Exploring
the Early History of the South Tibetan Fault System in
Bhutan
Our most recently funded project takes advantage of a rare
opportunity in the Himalaya: a chance to study putative klippen of
the South Tibetan fault system within the Lower Himalayan foothill
ranges of Bhutan. Most exposures of the South Tibetan fault
system occur at high elevations, near the crest of the Himalaya and
along the southern edge of the Tibetan Plateau. Detailed studies of
those structures show that most have experienced long and
kinematically complex deformational histories beginning at least as
far back as 23-20Ma. Late-stage deformation has largely obscured
the early structural history of most range-crest exposures of the
South Tibetan fault system, but a series of klippen in the
foothills of Bhutan appear to preserve pristine segments of the
system that have not experience late-stage slip. Frances Cooper
will lead our efforts will leverage these unique exposures to
better understand the early history of the South Tibetan fault
system.
Along-Strike
Variations in the Late Cenozoic Tectonic Evolution of the Bhutanese
Himalaya and the Potential Role of Climate
The topographic profile of western Bhutan is significantly
different from that of other sectors of the Himalaya in a way that
implies a distinctive tectonic evolution. The most strikingly
distinctive feature of western Bhutan is an extensive low-relief
"bench" at ~3000 m that interrupts the otherwise steep southern
flank of the Himalayan ranges. Preliminary geomorphic analyses have
led some researchers to suggest that this surface is a perched
paleo-erosion surface that has been uplifted to its present
position from a lower elevation. The postulated cause for the
implicit accelerated erosion is a Late Cenozoic (post-Middle
Miocene) change in deformation rate or kinematics or a change in
climate. East of the Kuru Chu, a major north-south river that
bisects Bhutan into western and eastern parts, the paleo-erosion
surface is either missing or too poorly defined to be obvious from
the analysis of remote sensing data.
PhD candidate Byron Adams is leading our work on the age, origin,
and tectonic significance of the physiographic bench of western
Bhutan. Building on research already begun at the University of
Texas – El Paso by former PhD student Jose Hurtado and his former
MS student Tobgay Tobgay, we are employing morphometric analysis of
bedrock streams to define domains of differential rock uplift and
we are constructing detailed structural maps of the boundaries
between domains in order to understand better the deformation that
accommodates differential uplift. A special focus of this work will
be the abrupt eastern termination of the paleo-erosion surface,
which may be a major, as-yet unmapped, fault. We are working to
constrain the age of the paleo-erosion surface, date the structures
that may bound it, and determine when the accelerated erosion that
resulted in the current topography of western Bhutan by
combining 40Ar/39Ar and (U-Th)/He
geochronology. Collectively, these data will be interpreted in the
context of the regional Late Cenozoic tectonic setting – especially
the Early Pliocene (?) development of the nearby Shillong Plateau
to the south, which blocks eastern Bhutan from the full impact of
summer monsoon storms sweeping northward from the Bay of
Bengal.
Constraining
the Age of India-Asia Collision Through Detrital Mineral
Thermochronology of the Indus Group, NW Indian
Himalaya
Continental sediments of the Indus Group of northwest India
unconformably overlie the tectonic collage produced by India-Asia
collision and thus provide a critical constraint on the age of the
collisional event. Although deposition of the Indus molasse is
widely regarded as having begun in Lutetian-Ypresian time, this
perspective is not explicitly supported by paleontological or
geochronological data. Other than the fact that most of the molasse
must be younger than Ypresian nummulitic limestones, age-diagnostic
fossils or volcanic horizons have not been identified throughout
most of the section, and the stratigraphic position of the only
well-documented fossil assemblage from the Indus Group (Upper
Oligocene ostracods) is debatable. there are no firm
constraints on
the ages of any deposits in this important sequence.
PhD candidate Alka Tripathy is using detrital mineral
thermochronology to greatly improve our understanding of the age
range, depositional history, and thermal evolution of this
important sedimentary succession. Her principal tools are laser
fusion 40Ar/39Ar biotite
thermochronology and both laser heating and laser ablation
(U-Th)/He zircon thermochronology. The first method capitalizes on
the wide distribution of biotite in Indus Group sandstones and on
the fact that post-depositional metamorphism of the sequence was at
a low enough temperature to eliminate the possibility of thermal
resetting of the 40Ar/39Ar biotite
chronometer. Thus, the youngest detrital biotites in each sample
provide an upper boundary of the sample's depositional age. The
(U-Th)/He zircon chronometer, on the other hand, has a closure
temperature range low enough that it should have been reset by
post-depositional metamorphism in the highest-grade portions of the
Indus Group sequence. The zircon ages from these parts of the
sequence will provide a lower boundary on depositional ages of all
parts of the sequence, since the entire depositional basin
experienced epizonal-anchizonal metamorphism at that time. In
addition to these important brackets on the age of sedimentation,
the results of this study also will provide better insights into
the provenance of Indus Group sediments, the likely developmental
age of the modern Indus River drainage system, and the age of
regionally important deformation that accompanied
epizonal-anchizonal metamorphism.
Development
of the Eastern Himalayan Fold and Thrust Belt, Bhutan
In collaboration with Professor Nadine McQuarrie of Princeton
University, postdoctoral researcher Frances Cooper will be using
the 40Ar/39Ar laser microprobe to date polyphase deformational
fabrics related to shortening in the Lesser Himalaya of Bhutan.
Integration of this work with structural mapping and metamorphic
petrology done by Nadine and her students should reveal how
deformation was partitioned through time and space in this part of
the Himalayan-Tibetan orogenic system.
