Structural Evolution of the Himalayan Thrust Belt, West Nepal

STRUCTURAL EVOLUTION OF THE HIMALAYAN THRUST BELT, WEST NEPAL

______

A Thesis

Presented to

the Faculty of the Department of Earth and Atmospheric Sciences

University of Houston

______

In Partial Fulfillment

of the Requirements for the Degree

Master of Science in Geology

______

An Li

August 2013 STRUCTURAL EVOLUTION OF THE HIMALAYAN THRUST BELT, WEST NEPAL

______

An Li

APPROVED:

______

Dr. Michael Murphy, Chairman

______

Dr. Alexander Robinson

______

Dr. Ran Zhang

______

Dean, College of Natural Sciences and Mathematics

ACKNOWLEDGEMENTS

I would like to express my sincere gratitude to my committee chair, Dr.

Michael Murphy for his patient guidance and persistent help for my thesis research.

Thanks him for taking me to the field trip at Big Bend and New Mexico. The most valuable thing I have learned from him is to be passionate to the work and be optimistic to the life.

I would like to thank my committee members, Dr. Alexander Robinson and Dr.

Ran Zhang for the support and assistance throughout this research project. Thanks to

Sylvia Marshall and Jay Krishnan for answering all my academic and IT questions.

Thanks to my classmate Yiduo Liu for giving me generous help in the field trips and constructive discussions about my thesis research. Thanks to my Uncle Faqi and

Aunt Fenghua for taking care of my life at Houston. And thanks to my classmates and friends for accompanying and encouraging me in both study and life.

Above all, I would like to thank my family, Zhenru, Jinbo, Dongran, Junfang.

Thank you for endless love and support that gave me the strength to conquer difficulties and courage to chase my dream. Thank you for helping me to find myself when I feel lost.

I dedicate this thesis to my family.

iii

STRUCTURAL EVOLUTION OF THE HIMALAYAN THRUST BELT, WEST NEPAL

______

An Abstract of a Thesis

Presented to

the Faculty of the Department of Earth and Atmospheric Sciences

University of Houston

______

In Partial Fulfillment

of the Requirements for the Degree

Master of Science in Geology

______

An Li

August 2013

ABSTRACT

Himalayan foreland basin strata were recently documented to crop out in a structural window in the central portion of the Himalayan thrust belt. Presently, structural interpretations of the thrust belt do not explain in detail how these strata were incorporated into the thrust belt and why they are not widely exposed throughout the

Himalaya. My research provides answers to these two issues.

The structural window is located in the Lesser Himalaya of western Nepal and exposes rocks which lie structurally beneath the Main Central thrust (MCT) and Ramgarh thrust (RT) sheets. The thrust sheet consists of Proterozoic metamorphic rocks. Below the thrust sheet, footwall rocks exposed in the window are unmetamorphosed sedimentary rocks which consist of the early foreland basin strata (the Suntar and Swat formations) and the pre-foreland basin strata (the Melpani and Lakharpata formations). These unmetamorphosed rocks are present in the foreland basin beneath Siwalik group, ~100km to the south (towards the foreland).

Two geologic maps were created; One covering the structural window and another covering the thrust belt from the High Himalaya to the MFT (Main Frontal

Thrust). Several cross-sections were constructed from both maps. Structural reconstruction of these cross-sections reveal the following: 1) ~ 75 km-long hanging wall flat extends northward from its surface trace to the southern margin of the Lesser

Himalayan duplex; 2) The geometry of the Jarjarkot klippe is narrower and structurally deeper than klippe to the west. The geometry of the northern flank of the klippe results from stacking of duplex horses, while the geometry of the southern flank results from slip over a ramp in the footwall of the MBT, 3) The early foreland basin strata in the window

v are modeled to have originated at the front of the thrust belt, and subsequently buried by the Ramgarh and MCT thrust sheets, and 4) Exposure of these strata results from growth of the duplex which brought them to a higher structural level than most parts of the thrust belt. This structural model explains why the only other exposure of foreland basin strata within the thrust belt is documented within a duplex on the north side of the Dadeldhura klippe. Moreover, this model predicts that foreland basin strata are likely to be exposed wherever duplexes exist.

