THESIS PROPOSAL DEGREE PROGRAMME: M. Sc. SUPERVISOR and COMMITTEE: Supervisor - Isabelle Coutand Committee Members - John Gosse, Martin Gibling, David Whipp TITLE OF PROPOSAL: Late Neogene Kinematics of the Sikkim Himalayas using ZHe Thermochronology and 3D Thermokinematic Modelling KEY WORDS Sikkim, (U, Th, Sm)/He, zircon, thermochronology, 3D thermokinematic modelling, exhumation, LIST INNOVATIONS or EXPECTED SIGNIFICANT OUTCOMES: 1. Isolating the effects of tectonics on Neogene upper crustal exhumation in Sikkim, India 2. Modelling the geometry of the Main Himalayan Thrust along which the Indian plate is underthrusted beneath Eurasia 3. Investigating the uplift of the Lesser Himalayan Duplex within the Rangit and Teesta windows SUMMARY OF PROPOSED RESEARCH: The exhumation of rocks in the Himalayas is controlled by tectonic and climatic processes both conditioning erosion rates. Contractional tectonics results in thrust faulting and crustal thickening generating high topography and increased relief while Indian summer monsoon precipitations become enhanced along the range front. Both processes induce mass movement (i.e. landslides), enhance fluvial incision and conspire to erode away the newly created topography to expose subsurface material. During its transport toward the surface (i.e. exhumation), subsurface material travels through the crustal thermal field and is then able to be studied using thermochronology. 18 rock samples were taken along a NW-SE transect through Sikkim India for (UTh- Sm)/He thermochronology on zircons. These rocks are processed in order to extract parent and daughter isotope concentrations and ultimately calculate cooling ages. Thermokinematic modelling will be used to investigate the contribution of tectonic forces to the cooling age distribution documented by our data. Two scenarios will be tested: 1) varying geometry of the Main Himalayan Thrust in Sikkim, India, and 2) introducing a localized uplift within the Lesser Himalayas associated with the uplift of the Rangit Duplex. If the results from the modelling do not match our calculated cooling ages then this will suggest that tectonics alone does not control exhumation. We will then qualitatively explore the effects of climate.
TIMETABLE Activity Start Date End Date Data acquisition - sample collection and zircon isolation June 2011 September 2011 Zircon selection and packing September 2011 November 2011 Helium extraction December 2011 April 2012 Parent isotope extraction May 2012 June 2012 Construct cross section July 2012 September 2012 Construct model domain September 2012 November 2012 Model Testing - MHT geometry November 2012 January 2013 Model Testing - Duplexing January 2013 March 2013 Qualitative climate testing March 2013 May 2013 Final Interpretations and Writing - Thesis completed May 2013 September 2013 Statement of Problem The complex feedback mechanisms that exist between tectonics, erosion and climate are linked at the surface by their impression on topography and effect on upper crustal exhumation. In the Himalayas, these forces are expressed by the interaction between: 1) convergence of the Indian and Eurasian plates, which causes crustal thickening and creates high topography, and 2) erosion due to mass movement, precipitation and fluvial incision which lowers topography and ultimately exhumes subsurface material (e.g. Whittacker 2012). The relative contribution of climatic forces to upper crustal exhumation is difficult to unravel; do climatic or tectonic forces play the larger role in the exhumation of material along the strike of the orogen? We will focus only on the tectonic influence as there are currently no numerical models that are able to accurately simulate the effects of climate on exhumation rates. The study area, Sikkim, India, represents within a transition zone between Nepal, to the west, and Bhutan, to the east, two better studied regions that differ drastically with respect to topography, precipitation, magnitude of upper crustal exhumation, geometry of the main tectonic structures and the structural levels exposed. Our own cooling ages, along with published data, will be used to create a thermokinematic model for the development of the Himalayan range front in Sikkim. If tectonics alone cannot explain the cooling age pattern calculated