Extensional
Fault Systems of the Ama Drime Range, southern Tibet
Much of the recent literature on the Himalayan-Tibetan orogenic
system has focused on the role of monsoon climate in its
development. In one controversial hypothesis, aggressive monsoon
precipitation currently attracts the southward flow of a channel of
middle crust from beneath the southern Tibetan Plateau to the
Himalayan orogenic front. Fortunately, this hypothesis makes a
number of predictions that are readily testable. One of these
predictions is that a recently active detachment occurs along the
southern edge of the Tibetan Plateau at the approximate position of
the Miocene South Tibetan fault (STF) system. A study of this kind
conducted several years ago in the Annapurna and Dhaulagiri ranges
of Nepal yielded evidence supporting the existence of such a
structure but exposures of critical contacts were poor – all were
just south of the plateau margin and thus in an area of heavy
vegetation – and both the reliability and the broader implications
of the results of that project have been questioned.
PhD candidate Jeni McDermott is addressing this issue in another
area that has far better exposure: the Ama Drime Range of southern
Tibet (N27˚54’-28˚30’; E87˚20’-87˚45’). Geologic mapping and
(U-Th)/He thermochronology thus far suggest at least three
generations of extensional structures in and around the range. At
least two of these are related to Early Miocene and Late Miocene
slip on the STF system, both older than Quaternary, N-striking
normal faults that bound the range. A discontinuity in the pattern
of cooling ages at the physiographic transition marking the
southern plateau margin may indicate active STF faulting. Exploring
this possibility further, Jeni is evaluating the case for
Quaternary channel extrusion with the intention of either
falsifying the hypothesis or establishing the structural and
geomorphic consequences of deformation related to the upper
bounding structure of the extruding channel. Regardless of the
outcome, the data obtained will provide new insights regarding the
development of transverse metamorphic culminations in collisional
settings.
Helium Diffusion and Alpha Ejection
The ArF excimer laser microprobe is an excellent tool for
high-precision profiling of 4He diffusion and alpha-ejection
gradients in minerals. In vacuo diffusion experiments on Durango
fluorapatite by research scientist Matthijs van Soest, postdoctoral
researcher Brian Monteleone, and former PhD student Jeremy Boyce
have yielded profiles consistent with bulk diffusion coefficients
obtained independently for that material. Natural alpha ejection
profiles, measured orthogonal to crystal faces in Durango
fluorapatite, indicate alpha stopping distances in that material
that are somewhat shorter than theory predicts, suggesting the need
for further empirical studies using laser ablation (or other)
techniques.
Laser
Microprobe (U-Th)/He Thermochronology
Research scientist Matthijs van Soest, postdoctoral researcher
Brian Monteleone, and former PhD student Jeremy Boyce (now at UCLA)
are leading efforts to develop microanalytical(U-Th)/He
thermochronology. We extract radiogenic helium using an ArF excimer
laser microprobe and we measure radioactive parent isotopes in situ
using secondary ionization mass spectrometry (SIMS). The principal
value of this technique is that it provides away to avoid the need
for alpha ejection corrections, thus minimizing analytical
uncertainties arising from such corrections. As a consequence, the
technique permits us to date non-euhedral grains such as those
commonly found in detrital samples. Thus far we have demonstrated
success with the technique for monazite, apatite, and
zircon.
Lunar
Field Geology
In
anticipation of NASA's return to the Moon, Kip Hodges is exploring
new, technology-enabled approaches to field geologic research on
planets other than Earth. Colleagues Chris Assad (JPL), Win
Burleson (ASU), and Dava Newman (MIT) are working with Kip to
design and test protocols for scientific extra-vehicular activities
by human-robotic exploration teams. Working with Apollo 17
astronaut/geologist Harrison Schmitt (right) and other members of
the Field Exploration Analysis Team (FEAT) – such as Mark Helper
(University of Texas) and Art Snoke (University of Wyoming) – Kip
is helping NASA prepare a new generation of astronaut
explorers.
Ultrahigh-Pressure
Metamorphism in the Tso Morari dome, NW Indian
Himalaya
Coesite-bearing
eclogites, indicative of metamorphism at pressures in excess of 22
kb, are found in two places in the Himalayan orogen: the upper
Kaghan Valley of Pakistan and the Tso Morari dome of northwest
India. These eclogitic terrains are thought to provide evidence for
continental subduction of the northern margin of India beneath
Eurasia during the early stages of Himalayan orogenesis. Following
on the work of former MS student Ryan Clark, postdoctoral
researcher Brian Monteleone has been studying the Tso Morari
examples in an effort to better understand the processes of
continental subduction and the exhumation of subducted continental
fragments.
The Tso Morari dome is a metamorphic core complex, structurally
analogous to metamorphic core complexes found in the western North
American Cordillera. Its core, consisting of eclogite-facies
gneisses and schists rocks that host the coesite-bearing mafic
eclogites, is surrounded by metasedimentary and metaigneous
units
that
were metamorphosed at higher structural levels and at different
times. The boundary between the core and surrounding rocks is a
complex system of normal faults, such as the north-dipping Ribil
fault shown on the right. (In this image, looking east, the layered
units to the left are an ophiolitic melange within the
Indus-Tsangpo suture zone, and the units without obvious layering
to the right are eclogite-facies rocks of the Tso Morari
core.)
Our work focuses on the movement history of such structures, as
well as their role in the exhumation of the ultrahigh-pressure
rocks of the core. To compliment our detailed structural research,
we are integrating the techniques of metamorphic petrology
and 40Ar/39Ar and (U-Th)/He
thermochronology in an attempt to develop a high-resolution record
of the pressure-temperature evolution of distinctive rock packages
bound by these structures. Ultimately, we hope to use the resulting
data to evaluate alternative thermomechanical models for the early
stages of Himalayan orogenesis.