CONTENTS

1. INTRODUCTION...... 1 1.1 GEOLOGY OF THE HIMALAYA ...... 1 1.1.1 Himalayan range and Himalayan fold thrust belt ...... 1 1.1.2 Timing of the Indo-Asian Collision ...... 4 1.1.3 Shortening of the Himalayan thrust belt ...... 4 1.1.4 Tectonostratigraphy of the Himalaya ...... 4 1.1.5 Himalayan foreland basin ...... 7 1.2 STRATIGRAPHY IN THE RESEARCH AREA ...... 12 1.3 MOTIVATION AND METHODS ...... 18 2. STRUCTURAL GEOLOGY OF THE RESEARCH AREA...... 21 2.1 FAULT ANALYSIS ...... 21 2.2 SHEAR ZONE ANALYSIS ...... 25 2.3 MAJOR STRUCTURAL FEATURES ...... 27 2.4 REGIONAL GEOLOGIC MAPS AND CROSS-SECTIONS ...... 35 3. DISCUSSION AND MODELS ...... 39 3.1 TIMING DATA IN RESEARCH AREA ...... 39 3.2 RECONSTRUCTION OF THE REGIONAL CROSS-SECTION ...... 41 3.3 COMPARISON WITH PUBLISHED CROSS-SECTIONS ...... 45 3.4 PREDICTION ...... 48 4. CONCLUSIONS ...... 50 5. APPENDIX ...... 51 6. REFERENCES ...... 59

vii

FIGURE LIST Figure 1. Topographic map of the Himalaya ...... 2 Figure 2. Tectonostratigraphic Map of the Himalaya ...... 3 Figure 3. Foreland basin system ...... 7 Figure 4. Regional geologic map of the research area ...... 14 Figure 5. The concepts of window and klippe ...... 19 Figure 6. The location of field maps ...... 20 Figure 7. Tectonic map of the research area ...... 22 Figure 8. Structural map of the structural window ...... 23 Figure 9. Stereoplot of the fault surface at station L-4, L-5 and R-7 ...... 24 Figure 10. Steoreoplot of the S-C fabric at station L-1 and L-6 ...... 26 Figure 11. Cross-section C-C’ ...... 28 Figure 12. Cross-section B-B’ ...... 30 Figure 13. Cross-section D-D’ ...... 31 Figure 14. Geologic map of the structural window ...... 33 Figure 15. Cross-section A-B south of the structural window ...... 34 Figure 16. Cross-section B-C through the structural window ...... 34 Figure 17. Characteristics the Ramgarh thrust sheet imbricate ...... 35 Figure 18. Regional cross-section A-A’ ...... 36 Figure 19. Sequential reconstruction of regional cross-section from 20Ma to 10Ma . 42 Figure 20. Sequential reconstruction of regional cross-section from 6Ma to Recent . 43 Figure 21. Talkot cross-section of DeCelles et al. (2001) ...... 46 Figure 22. Simikot cross-section of Robinson (2008) ...... 46 Figure 23. Kathmandu cross-section of Yin (2006) ...... 47 Figure 24. Tectonostratigraphic map of Nepal ...... 49

TABLE LIST Table 1. Stratigraphic column of Cenozoic formations at the research area ...... 9 Table 2. Stratigraphic column for the research area ...... 13 Table 3. Timing data of major faulting and deposition of foreland basin strata ...... 40

viii

1. INTRODUCTION

1.1 GEOLOGY OF THE HIMALAYA

1.1.1 Himalayan range and Himalayan fold thrust belt

The Himalayan range lies between Namche Barwa peak to the east and Nanga

Parbat peak to the west (Figure 1). To the north, the Himalayan range is bounded by the east-flowing Yalu Tsangpo River and west-flowing Indus River. To the south, the

Himalayan range is bounded by the Main Frontal Thrust which is also the northern margin of the Indo-Gangetic depression (Heim and Gansser, 1939).

