3.3 Tectonic collisional and post-collisional phases in the
Himalayan chain
Dipartimento di Scienze della Terra, Università di Pisa, Italy
Dipartimento di Mineralogia e Petrologia, Università di Padova
Istituto di Geoscienze e Georisorse (CNR- Pisa)
Department of Irrigation (Kathmandu – Nepal)
Principal investigator: Prof. Rodolfo Carosi
Researchers involved:
Rodolfo Carosi - Dipartimento di Scienze della Terra (University of Pisa, Italy), geological
mapping, meso and microstructural analyses;
David Iacopini - Dipartimento di Scienze della Terra (University of Pisa, Italy), geological
mapping, vorticity analyses;
Chiara Montomoli - Dipartimento di Scienze della Terra (University of Pisa, Italy),
geological mapping, microstructural analyses;
PierCarlo Pertusati - Dipartimento di Scienze della Terra (University of Pisa, Italy),
geological mapping, mesostructural analyses;
Dario Visonà - Dipartimento di Mineralogia e Petrologia (Università degli Studi di
Padova), geological mapping, geothermobarometric analyses;
Gianfranco Di Vincenzo -
Istituto di Geoscienze e Georisorse (CNR- Pisa),
geochronological analyses
Dr. Dibya Ratna Kansakar - Department of Irrigation (Kathmandu – Nepal), geological
mapping
Research description
This project is part of an earth science research project focused on the Himalayan
chain and developed during the last decades from some Italian universities (Pisa,
Padova, Milano, Torino). The Himalayan chain represents an extraordinary natural
laboratory for the study of different natural phenomena that interest all the mountain
belts such as geological, tectonic, magmatic and metamorphic evolution. During the
last decades the main scientific results coming from the study of the Himalayan chain
had very strong consequences in the understanding of the geological evolution of the
other belts all around the world too and more generally in the earth science
development. It is worth noting to remember the discovering of inverted metamorphism
(Le Fort, 1975) or the recognition of low angle extensional faults coexisting with
compressive tectonics in the chain (Caby et al., 1983; Burg et al., 1984; Burchfiel et al.,
1992) or the tectonic extrusion of the Higher Himalayan Crystallines with important
consequences on the exhumation of high pressure rocks (Hodges et al., 1992;
Chemenda et al., 1995). Many scientific works, dealing with the Himalayan geology, are
published every year on international papers expecially from the more industrialized
countries. These works are very modern in concept and often deal with modelisations of
geological processes involving large sectors of the belt, using very sophisticated
methodologies. By the way the direct knowledges of the chain are still uncompleted and
largely hetereogeneous, both for the huge size of the study object and for the
difficulties we have to reach many outcrops. The published tectonic models have some
limits due to the lack of a deeply field knowledge of the chain and to its threedimensional development. A question arises: Could a single geological section of the
chain be taken as representative of the whole structure of the chain? Is it reasonable to
assume a so large cylindrism? We think that a larger knowledge of the chain, based on
direct observations on the field with the collection of geological and structural data
could greatly improve the researches.
In Nepal some geological maps already exist, but for many areas, especially for the
highest zones, a complete geological survey is still lacking. In many areas, such as the
Everest park area, where the pyramid laboratory has been built, no geological maps with
structural, petrographic and tectonic informations are still available. The proponents of
this project gained important geological data during past scientific expeditions that
could constitute a valid starting point for further researches both for the understanding
of the tectonic evolution of the Higher Himalayan Crystallines and for compiling a new
geological map of the Mt. Everest area. The map, realized in collaboration with some
nepalese researchers, will be compiled at 1:100.000 scale and will be supported by a
GIS database, containing all the available tectonic informations. This map could be
realized during at least three geological expeditions where four researchers could be
involved for the geological mapping.