then some other forcing, such as focused monsoon precipitation, may have played a larger role. The effects of climate will be investigated qualitatively. Background Study area: The Himalayan orogen was formed by the collision of the Indian and Eurasian plates at ~55 Ma (Le Fort 1975) and continues to be deformed by ongoing convergence. The Himalayas are characterized by a set of laterally continuous lithotectonic units and their bounding structures found along strike of the orogen (eg. Hodges 2000). The Sikkim Himalaya is situated in the East-central part of the orogen (Figure 1); the main units were emplaced on top of each other by a series of southward propagating thrust faults: the Main Central Thrust (MCT), the Main Boundary Thrust (MBT) and the Main Frontal Thrust (MFT). These 3 faults apparently branch at depth from a single basal decollement, the Main Himalayan thrust (MHT) (e.g. Lavé and Avouac 2001) which represents the base of the overthrusting Eurasian crust. The main units of Sikkim are separated by the MCT, MBT, MFT and the
normal sense South Tibetan Detachment Zone (STDZ) to the north (Figure 2). From north to south the main geologic units of Sikkim are: 1) the low grade meta-sedimentary Tethyan Sedimentary Sequence, the northern most unit bounded to the south by the STDZ. 2) The high grade gneisses and associated leucogranites of the Greater Himalayan Sequence (GHS) and Main Central Thrust Zone (MCTZ) which are thrust along the MCT over 3) the low grade meta-sedimentary Lesser Himalayan Sequence (LHS). Between the GHS and LHS is the main Central Thrust Zone (MCTZ) The LHS is thrust atop 4) the synorogenic Siwalik sediments of the foreland basin which is in turn thrust along the MFT atop 5) quaternary sediments in front of the collision zone. Figure 1 A simplified geological map showing the main geologic units and structures of the Himalayan orogenic front in Bhutan, Nepal and various parts of northern India. The black box indicates the location of the study area (Figure 3). Modified Regional Geology: To produce a tectonic model for Sikkim, structural, geophysical and topographic data from previously published studies will be used as guides. Within Sikkim the GHS has been eroded such that the trace of the MCT has migrated significantly northwards exposing the lower LHS member within a structure called the Teesta half-window (Figures 2 and 3). Within the Teesta a second full tectonic window, called the Rangit window has been formed exposing the upper units of the LHS through erosion of the folded Ramgarh Thrust (RT). Thickened and possibly repeated slabs of LHS coupled with surficial strike and dip data have led to the interpretation that a duplex system exists beneath these windows (Bhattacharrya and Mitra 2009, figure 3). The geometry of the duplex has been interpreted to contain a foreland dipping component on the southern flank, an antiformal stack in the center and a hinterland dipping component to the north. Two major rivers, the Rangit and the Teesta, flow N-S through their respective windows and have likely contributed heavily to the erosion that
Figure 2 A Geologic map of Sikkim showing the major units and structures including the mushroom shaped double tectonic window. Black stars denote sample locations. The black line shows the approximate location of figure 3. Modified from Grujic et al. 2011. Geophysical studies (e.g. Acton et al. 2010 and Alsdorf et al. 1998) have imaged the subsurface in Sikkim; both teams were able to image what is thought to be the trace of the MHT. The MHT appears to have a complex geometry beneath Sikkim with ramp and flat portions rather than a direct descent into the subsurface, which is consistent with views in literature (e.g. Hauck et al. 1998). The trace of the MCT appears first below the LHS in the imaging at a depth of 10-15 km as a nearly flat structure; the MHT connects to its current surface trace, the MFT, somewhere south of this but is not exposed in at the surface in Sikkim (the frontal ramp likely dips at ~15-30 o (Robert 2011)). The feature begins to dip northwards (~10 o ) beneath the GHS before disappearing at a depth of 20-25 km. The INDEPTH survey imaged the MHT even further north and into Tibet showing that it continues to dip northwards until at least 40 km depth where the signal is lost.
