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ABSTRACT: The Himalayan range is commonly presented as largely laterally uniform from west to east. However, geological structures, topography, precipitation rate, convergence rates, and low-temperature thermochronological ages all vary significantly along strike. Here, we focus on the interpretation of thermochronological data sets in terms of along-strike variations in geometry and kinematics of the main crustal detachment underlying the Himalaya: the Main Himalayan Thrust (MHT). We report new apatite fission track (AFT) ages collected along north-south transects in western and eastern central Nepal (at the latitudes of the Annapurna and Langtang massifs, respectively). AFT ages are consistently young (<3 Ma) along both N-S transects in the high-relief zone of the Higher Himalaya and increase (4 to 6 Ma) toward the south in the Lesser Himalaya. We compare our new data to published low-temperature thermochronological data sets for Nepal and the Bhutan Himalaya. We use the full data set to perform both forward and inverse thermal kinematic modeling with a modified version of the Pecube code in order to constrain potential along-strike variations in the kinematics of the Himalayan range. Our results show that lateral variations in the geometry of the MHT (in particular the presence or absence of a major crustal-scale ramp) strongly control the kinematics and exhumation history of the orogen.
Journal of Geophysical Research. 01/2011; 116:B05202.
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ABSTRACT: Although the Himalayan range is commonly presented as cylindrical
along-strike, geological structures, topography, precipitation rate,
convergence rates and low - temperature thermochronological ages all
vary significantly from west to east. Here, we focus on the
interpretation of thermochronological datasets in term of cylindricity
in geometry and kinematics of the MHT along the Himalayan range. We
propose a structural and kinematic model of the major crustal Himalayan
thrust, the MHT, based on apatite fission-track (AFT) ages collected
along north - south transects in western and eastern - central Nepal
(Kali Gandaki and Trisuli Rivers). AFT ages are consistently young
(≤3 My) along both N-S transects in the MCT zone and increase (4 to 6
My) toward the south in the Lesser Himalaya. We constrain the geometry
of the MHT ramp with 2 age-elevation transects, one in the MCT zone and
one in the outer Lesser Himalaya, interpreted in terms of exhumation
rate. The data can be fit without invoking out-of-sequence thrusting in
the Main Central Thrust zone by varying the geometry of the MHT along
strike, in accord with independent geodetic and geophysical data. We
compare our data to published low-temperature thermochronological
datasets for western - central Nepal, eastern - central Nepal, western
India and the Bhutan Himalaya. We use these data to constrain numerical
thermal-kinematic models using a modified version of the PECUBE code, in
order to quantify potential along-strike variations in the kinematics of
the Himalayan range. Our results show that lateral variations in
geometry of the MHT (in particular the presence or absence of a major
ramp) strongly control the kinematics, the exhumation history and the
topography of the orogen. Where a major crustal ramp is present, the
topography shows a steep gradient that focuses exhumation and orographic
precipitation whereas the topography is more gentle and exhumation less
focused in the absence of a ramp. Our results imply that along-strike
climatic variations in the Himalaya respond to tectonics rather than
driving it.
AGU Fall Meeting Abstracts. 11/2009; -1:05.
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ABSTRACT: We study the recent dynamics of the central Nepal Himalaya, focusing on possible reactivation of the footwall of the Main Central thrust, which is marked by an abrupt topographic transition. Different tectonic mechanisms, such as overthrusting of a major crustal ramp, underplating, or out-of-sequence thrusting, have been suggested to explain the morphology and exhumation patterns in this area. We present 25 new apatite fission-track ages collected along a north-south transect in central Nepal, as well as two age-elevation profiles. Ages are consistently younger than 3 Ma old in the Main Central thrust zone and increase continuously to 4?6 Ma old in the south. No jump in apatite fission-track ages is observed across the topographic transition. Apparent exhumation rates from age-elevation relationships vary from 0.46 +0.13/?0.09 km/Ma in the Palung granite south of Kathmandu to 4.4 +4.8/?1.5 km/Ma in the Main Central thrust zone; the latter rate is probably overestimated by a factor of two due to topographic effects. As shown by a new numerical model, these strongly varying exhumation rates can be explained by overthrusting of a crustal ramp, which exerts a primary control on age patterns, and do not require out-of-sequence reactivation of thrusts in the Main Central thrust zone.
Geology. 01/2009; 37(8):731.