Another crucial point fundamental for the comprehension of the tectonic evolution of
mountain belts, expecially for the understanding of the modalities of the exhumation of
metamorphic rocks, is represented by the discover in the 80’s, in the Himalayan orogen,
of a system of extensional faults located near the boundary between the Higher
Himalayan Crystalline (HHC) and the Tibetan Sedimentary Sequence (TSS) (Caby et al.,
1983; Burg et al., 1984, Burchfiel et al., 1992). The study of these structural elements
gave a huge contribution also to the understanding of other orogenic belts (Chemenda
et al., 1995, 1996, 2000). According to many Authors (Burchfiel et al., 1992, Hodges et
al.,1997; Dézes et al.,1999) the Main Central Thrust Zone (MCTZ) and the South Tibetan
Detachment System (STDS) developed contemporaneously between 19 and 22 Ma ago,
causing the tectonic extrusion, toward the South, of the HHC with important
implications on the teconic and metamorphic evolution of both the HHC and the Lesser
Himalaya and on the genesis of the Himalayan granites. Tectonic models on the tectonic
extrusion of the HHC assume a general non coaxial deformation with a simple shear
component
localized in the Main Central Thrust Zone and in the South Tibetan
Detachment System, while in the central portion of the HHC a pure shear component of
the deformation prevailed, causing the extrusion toward the South (Grasemann &
Vannay, 1999; Grasemann et al., 1999). By the way in the Khumbu valley, in the HHC,
some south verging extensional ductile shear zones have been recognized. These shear
zones have been dated to 15 Ma (Carosi et al., 1999 a, b). Their geometry and
kinematic highlight a more complex tectonic evolution of the HHC so that the study of
its structural evolution need to be
improved. Besides all the proposed models
regarding the tectonic evolution of the chain assume that the deformation is cylindrin
over large areas. Recent studies, performed along some transects oriented
perpendicular respect to the structural directions of the belt in western Nepal, pointed
out some structural differences respect to the study transects located in eastern and
central Nepal (Carosi et al., 2002). The main differences are: a) a lower thickness of the
HHC; b) extensional faults, correlated to the STDS, are less developed; c) minor
quantities of High Himalayan granites
The research unit will perform a structural study focused on the comprehension of the
geometries of geological structures, on their kinematics, their superposition and
tectonic evolution during the deformation history. The different foliation surfaces and
extension lineation developed during the different deformation phases, together with
the kinematic indicators and the metamorphic parageneses, will be analized.
A comparative structural study will be performed along the two proposed transects and
the differences of deformation mechanisms and type of flow will be investigated. These
data will permit to compile a more realistic model for the structural evolution, taking
into account the geological and structural differences along the chain. To investigate
the characteristics of the flow during the structural evolution and to gain informations
about the mechanisms and modalities of exhumation, vorticity analyses will be
performed on samples collected in different structural positions in the HHC. Suitable
lithotypes will be sampled near the MCT and near the STDS, but also in the central
portion of the tectonic unit. If for example metamorphic rocks have been exumed
according to a channel flow model (Gruijc et al., 1996; Grasemann et al., 1999), we
would expect to find, in high strain shear zons, spatial differences of the kinematic
components of the flow (Xypolias & Koukovelas, 2001). Through the application of
appropriate microstructural analyses, such as vorticity analyses, crystallographic and
shape preferred orientation, analyses of syntectonic vein systems, flow maps will be
drawn and the type of deformation during the exhumation process could be
constrained. These studies will led also to check the cylindrism of geological structures
and the differrences in the entities of extrusion.
The presence of extensional top-to-the SW shear zones recognized in the HHC, younger
respect to the ones located in the STDS could be referred to a much more complex
polyphasic extrusion history than that highlighted by many authors till now. The
structural position of these shear zones, far away from the STDS, let us to hypothesize
that the characteristics of the flow changed in space and time during the extrusion
process. From this point of view geochronological constraints of deformations become
essential, so in situ datings will be focused on mineralogical phases (such as muscovite,
biotite, amphibole) developed along the different foliation surfaces and in the
recognized high strain zones.
Objectives
Aim of this project is to reconstruct the sin and post-collisional tectonic evolution of the
Himalayan chain in Nepal. The new geological data, acquired during this project,
combined with the data already acquired during past scientific
expeditions by the
research proponents will permit to compile a new geological 1:100.000 map of the
Khumbu valley with its relative GIS database. The structural data collected for each
deformation phase will permit to constrain a tectonic model for the exhumation of the
metamorphites. The main structural characteristics of the recognized deformation
phases will be outlined for each tectonic unit and we will try to obtain radiometric
datings for the development of the different foliation surfaces.