Figure 3 A balanced cross section through Sikkim, India showing the major units and structures present. The structure of the duplex system that exists below the LHS is an interpretation made by the authors, other structurations are possible. Modified from Mitra et al. 2010. Topography: In Nepal, as well as Sikkim, the mean elevation profile of the orogenic front is characterized by a two-step morphology whereas in Bhutan the profile is straight showing a near constant, gradual gain in elevation from the foreland to the plateau (Duncan, 2003, Figure 4A and 4D). Looking at mean relief profiles (Figures 4B and 4E) it is evident that Nepal has two high angle slope zones separated by a narrow zone of low relief; elevations are generally less than 2,000 m until ~125 km north of the MFT where elevation increases rapidly to 4,000-6,000 m near the plateau. In Bhutan, mean relief is relatively high and constant along the profile; elevations less than 2,000 m are mostly restricted to the first 25 km north of the MFT showing a more gradual increase to higher elevations compared to Nepal. Interestingly, areas of high relief correlate well with areas where the upper nappe of the GHS is preserved at surface (Figure 4C and 4F). The differences between these topographic profiles have caused the spatial distribution of precipitations to be restricted to the southern foothills of Bhutan while they penetrate further north into the range in Sikkim and Nepal (Bookhagen and Burbank 2006).
Figure 4 Two swath sets showing false color perspectives across Nepal (A,B,C) and Bhutan (D,E,F) from the foreland to the Tibetan Plateau. A and D show comparative DEM elevations; B and E show DEM calculated mean slope angle; C and F shows composite geology. From Duncan et al 2003. Climate: The largest climatic force in the region, the Indian Summer Monsoon, is one of the most intense seasonal rainfall events on earth and each year brings several meters of concentrated precipitation to the range front making it a source of significant erosion (eg. Bookhagen and Burbank 2010). This event occurs in large part due to a low pressure zone located above the Tibetan Plateau that draws moist air northwards from the Bay of Bengal until it reaches the southern ramparts of the Himalayan front where it is diverted westward along strike of the orogen, condensing and falling in the form of heavy seasonal rains (Harris 2006). Bookhagen and Burbank (2006) monitored mean annual rainfall along the entire Himalayan orogen and Tibet and detected several differences between Bhutan, Sikkim and Nepal. In Bhutan, there is a single thick band of high precipitation (>3 m/yr) that is restricted to the southern foothills while in Sikkim the single band breaks up and begins to form 2 thinner bands of high precipitation with a swath of lower precipitation in between. This trend continues and strengthens into Nepal; the two new bands of high precipitation clearly follow the trend of Nepal's two bands of high relief mentioned earlier. Upper Crustal Exhumation: While considerable attention has been given to Nepal, in the west, (eg. Herman et al. 2010) and Bhutan, to the east, (eg. Grujic et al. 2006), little to no low temperature thermochronometric data is available for Sikkim. This region was chosen specifically because it represents a transition zone between two better documented areas which have drastically different low temperature cooling age patterns. Mean apatite fission track (AFT) cooling ages (closure temperature (Tc) ~120 o C) in Bhutan span ~2-8 Ma while in Nepal to the west AFT ages are comparatively younger at
~1-4 Ma on average. Ar/Ar cooling ages (Tc~400 o C) show the same disparity with Nepalese ages ranging from ~4-6 Ma and the Bhutanese ages ranging from ~10-11 Ma (Kellet et al 2009). Many of the Ar/Ar ages from Nepal are younger than the AFT ages from Bhutan which shows that the exhumation of even much deeper material in Nepal occurred at a faster rate than in some parts of Bhutan. Finally, Sikkim and Nepal are marked by the presence of tectonic windows (like the Rangit and Teesta) exposing the LHS beneath GHS rocks while TSS meta-sediments are preserved as klippen atop the GHS in Bhutan (Figure 1). In Bhutan there is also a much larger, and more continuous, portion of the GHS preserved on the surface compared to Nepal and Sikkim. These differences in exposed structural levels, in conjunction with the occurrence of younger AFT and Ar/Ar ages in the Nepal, suggest a higher magnitude of exhumation in Nepal as compared to Bhutan. Our cooling ages from Sikkim will