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ABSTRACT: We study the recent dynamics of the Himalayan orogen in central Nepal
with the specific goal of quantifying the onset of activity and the
deformation history recorded by the different major thrusts along the
Himalayan range, and propose a structural and kinematic model of the
major crustal Himalayan thrust, the MHT. We report 27 new apatite
fission-track (AFT) ages collected along north - south transects in
western and eastern - central Nepal (Kali Gandaki and Trisuli Rivers).
AFT ages are consistently young (< 3 My) along both N-S transects in
the MCT zone and increase (4 to 6 My) toward the south in the Lesser
Himalaya. We present and compare 2 age - elevation transects, one in the
MCT zone and one in the outer Lesser Himalaya, and interpret them in
terms of exhumation rate that we use to constrain the geometry of the
MHT ramp. The Himalayan range is commonly presented as a cylindrical
structure from west to east. However, geological structures, topography,
precipitation rate, convergence rates and low - temperature
thermochronological ages all vary significantly along strike. Here, we
focus on the interpretation of thermochronological datasets in term of
cylindricity in geometry and kinematics of the MHT along the Himalayan
range. We compare our new data to published low-temperature
thermochronological datasets for western - central Nepal, eastern -
central Nepal and the Bhutan Himalaya. We use these data to perform
numerical thermal- kinematic modelling with a modified version of the
PECUBE code, in order to constrain potential along-strike variations in
the kinematics of the Himalayan range. Our results show that lateral
variations in geometry of the MHT (in particular the presence or absence
of a major ramp) strongly control the kinematics and exhumation history
of the orogen.
AGU Fall Meeting Abstracts. 11/2008; -1:2045.
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ABSTRACT: We review recent developments in thermal modeling methods - from simple
one-dimensional solutions of the heat transport equation to
sophisticated three-dimensional finite element models that include the
effect of an evolving, finite amplitude topography or the movement of
crustal blocks along an array of faults - that have led to a more
quantitative interpretation of thermochronological datasets in orogenic
settings. We will show the importance of incorporating the effects of
the advection of heat due to rock exhumation and lateral tectonic
translation, as well as the perturbation caused by surface topography.
We will also focus on the establishment and relaxation of the thermal
state of the crust at the onset and end of a tectonic event. We will
show how different thermochronoligical datasets can be modeled,
including multi-system and detrital datasets. We will demonstrate how
modeling the thermal structure of the crust strongly alters our
interpretation of thermochronological data, and how numerical models
combined with appropriate inverse methods can not only provide
quantitative constraints on the parameters that we have often
arbitrarily chosen to describe a given tectonic and geomorphic scenario,
but also help refine the parameterization or define appropriate targets
for further data collection. These points will be illustrated by a wide
range of case studies.
AGU Fall Meeting Abstracts. 11/2008; -1:05.
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ABSTRACT: The development of topography and drainage networks in active tectonic
regions is controlled by, and contains the record of, the interactions
between tectonics, climate and surface processes. In order to study
these interactions in the Nepal Himalayas, we are developing coupled
numerical models of tectonics, erosion and drainage development on
different spatial and temporal scales. We have studied drainage
development associated with a single fold ridge using a numerical model
that couples a kinematic description of fault-related folds to a surface
processes model taking into account diffusive and fluvial erosion as
well as bedrock landsliding. This model has demonstrated the fundamental
control of tectonic and geometric parameters on the evolution of relief
and the associated drainage networks. In particular, we show that the
dip of the detachment underlying the fold provides a major control on
this evolution. In order to study the controls on drainage development
at the scale of the foreland fold-and-thrust belt, the model was
extended to include several ramps that branch onto a single detachment.
A minimal-work approach permits to define which ramp will slip at each
time step and provides the coupling between tectonics and surface
processes. We observe a general forward propagation of deformation, with
out-of-sequence thrusting. Initial results demonstrate the influence of
internal and basal friction on the rate of forward propagation of
deformation, but also the influence of surface processes. This model
allows us to study the controls on the important lateral variations in
the width and structure of the Himalayan foreland fold-and-thrust belt.
Finally, we use a thermo-mechanical model to study the interactions
between tectonic and surface processes at the scale of the entire
mountain belt. Starting with an initial geometry consisting of a
"normal" (Indian) crust separated from a region of thickened (Tibetan)
crust by the Himalayan topographic front, we show that extrusion of
Tibetan crust is favored by strong erosion without foreland deposition.
In other cases, the rocks present at the front originate from the Indian
crust and have medium metamorphic grades. We will use this model,
constrained by thermobarometric and thermochronological data, to study
the transition from the Miocene 'extrusion' mode to the post-Miocene
'frontal propagation' mode of evolution of the Himalayan orogen.
03/2003; -1:2313.
