesting a kinematic link among these three structures (Lee et al., in review a).
3.3 Mabja Dome
The regional setting of the Mabja Dome is similar to that of the Kangmar Dome (Fig. 1). The Mabja
Dome is exposed within the core of the North Himalayan antiform and lies in the hangingwall of the
GKT. Our preliminary studies within the west-central part of the Mabja Dome (Fig. 2) reveal that
its geologic history is also similar to the Kangmar Dome (Lee et al., 1998b). However, significant
differences suggest that the Mabja Dome did not form by thrusting of the core southward over a
north-dipping ramp along the GKT (e.g. Lee et al., 1999, in review a). The deepest structural levels
of the Mabja Dome are underlain by a K-feldspar augen biotite orthogneiss and incipient
migmatites, which are in turn are overlain by high-grade metasedimentary rocks and granitic
orthogneisses (Fig. 2). The grade of metamorphism, defined by sillimanite-in, kyanite-in, staurolitein, garnet-in, and chloritoid-in isograds, decreases upsection and dies out at the highest structural
levels, where unmetamorphosed clastic rocks are exposed. Mesoscopic fabrics indicate that peak
metamorphism and isograd development occurred after D1 deformation and prior to or during the
early stages of D2 deformation.
Preliminary studies have identified two major penetrative deformational events within the
Mabja Dome. The first deformational event, D1, exposed at structural levels above the garnet-in
isograd, resulted in WNW–ESE trending, open to tight to isoclinal F1 folds of S0 and an associated
moderately NE-dipping axial planar foliation, S1.
The second event, D2, exposed at structural levels below the garnet-in isograd, crenulated
bedding and S1 foliation at high structural levels and transposed bedding and S1 into parallelism to
a high strain mylonitic foliation, S2, at deeper structural levels. Associated with the S2 foliation is a
~N–S stretching and mineral alignment lineation. On the basis of our mapping to date, the S2
foliation is somewhat domed across the area: it dips moderately to steeply SSW on the southern
flank of the dome and moderately NW on the northern flank. Mesoscopic kinematic indicators
associated with the S2 mylonitic foliation record top-to-the-S shear on the south-dipping flank and
both top-to-the-N and top-to-the-S shear on the north-dipping flank of the Mabja Dome. The Kfeldspar augen orthogneiss exhibits predominantly symmetric fabrics. Faults are scarce, and there is
no evidence for significant offset of units or metamorphic isograds throughout the region mapped to
date.
A pegmatite dike swarm and 2 two-mica granites have also been mapped. The dike swarm,
exposed at fairly deep structural levels, appears to have been emplaced during or after the D2
deformation. The two-mica granites are undeformed and cut across isograds, unit contacts, and the
D2 structural fabrics, indicating that they were emplaced after the D2 deformation. Andalusite in
the contact aureole of one of these plutons implies emplacement at relatively shallow depths.
C-8
Reconnaissance suggests that at least one additional syn(?)- to post-tectonic granite is exposed
along the western flank of the dome.
U/Pb geochronology by Scharer et al. (1986) on monazite yielded ages of 9.2±0.9 Ma and
9.8±0.7 Ma from slightly deformed granites (Maluski et al., 1988) within the Mabja dome;
unfortunately, the location of these samples was not reported. 40Ar/39Ar thermochronology on biotite
and muscovite from both the granitic rocks and orthogneissic rocks from the Mabja Dome yielded
disturbed spectra with total gas ages of 6-8 Ma, although the location of these samples was also not
reported (Maluski et al., 1988). Our preliminary U/Pb geochronology on zircons from one of the
orthogneiss bodies shows that it is Paleozoic and represents basement similar to that seen in
Kangmar Dome. Preliminary U/Pb geochronology on igneous monazite from one of the posttectonic 2-mica granites yielded an intrusive age of 14.5±0.1 Ma (Fig. 2) (Lee et al., 1998b)
indicating that D2 deformational fabrics and the domal form developed prior to the middle
Miocene.
The striking similarity of the structural geology and metamorphic history between the Mabja
and Kangmar domes implies that they formed by the same process: contraction, followed by
extension, and ending with exhumation by thrust faulting and erosion (Lee et al., 1999; in review a).
However, because migmatitic rocks, syn- to post-tectonic pegmatites, and syn(?) to post-tectonic
granites are exposed within the Mabja Dome, but not in the Kangmar Dome, we cannot yet rule out
a diapiric origin or diapirism accompanied by regional deformation.