Materials and methods
This research is fundamentally based on field work during at least three field
expeditions lasting each one approximately one month during which italian reserchers
and nepalese researchers will be involved (four researchers for each field expedition).
During the expeditions geological mapping accompanied by mesostructural analyses
will be performed and oriented samples will be collected. As many areas are not
accessible a detailed study of aerial photos is fundamental to identify the main
geological features and the main brittle structures. Thin sections will be obtained from
the collected oriented samples and they will be analized under an optical microscope to
investigate structures, kinematic indicators from shear zones and the complex relations
between deformation and metamorphism that in the HHC is strongly polyphasic.
To investigate the type of flow acting during the different deformation phases specific
microstructural analyses will be performed on suitable lithotypes (quartzites, gneisses
and milonitic micaschists) outcropping along the study transects. In particular the
analyses of stable porphyroclasts, the measurement of shape and crystallographic
preferred orientation (for example in quartz crystals) and the study of syntectonic veins
will be applied to gain informations about the non-coaxiality of deformation (Wallis,
1995; Passchier, 1977; Passchier & Trouw, 1996). If a sufficient number of samples will
be homogeneously collected in the study area it will be possible to draw some maps of
the flow acting during the deformation. After a carefull microstructural analyses, during
which
the
relations
between
blastesis
and
deformation
will
be
studied,
geothermometers analyses will be performed on selected samples to investigate the
pressure and temperature conditions existing during the structural evolution of the
chain. The analyses will be performed in collaboration with D. Visonà from the
Department of Mineralogy and Petrology of the University of Padova.
Geochronological datings will be performed in collaboration with G. Di Vincenzo (CNR,
Pisa) through Ar/Ar in situ analyses on single mineralogical phases, for example micas
and amphiboles, developed along different foliation surfaces and lineations.
The GIS database will be realized using Archview program.
Results
- 1:100.000 geological map of a sector of the Khumbu region;
- GIS geological database;
- Understanding of the tectonic evolution of the geological structures in the HHC
recognized along the Mt. Everest transect and comparison with the structural data
collected along the transects in central and western Nepal; testing of the published
models for the tectonic extrusion;
- Geochronological dating of selected shear zones developed in the HHC and their
relations with extensional and compressive structures.
Diffusion of results
- publication of both geological map and GIS database on the web site of Earth Science
Department of the University of Pisa (Italy);
- publication of scientific papers containing our main results on international reviews,
such as for example Journal of Asian Earth Sciences and Comptes Rendus
Geosciences, and on nepalese geological reviews;
- depending on the available funds we will attend to international scientific meeting,
such as the annual Himalayan-Karakorum-Tibet workshop, and to national and
international meeting held in Nepal.
Work program
Period of the research: three years, starting from 2003.
Three field expeditions will be done during which a geological map will be compiled and
structural data, regarding both brittle and ductile structures, will be collected. During
the field work oriented samples will be collected too. Two field expeditions will be done
in the Khumbu valley while the third one will be in western Nepal, in the Dolpo region
where the research proponents collected geological and structural data during a past
expedition in autumn 2000 (Carosi et al., 2002)
Also in the Khumbu valley geological and structural data have been already aquired by
the research proponents during past expeditions (Carosi et al., 1999 cum bibl.), these,
data joined with the ones that could be collected during two other expeditions, each
one lasting one month, could led to the compilation of a 1:100.000 geological and
structural map.
During the second and the third years of the research specific analyses will be
conducted on the collected samples:
- microstructural analyses to investigate the main features of the different tectonic
phases and to understand the relations between metamorphic blastesis and
deformation;
- kinematic vorticity analyses;
- geothermobarometric analyses to investigate the P-T paths followed by the different
tectonic units;
- geochronological analyses in situ to determine the ages of the different tectonic
phases;
During the third year of the research a GIS database will be realized with Archview
program containing all the geological and structural data of the study area.