Geologically, the Himalayan fold thrust belt is bounded by the Indus-Tsangpo suture to the north, the Main Frontal Thrust (MFT) to the south, the left-slip Chaman fault in the west and the right-slip Sagaing fault to the east (Figure 1) (LeFort, 1975). The

Himalayan fold thrust belt consists of several south-vergent thrust sheets and related folds.

The main thrusts in the Himalayan fold thrust belt include the Main Frontal Thrust (MFT),

Main Boundary Thrust (MBT), Main Central Thrust (MCT) and South Tibetan

Detachment (STD). These faults divide the region into four tectonostratigraphic zones which includes the Subhimalayan, Lesser Himalayan, Greater Himalayan, and Tethyan

(or Tibetan) Himalayan zones (Figure 2) (Heim and Gansser, 1939).

Karakoram orogen Karakoram

Himalaya

howing the Tibet the howing

(red frame shows the research area)Reference (Yin, 2006). the shows research (Yin, (red area)Reference frame

Topographic map of the Himalaya map Topographic

Figure 1. Figure

ference (Yin, 2006). ference (Yin,

sections) Re sections)

Figure Figure

location of published cross location of published

Tectonostratigraphic Map of the Himalaya. (Red frame shows the research area, colored lines shows the research shows area, the (Red the Map frame Himalaya. of Tectonostratigraphic

Figure.2 Figure.2

1.1.2 Timing of the Indo-Asian Collision

To better understand of the Cenozoic history of the Himalayan-Tibetan orogeny, we need to have a broad view of the context of the Indo-Asian collision. Paleo-magnetic evidence and stratigraphic and sedimentologic evidence have been used to understand the timing of the Indo-Asian collision. According to Patriat and Achache (1984), Cenozoic magnetic anomalies in the Indian Ocean show that the relative velocity between the

Indian and Eurasian plates decreased rapidly ~50 Ma. They interpreted this as the onset of the Indo-Asian collision. Gaetani and Garzanti (1991) showed an abrupt change from marine to terrestrial deposition occurred at the end of the early Eocene in the Zanskar region of northeastern India (~52Ma) which has also been interpreted as the initial age of the collision.

1.1.3 Shortening of the Himalayan thrust belt

Approximately 2500 km of convergence has taken place between the Indian and

Asian plates since 50Ma (Patriat and Achache, 1984). About one half to one third of the convergence between the Indian and Asian plates may be accommodated through crustal shortening in the Himalayan thrust belt (DeMets et al., 1994; Bilham et al., 1997; Larson et al., 1999; Lavé and Avouac, 2000). Shortening of the northern Indian margin is accommodated by several south-vergent thrust faults (Gansser, 1964; Powell and

Conaghan, 1973; Murphy and Yin 2003; Long et al., 2011). The crustal shortening varies along strike: increasing from west to east (Patriat and Achache, 1984; Dewey et al., 1989;

Schelling, 1992).

1.1.4 Tectonostratigraphy of the Himalaya

Gansser (1964) divided the tectonostratigraphy of Himalaya into 4 zones by major

faults. From north to south, those faults are: the South Tibetan Detachment System

(STDS), Main Central Thrusts (MCT), Main Boundary Thrusts (MBT) and Main Frontal

Thrusts (MFT). These major faults divide the Himalayan thrust belt into four divisions which are, from north to south, the Tibetan Himalaya, Greater Himalaya, Lesser

Himalaya and Subhimalaya. The following paragraphs introduce the major characteristics of these divisions.