4. Proposed Research
4.1 Introduction
The results of our preliminary geologic mapping and structural studies within the Mabja Dome
provide plausible evidence for a combination of contractional and extensional deformation leading
to the domal form. Furthermore, these studies are compatible with, but do not prove, our hypothesis
that doming occurred as a consequence of thrust faulting contemporaneously with normal slip along
the STDS. However, because these preliminary studies raise a number of questions and because
critical areas have not been mapped, we cannot yet rule out a diapiric origin or diapirism
accompanied by a significant component of regional deformation (contraction and/or extension).
Our proposed research focuses on documenting the mechanism by which the Mabja Dome
formed by a combination of geologic mapping, detailed structural, kinematic, metamorphic
petrology, geochronologic, and thermochronologic investigations. Our proposed field-based
investigations will provide a three-dimensional view of the nature, geometry, and kinematics of
ductile and brittle deformation of plutonic rocks and country rocks, and relative ages of pluton
emplacement, deformation, and metamorphism. Metamorphic petrology will provide quantitative
constraints on peak temperatures and pressures prior to exhumation. U/Pb geochronology and
40
Ar/39Ar thermochronology will provide age constraints on the timing of pluton emplacement, peak
metamorphism, brittle and ductile deformation, and cooling histories. This strategy will document a
time-integrated view of gneiss dome development within an overall lithospheric convergent setting.
Furthermore, it will allow us to test our hypotheses that normal slip along the STDS was
accompanied by contraction in the hangingwall of the STDS and that there is a kinematic link
among the North Himalayan gneiss domes, the STDS, and the RZT. Finally, if we confirm our
hypotheses, our research will provide much needed geologic data for characterizing the mechanisms
of formation of the Tibetan plateau.
4.2 Geologic Mapping, and Structural and Kinematic Studies
C-9
With the exception of the Kangmar Dome (e.g. Burg et al., 1984; Chen et al., 1990; Gans et al.,
1998; Hacker et al., 1998; Maluski et al., 1988; Scharer et al., 1986; Wang et al., 1997; Lee et al.,
1998a, 1999, in review a, b), little is known about the geology of the North Himalayan Gneiss
Domes. Preliminary mapping within the Mabja Dome has established that its geology, structure,
and metamorphic histories are similar to the Kangmar Dome, but there are important differences,
including exposures of migmatites, syn- to post-tectonic pegmatites, and syn(?)- to post-tectonic
granites (Lee et al., 1998b). In addition, most of the dome remains unmapped, including such
critical areas as the core, the western flank around a syn(?)- to post-tectonic granite, the southern
flank where the GKT is exposed(?), and the northeastern part of the dome (Fig. 2). Mapping these
areas may reveal crucial field relations that will shed light on the evolution of Mabja Dome.
We will undertake geologic mapping at 1:50,000 scale on topographic maps and aerial
photographs, and collect structural and kinematic data to constrain the nature, geometry, kinematics
and magnitude of deformation, and timing of magmatism relative to deformation and
metamorphism. Such an approach will provide the necessary framework for understanding the
nature of deformation with respect to the mechanisms proposed for gneiss dome formation, for
understanding the results of our thermobarometric, geochronologic, and thermochronologic studies,
and for understanding the nature of deformation in this area with respect to the STDS in particular,
and the Tibetan plateau in general.
The primary goals of this aspect of our research are to: (1) Map the core of the dome to
determine if a syntectonic granitic body and/or an extensive zone of migmatites are exposed. Is the
core dominated by a plutonic body? If so, what is the age of emplacement of this pluton relative to
peak metamorphism and deformation? Or, is the core dominated by migmatites, and, if so, are the
migmatites the source of the pegmatites exposed at higher structural levels? (2) Map metamorphic
isograds to ascertain whether they are related to structural or plutonic features. Is the metamorphism
the result of conductive relaxation of isotherms or is it the result of convection triggered by the
emplacement of plutons? If the latter, is the source of the heat exposed in the core of the dome? (3)
Map the southern flank of the Mabja Dome to discover if it lies in the hangingwall of a northdipping thrust fault, similar to the Kangmar Dome which lies in the hangingwall of the northdipping GKT (Burg et al., 1984; Lee and Dinklage, unpubl.; Ratschbacher et al., 1994). (4) Map the
western part of the dome to document whether the granite pluton exposed there is syn- or posttectonic.