bridge a spatial gap between the Nepal and Bhutan data. Exhumation: Exhumation is defined as the upwards movement of rocks with respect to the surface (England and Molnar 1990). Rock uplift alone cannot directly exhume material, an erosional component must be present in order to remove surface material and expose what lies beneath. In compressional settings, exhumation is achieved through tectonic rock uplift and erosion through rock sliding, heavy precipitation and fluvial processes (figure 5). This material has a thermal history that can be observed using thermochronology. Whether the main erosive force contributing to Himalayan exhumation is tectonic or climatic in nature is one of the broader questions this study will help to answer. Thesis Objectives 1) Data Acquisition: The first phase is to collect the whole rock samples we need and to process them to obtain zircon grains that are usable for (U-Th-Sm)/He thermochronology. 2) Isotope extraction: Phase 2 entails the extraction of parent and daughter isotopes from zircon grains using mass spectrometry techniques at the DAL Noble Gas Extraction lab and the UCSC Keck Isotope lab. Zircon cooling ages for the samples will also be calculated during this phase. 3) Simplified Cross Section: We will construct a simplified cross section across the study area, Sikkim, India, based on geological, structural, topographical and geophysical data available in the literature. 4) Construct Model Domain: Relying on the cross section built in phase 3 as a basis, along with known thermal and material properties of the crust, a thermokinematic model domain for Sikkim will be produced using the PECUBE software. 5) Model Testing: Using the model domain constructed in phase 4 we will test two scenarios in order to constrain the development of the Sikkim Himalaya. We will test: a) varying the geometry and kinematics of the Main Himalayan Thrust at depth and b) the development of a duplex at mid-crustal levels. If the modelling outputs do not fit our own cooling age data, we will qualitatively explore the effects of erosion due to enhanced precipitation and river incision. Methodology There are two main methods I will use to complete the objectives of my thesis: 1) (U-Th-Sm)/He thermochronology on zircons (ZHe) and 2) Thermokinematic modelling using the PECUBE software. We use ZHe because its relatively low closure temperature (Tc~180 o C) allows us to evaluate the low temperature cooling histories of rocks from the mid-upper crust. They are also relatively abundant in the rocks in which we are interested. The PECUBE software generates sets of artificial cooling ages based on model parameters that are then compared to the cooling ages we obtain through ZHe so that we may constrain our parameters and find the best fit model to describe our data. (U-Th-Sm)/He dating of Zircons: ZHe thermochronology is a type of radiometric dating that involves measuring the concentration of parent U, Th and Sm and daughter 4He in zircons in order to calculate
the cooling age of the sample. Cooling ages represent the amount of time taken for a rock particle to cool from its closure temperature at depth (Figure 5), along its exhumation pathway to the surface where it can be sampled; the closure temperature of the ZHe systems sits at ~180 o C (Reiners 2005). Above this closure temperature, the crystal lattice of zircon crystals is open to the diffusion of 4 He atoms allowing for no accumulation of daughter products. Below the closure temperature the crystal lattice of the zircons is essentially closed to the diffusion of 4 He allowing for accumulation which is what allows us to date the crystals. Cooling ages are only equal to crystallization ages in the case of very rapid cooling such as solidifying lavas. There are some necessary assumptions that must be taken in order for cooling ages to be geologically meaningful (Farley, 2002). 1) It is assumed that there is no initial 4He present within the crystals from other geologic or atmospheric sources. 2) Secular equilibrium of parent isotopes is assumed so that the equation to calculate cooling age can be solved correctly. 