As part of these studies, we will also: (a) Create a detailed (1:50,000 scale) geologic map of the
dome. (b) Create a detailed map of foliation and lineations orientations to determine the threedimensional geometry and kinematics of foliations and lineations within the Mabja Dome.
Preliminary mapping and structural studies suggest that Mabja Dome possesses two major
penetrative deformational fabrics (Lee et al., 1998b). However, whether the mylonitic foliation is
domed and the stretching lineation unidirectional is not known. If the stretching lineation is
unidirectional, then this suggests a tectonic origin. If however, the stretching lineation is not
unidirectional, as has been documented in the Kigluaik gneiss dome, Alaska (Amato et al., 1994;
Calvert et al., in press), then this suggests a diapiric origin. (c) Collect structural and kinematic data
on finite strain and shear within the penetratively deformed rocks using conventional mesoscopic
and microscopic investigations. In addition, complete electron back-scatter diffraction (EBSD)
studies of selected, chiefly monomineralic, samples to assess sense of shear through asymmetrical
lattice and shape preferred orientations, as well as the contributions of dislocation glide and creep
relative to cataclasis and grain-boundary sliding through variations in preferred orientation strength.
Is the bulk strain coaxial or non-coaxial? If non-coaxial, is there a consistent sense of and direction
of shear across this dome, suggesting a tectonic origin? Or, is the direction of shear down-dip and
C-10
radially oriented, compatible with a diapiric origin? What is the magnitude of finite strain within the
tectonites of this dome and how does it vary in three dimensions? (d) Document field evidence for
timing of magmatism relative to penetrative deformation and metamorphism to address whether
metamorphism was the result of conduction or advection.
4.3 Metamorphic Petrology
Quantitative thermobarometry investigations within the Kangmar Dome provided critical
information on the tectonic evolution of the dome. Most importantly, not only were the rocks buried
to depths of 30 km, but the apparent pressure gradient is ~125 MPa/km, well in excess of the
expected gradient of 27 MPa/km. Development of such a subvertically shortened pressure gradient
requires a factor of five subhorizontal stretching after the pressure gradient was “frozen in”. The PT
data, along with structural data, imply a tectonic history of burial by contraction, followed by peak
metamorphism, subvertical thinning and tilting, and ending with doming by thrust faulting (see
section 3.2).
Quantitative thermobarometry should place similarly strong constraints on the tectonic
evolution of the Mabja dome. We will undertake petrographic examination of pelitic rock samples
to document the prograde sequence of mineral assemblages that define a series of isograds. Are the
isograds concentric, as documented in the Kangmar Dome? In addition, petrographic examination
will establish the relative ages between mineral growth and deformational events—did peak
metamorphism occur during the early stages of D2 deformation, as documented in the Kangmar
Dome? On the basis of these studies, we will select 10 samples that provide three-dimensional
coverage of the dome for detailed thermobarometry. Each sample will be examined carefully with
back-scattered electron imaging and element mapping prior to quantitative line scans of selected
mineral groups. Temperatures and pressures will be calculated with well-calibrated
thermobarometers (e.g., garnet–biotite (Ferry and Spear, 1978; Hodges and Spear, 1982), garnet–
aluminumsilicate–quartz–plagioclase (Ghent, 1976; Ghent et al., 1979)), using principally
Thermocalc (Holland and Powell, 1998), supplanted with a healthy dose of geologic sense
regarding the application of such calculations in light of reaction textures and mineral zoning.
If we establish that peak pressures and temperatures developed asymmetrically across the
dome—for example the north flank of the dome was deeper and hotter—and were not spatially
associated with a pluton, such relations would support conductive relaxation of isotherms and a
contractional origin for the dome. If, on the other hand, we show that peak pressures and
temperatures are symmetrically disposed around a pluton exposed within the core of the dome, this
finding would support an advective source for metamorphism and a diapiric origin for the dome.
4.4 Geochronology and Thermochronology
4.4a Introduction
A critical aspect of testing the various models proposed for the formation of the Himalayan gneiss
domes, as well as addressing questions concerning the evolution of the Tibetan plateau, involves
acquiring chronological constraints on intrusive, metamorphic, and deformational events. We
propose to use U/Pb and 40Ar/39Ar methods to date igneous and metamorphic rocks in the Mabja
Dome to build on our reconnaissance U/Pb geochronology and as well as the reconnaissance U/Pb
geochronology and 40Ar/39Ar thermochronology of Scharer et al. (1986) and Maluski et al. (1988),
respectively. Just such an integrated geochronologic and thermochronologic study at Kangmar
Dome provided absolute age constraints on the timing of intrusive, metamorphic, and deformational
events, and can provide a T-t history ranging from temperatures as high as ~800°C (approximate
temperature at which zircon crystallizes) to as low ~150°C (closure temperature for the smallest
C-11
domains in K-feldspar) (e.g. Heizler et al., 1988; Lovera et al., 1989). Provided we can eliminate
cooling mechanisms such as fluid convection and/or lateral heat flow, calculated cooling rates may
either reflect tectonic denudation resulting from motion along shear zones and faults (asymmetric
cooling) or cooling following emplacement of plutonic bodies (symmetric cooling).
4.4b Zircon and Monazite U/Pb Geochronology
In the Mabja Dome at least three two-mica granites are exposed, two of which have been partially
mapped (Fig. 2). The relative age relations between granite emplacement and the deformational
events, for at least these two plutons, indicate that the plutons were emplaced after the D2
deformation. Our proposed geologic mapping and structural studies will establish how many
additional plutons might be present in this dome, especially in the core of the dome, and most
importantly, their emplacement ages relative to formation of the deformational fabrics and the
growth of metamorphic minerals. Geochronologic studies of these rocks will provide some of the
most critical data on the timing of deformational and metamorphic events and thereby allow us to
place these events into a regional tectonic context.
To establish absolute ages for pluton emplacement and place age constraints on deformational
structures and metamorphic events, we propose to use U/Pb geochronology on igneous zircon and
monazite to determine crystallization ages of four plutonic rocks and on metamorphic monazite
from four metasedimentary rocks to constrain the age of peak metamorphism. U/Pb ages are critical
because they will establish whether metamorphism and plutonism were synchronous and, along
with our mapping and petrologic studies, assess whether plutonism was the heat source for
metamorphism. We were unable to solve this problem at the Kangmar Dome because Tertiary
plutons are not exposed there. Secondly, on the basis of numerical simulations, it is plausible that
the North Himalayan leucogranites were diapiric in origin (Harrison et al., 1997). If true, then the
location of the domes within a belt of leucogranites is merely coincidence if our ramp model is
correct. Therefore, establishing the age of pluton emplacement, metamorphism, and exhumation
(see section 4.4c below) is important for differentiating among these different mechanisms. For
example, if peak metamorphism and pluton emplacement were not related in time and space, then a
diapiric origin seems unlikely. Furthermore, if we establish that the age of formation of the Mabja
Dome is ~17-12 Ma, then we will have established a temporal link to normal slip along the STDS
(Edwards and Harrison, 1997; Wu et al., 1998; Murphy and Harrison, 1999), which would support
our working hypothesis. Finally, these studies will also establish the duration of Tertiary
magmatism. If the plutons were emplaced over a several m.y. period, this will have different
implications than if the plutons were emplaced during a single episode following D2 deformation.
Inheritance and/or Pb loss are problems common among the attempts to use U/Pb
geochronology to date refractory accessory minerals from the High Himalayan leucogranites. To
address this potential problem in the Mabja Dome, we will analyze different fractions utilizing a
combination of abrasion techniques and partial dissolution experiments (McClelland and Mattison,
1996) to establish and remove the effects of inheritance and/or Pb loss.
4.4c 40Ar/39Ar Thermochronology
Reconnaissance 40Ar/39Ar thermochronology on biotite and muscovite from both the granitic rocks
and orthogneissic rocks from the Mabja Dome yields disturbed spectra with total gas ages of 6-8
Ma (Maluski et al., 1988). Because the locations of these samples are unknown, we do not know
whether these ages reflect cooling following the emplacement of a post-tectonic pluton, cooling due
to exhumation along a thrust ramp, or cooling due to buoyancy driven exhumation. What were the
timing and rates of cooling across the dome? Do the timing and cooling rates vary spatially? In the
C-12
Kangmar dome, our thermochronologic data (Lee et al., in review a) show a northward decrease in
mica ages, suggesting either that the north flank of the dome resided at slightly deeper structural
levels before exhumation, consistent with the geothermobarometric data, or that there was a greater
degree of reheating on this flank. In addition, micas ages increased with depth indicating
refrigeration by underthrusting cold Tethyan sediments. In the Mabja Dome, was cooling radially
symmetric, suggesting a diapiric origin, or asymmetric, suggesting a tectonic origin? If cooling was
asymmetric, do ages young in the direction opposite to the shear sense, suggesting exhumation
during contraction? And do ages increase with depth suggesting refrigeration by underthrusting of
cold Tethyan sediments? We propose to address these problems by undertaking detailed 40Ar/39Ar
analyses.