3) Finally, it is assumed that there is no significant zonation of parent isotopes within our crystals as this can significantly alter the value of the a-ejection correction that will be calculated later. Figure 5 A 3D crustal block showing the exhumation of 2 samples (white dots) from the same closure temperature T3 through the overlying isotherms to surface. This shows the disturbances in the subsurface thermal field caused by topography and advection of material and heat parallel to thrust faults that can affect cooling ages and exhumation pathways. From Ehlers and Brandon (2003). The ZHe methodology can be broken down into 4 steps: 1) crystal isolation, 2) grain selection, 3) measurement and packing and 4) Isotope extraction, age calculation and correction; each of these steps will be described herein. Crystal Isolation: First, whole rock samples are crushed down to a mesh size of >425 µm using a Jaw crusher and then a disk mill grinder. The samples are then run through a Wilfley table in order to separate out the least dense fraction. The heavy fraction remaining is then immersed in Sodium Polytungstate liquid (SPT). SPT has a density of ~2.8 g/cm 3 so everything with a density less than that will float and anything denser will sink. This generally separates quartz (d~2.7 g/cm 3 ) and other similar light minerals, from the heavier apatite (d~3.2 g/cm 3 ) and zircon grains (d~4.6 g/cm 3 ). After the SPT separation, a magnetic separation using a hand magnet and the Frantz magnetic separator is done. This gets rids of most metallic minerals as well as the majority of micas. After magnetic separation, a final density separation is performed using Diiodomethane (MI). This heavy liquid has a density of ~3.3 g/cm 3
and is used to separate a fraction containing predominantly apatite from the fraction that contains predominantly zircons. Once a zircon separate is achieved, a final step of sieving the zircons into separate size fractions can be done so grains that are too small to be properly dated (< 60 µm) can be removed immediately. Grain Selection and packing: Individual zircon grains are selected for analysis on the basis of size, morphology and general clarity and freedom from mineral inclusions (Reiners et al. 2005). This is a highly important step when doing any type of (U-Th-Sm)/He dating as the size and morphology of the grains are directly related to the magnitude of the a-particle ejection correction, which will be outlined in the age calculation section. Generally, the zircon grains selected for this study ranged from about 75-150 µm in width; larger grains than this tended to be less euhedral or contained many inclusions. The morphologies of the grains are also important. For each sample that we are analyzing (18 in total) 5 acceptable grains were selected, for age reproducibility, for a total of 90 grains. Each single zircon crystal needed to be photographed and have its dimensions recorded before being wrapped in a thin strip of platinum foil. This foil serves two purposes, first, it makes it easier to move the grains and manipulate them during transport and second, it shields the grains from direct heating during laser extraction of 4He gas to prevent shattering of the grains or uneven heating. Isotope Extraction: Isotope extraction will be done in 2 different phases: Non-destructive extraction of 4 He at the Dalhousie Noble Gas Extraction Lab and Destructive U, Th and Sm measurement at the Keck Isotope Lab at UCSC. 4 He extraction is done using a ND-YAG IR laser and quadrapole mass spectrometer. The sample, in Pt foil placed on a small planchet within a vacuum sealed chamber, is heated to ~1300 o C for 30 minutes in order to extract all the helium. Meanwhile a pressure line attached to the planchet is voided to a near total vacuum and then injected with a measured amount of 3 He spike which is allowed to equilibrate. The sample 4He is then introduced to the pressure line where it equilibrates and is then introduced to the mass spectrometer for measurement. The grains will then be taken to the Keck Isotope lab for extraction of U, Th and Sm. Parent isotope extraction is a destructive process so our grains are lost during this process. The grains first need to be removed from their Pt foils. This is due to the Ar plasma used in the ICP-MS; the plasma can combine with dissolved Pt introduced with the sample to produce PtAr compounds that are of highly similar mass to one of our parent isotopes ( 235 U). Once removed from the foil, the grains are dissolved in a solution of nitric and hydrofluoric acid. The solution, along with an isotopic spike, is nebulized and introduced to a Thermo X-series II ICP-MS. Age Calculation and Correction: The cooling age of a sample is represented by the variable t in the following 4He ingrowth equation (Farley. 2002): The cooling age cannot however be solved for directly due to the fact that the daughter 4 He has 4 potential parent isotopes that produce it and we cannot differentiate between the 4 He produced by each parent. The solution to this is to approximate the value of t using a Taylor series approximation. This is an approximation of a function, t in this case, as a sum of terms calculated from the values of its derivatives at a single point. The Taylor approximation is applied to equation 1 to produce the following (simplified) expansion (M. Fellin, personal communication):