The Mabja Dome is well suited to 40Ar/39Ar studies because the orthogneissic rocks,
metasedimentary rocks, and leucogranites contain abundant mineral phases suitable for such
studies. We will use conventional resistance-furnace step-heating experiments to obtain 40Ar/39Ar
age spectra on hornblende, muscovite and biotite, and multiple diffusion domain analyses of Kfeldspar Arrhenius data and 40Ar/39Ar age spectra. Careful selection of samples, based on our
geologic mapping, structural and kinematic studies, will provide data essential to assess variations
in cooling history across this dome, with depth, and with proximity to plutons.
Well-behaved, conventional step-heating 40Ar/39Ar experiments on hornblende, muscovite and
biotite will provide a t-T point for each sample (argon closure temperatures for hornblende,
muscovite and biotite depend on composition, grain-size, and cooling rate, among other factors, but
are roughly 525°C, 400°C and 300°C, respectively) (Harrison et al., 1985; Harrison and McDougall,
1980; Snee et al., 1988; Hodges, 1991). K-feldspars, on the other hand, may be characterized by a
discrete distribution of diffusive length scales (Fitz Gerald and Harrison, 1993; Harrison et al.,
1991; Lovera et al., 1993; Lovera et al., 1989; Lovera et al., 1991). As such, analyses of Arrhenius
and 40Ar/39Ar age spectrum data from a single sample provide a segment of its thermal history. This
technique provides more detail on cooling histories, over a range in temperatures from as high as
~400°C to as low as ~150°C, than can be provided by conventional 40Ar/39Ar age spectra (e.g.
Heizler et al., 1988; Lovera et al., 1989).
Assuming well-behaved samples, we will be able to calculate a well-constrained cooling history
for each dome from as high as the argon closure temperature of hornblende, 525°C, to as low as
argon closure temperatures of ~150°C for the smallest K-feldspar domains. Furthermore, if we
discover and successfully date a synextensional granite, this cooling history may be extended to as
high as ~800°C. This is a temperature range over which both crystal-plastic processes (above
~300°C or so) and brittle processes (below ~300°C) are the dominate deformation mechanisms in
continental rocks. Therefore, younger ages in cover rocks and rapid cooling rates that young in the
direction opposite to the sense of shear will largely reflect motion along contractional shear zones
and/or thrust faults, whereas older ages in cover rocks and rapid cooling rates in gneissic core rocks
that young in the direction of shear will largely reflect motion along normal sense shear zones
and/or normal faults. In contrast, symmetrically distributed cooling rates that decrease
asymptotically from rapid (on the order of 100-500°C/Ma) to slow (≤5°C/Ma) are indicative of
cooling following pluton emplacement. However, a number of mechanisms such as fluid convection
and lateral heat flow can cause crustal rocks to rapidly cool. Our proposed geologic mapping and
structural studies will also allow us to assess the importance of these complications as well as
eliminate or minimize their effects when sampling.
It is our experience, based upon our thermochronologic studies in the Kangmar Dome, that six
K-feldspar samples, four hornblende, and 10 mica samples should be analyzed to obtain sufficient
detail on the three-dimensional variation in cooling history.
C-13
4.5 Implications for the Evolution of the Tibetan Plateau
The regional spatial and temporal setting of the North Himalayan Gneiss Domes hint at different
interpretations. For example, the North Himalayan Gneiss Domes are exposed within the core of the
North Himalayan antiform, suggesting that the domes formed as a result of contraction. INDEPTH
reflections have been interpreted to show an antiformal duplex in the hangingwall of the Main
Himalayan Thrust (MHT) that extends to a depth of ~35 km beneath the Kangmar Dome (Hauck et
al., 1998), also implying a contractional origin. On the other hand, the domes are also exposed
within a belt of Miocene leucogranites, which have been suggested to be anatectic melts that
diapirically rose to the middle crust (Harrison et al., 1997).