The value of t1 is approximated to be the uncorrected cooling age of the sample. The age is uncorrected because of α-particle ejection. When a radioactive isotope goes through α-decay, the daughter products are emitted a certain distance from the site of decay due to the kinetic energy of the reaction; this distance in zircons is ~20 µm. When decays occur within 20 µm of the crystal edge there is a statistical certainty that some daughter product will be ejected from the crystal lattice and lost to the environment (Farley and Wolf 1996). This results in an underestimation of the true cooling age of the sample and needs to be accounted for. The correction is based on the surface-area-to-volume ratio of the grain, geometry and medium specific diffusion parameters. Correction values of 0.75-0.90 are common (Farley and Wolf 1996) however this value can increase or decrease significantly with changes in grain size or poor grain geometry. With this correction, we can now calculate the final cooling ages for each of our samples by dividing the uncorrected age by the correction factor. Once the ages are calculated we will use them for comparison to our modelling results. PECUBE Thermo-kinematic Modelling: For the modelling phase of this project we will be using the PECUBE software, developed by Jean Braun (Braun. 2003), to model thermochronometric cooling ages and try to constrain part of the tectonic model of the Sikkim Himalayas. To begin building our model we first need a base to work from; for this we will create a simplified cross section across Sikkim using structural, geological and geophysical measurements from published data sets. With this cross section and other known thermal and material properties we will use PECUBE to construct a model block representing the study area. The PECUBE program itself solves the equation describing heat transfer in 3D, taking into account production, advection and diffusion of heat over time and space. Individual rock particle paths are tracked from a point at depth (eg. Figure 5), through their closure temperature and along their exhumation pathway to the surface over a pre-set number of time steps (Braun et al. 2011). This provides predicted cooling ages, and due to a known exhumation pathway, an exhumation rate. These models are run several hundreds of times with subtly different variables to try and find the best possible fit to the actual data. Model Testing: The first model we will test will be to vary the geometry and kinematics of the MHT, the main basal thrust separating the Eurasian plate from the underlying Indian plate. The MHT in Sikkim is thought to have a ramp and flat morphology rather than a linear descent into the subsurface (e.g. Alsdorf et al. 1998) The angles at which the different sections of the MHT decline will be varied in order to test the effects of fault geometry on exhumation rates. In addition, the partitioning rate of shortening along portions of the MHT will also be tested for its effect on exhumation. The Second model will explore the development of a duplex in the subsurface beneath the LHS in Sikkim by testing different rates of localized exhumation within the Teesta and Rangit windows. This exhumation corresponds to erosion and the removal of material at the surface exposing the underlying rocks in a window (e.g. Herman et al. 2010). If the results of these models do not give us statistically meaningful results when compared to our calculated cooling ages we will qualitatively explore the effects of climate by comparing 1) the modern precipitation distribution to cooling ages across Sikkim and 2) river induced incision due to the Teesta and Rangit rivers which run generally N-S through their respectively named windows. REFRENCES Acton, C., Priestley, K., Mitra, S. and Gaur, V. 2011. Crustal structure of the Darjeeling-Sikkim Himalaya and southern Tibet. Geophys. J. Int., 184, 829-852. Alsdorf, D., Makovsky, Y., Zhao, W., Brown, L.D., Nelson K.D., Klemperer, S., Hauck, M., Ross, A., Cogen, M., Che, J. and Kuo, J. 1998. INDEPTH (International Deep Profiling of Tibet and the Himalaya) multichannel seismic reflection
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