The results of detailed, integrated geologic investigations of the Kangmar Dome provide
plausible evidence that this dome formed as a result of contraction, then extension, and finally
doming by thrusting upward and southward over a north-dipping thrust ramp. Middle to late
Miocene thrusting in the Kangmar Dome region was contemporaneous with normal slip along the
STDS and thrust motion along the RZT, suggesting a kinematic link among these structures. This
exciting new kinematic picture for the evolution of the Himalaya and southern Tibet needs to be
confirmed. For example, limited geologic mapping and structural studies within the Mabja Dome
suggest that it exposes a similar deformational and metamorphic history as in the Kangmar Dome.
However, the Mabja Dome exposes significant differences that hint at buoyancy-driven
exhumation. What factors controlled the spatial and temporal development of these gneiss domes?
Was it solely structural, or did magmatism play a critical role? Was the formation of these domes
linked temporally and structurally to the STDS? Determining the processes that lead to gneiss dome
development in southern Tibet, therefore, are key to gaining a better understanding of the
development of overthickened crust and perhaps the timing that high elevations were achieved on
the Tibetan plateau. For example, we may be able to address such important issues as:
•If these domes are the result of crustal contraction, then documenting the geometry and
quantifying the magnitude of contraction involved in bringing them to the surface will yield
better constraints on the degree of strain partitioning between crustal contraction and lateral
extrusion, both of which accommodate convergence between Asia and India (e.g. Tapponnier et
al., 1982). In addition, if contraction occurred coevally with normal slip along the STDS, this
will confirm our working hypothesis that the hangingwall of the STDS was shortening during
normal slip along the STDS.
•If these domes are the result of diapirism, then documenting the geometry and timing of
diapirism will lead to a better assessment of the role of deformational heating along the basal
decollement and subsequent anatexis (Harrison et al., 1997) in the evolution of the Tibetan
plateau.
4.6 Role of Personnel and Work Plan
For Jeff Lee, Brad Hacker, Bill McClelland, and Wang Yu, this project is a natural extension of
their work completed in the Kangmar Dome and started in the Mabja Dome. Lee, McClelland, a
UCSB graduate student, and Wang will complete the field mapping, structural studies, and
collection of samples for structural, kinematic, metamorphic petrology, geochronology, and
thermochronology investigations. Two months of field work focusing in the areas shown in Figure
2a will be conducted during Spring, 2001. These studies will provide the structural and geometric
framework for the samples collected for kinematic, metamorphic petrology, geochronology and
thermochronology studies. Dr. Wang will be in charge of organizing logistics for the field work.
C-14
At UCSB, efforts will focus on map and structural data compilations, preparing samples for
petrologic, geochronologic and thermochronologic analyses, and completing analyses. Lee will be
in charge of compiling map and structural data, undertaking structural and metamorphic
petrography, and coordinating various components of the research. The UCSB graduate student,
under the tutelage of Hacker, will complete the quantitative thermobarometry and EBSD studies.
Lee, the graduate student, and Hacker will participate in sample preparation and analyses for
conventional and diffusion domain Ar/Ar studies. McClelland will complete the U/Pb zircon and
monazite analyses. The results of the proposed research will be presented at GSA and similar
meetings, and will be submitted as manuscripts for publication in journals in fields of regional and
structural geology, and tectonics.
5. Summary and Significance
This proposal outlines detailed geologic mapping, structural, kinematic, metamorphic petrology,
geochronologic, and thermochronologic investigations to address the origin and nature of the North
Himalayan gneiss domes of southern Tibet. Our investigations will characterize the nature,
geometry, and kinematics of ductile and brittle deformation, and document the pressure/temperature
conditions of metamorphism, the timing of pluton emplacement and peak metamorphism, and the
cooling of plutonic rocks and country rocks within these gneiss domes. These studies are designed
to evaluate the role of contractional, extensional, or buoyancy driven processes in the formation of
these gneiss domes. Our preliminary results suggest that alternating contraction and extension might
be responsible for two of the domes, indicating that these domes have complex histories and
polygenetic origins. Our studies will provide important data necessay to assess models proposed for
the formation of overthickened crust and high elevations within the Tibetan plateau.
C-15