Subhimalaya

The Subhimalaya Zone is bounded by Main Frontal Thrusts (MFT) to the south and Main Boundary Thrusts to the north (Figure 2). Since Middle Miocene to Pleistocene, detritus shed from the rising Himalaya accumulated and formed the foreland basin deposits of the Siwalik Group (Upreti, 1999). The Siwalik Group has been informally subdivided into three units-the lower, middle and upper members. The entire package of sediments in the Siwalik Group is un-metamorphosed and coarsens upward (Upreti,

1999).

Lesser Himalayan Zone

The Lesser Himalayan Zone lies between the Main Boundary Thrust (MBT) in the south and the Main Central Thrust (MCT) in the north (Figure 2). This zone occupies a relatively broad region, especially in western Nepal (Upreti, 1999). Separated by regional unconformities, this zone is divided into three stratigraphic successions which are: the Lesser Himalayan sequence, the Gondwana sequence and the Eocene-Lower

Miocene foreland basin sequence (Robinson et al., 2006). Detrital and igneous zircon U-

Pb ages indicate that the Lesser Himalaya in Nepal are Proterozoic in age (Parrish and

Hodge, 1996; DeCelles et al., 2000; Martin et al., 2005). Older Lesser Himalayan meta-

sediments have been exhumed and thrust over rock sequences along thrusts like the

Ramgarh thrust (Sakai, 1983; Upreti, 1999).

Greater Himalayan Zone (Higher Himalayan Zone)

The Higher Himalayan Zone lies between the Main Central Thrust (MCT) to the south and South Tibetan Detachment System (STDS) to the north (Figure 2). In west

Nepal, Greater Himalayan rocks are exposed at the surface in two regions: north of the

Main Central thrust and in the Dadeldhura and Jajarkot klippen which lie south of the

MCT and south of the Lesser Himalayan duplex zone respectively (Figure 2) (Robinson et al., 2006). The Dadeldhura and Jajarkot klippen are referred to as the Lesser

Himalayan crystalline nappes (Gansser, 1964; Upreti, 1999).

Tethyan (or Tibetan) Himalayan Zone

The Tibetan Himalayan Zone is bounded by the Indus-Yalu Fault to the north and the STDS to the south (Figure 2). This zone consists of sedimentary rocks known as the

Tethyan (or Tibetan) Himalayan sequence.It has undergone very little metamorphism except at its base where it is close to the Higher Himalayan Crystalline rocks of the

Higher Himalayan Zone (Upreti, 1999). In the research area, this tectonic zone has a very limited exposure and therefore is not shown on the geologic map.

1.1.5 Himalayan foreland basin

A foreland basin generally is defined as an elongate trough that forms between a contractional orogenic belt and the adjacent craton, mainly in response to flexural subsidence that is driven by thrust-sheet loading in the orogeny (DeCelles and Giles,

1996). The foreland basin system consists of four depozones which include back-bulge, forebulge, foredeep and wedge-top (Figure 3).

The Himalayan foreland basin lies between the Himalayan thrust belt and the

Indian craton. After initial collision between the Asian and Indian plates, deformation propagated southward and the evolving thrust belt system shed detritus into the adjacent foreland basin, which then became a part of the orogenic system as the Foreland Basin sequence and Subhimalaya (Burbank et al., 1996, 2003; DeCelles et al., 1998, 2004).

Figure 3. Foreland basin system (DeCelles and Giles, 1996). (D= duplex, TF= topographic front and TZ= triangle zone)

The shallow-marine, Eocene Swat (or Bhainskati) Formation accumulated in a back-bulge depozone (Figure 3) between a southward migrating forebulge and the Indian craton (DeCelles et al., 2001). Migration of the forebulge through this region during

Eocene-Oligocene produced a regional unconformity that spans ~15-20 m.y. By Early

Miocene, the forebulge unconformity was onlapped by the distal fringes of the southward migrating foredeep depozone which is represented by the fluvial deposits of the Suntar

(or Dumri) Formation. Continued southward migration of the foredeep after Middle

Miocene formed the fluvial Siwalik Group (Table 1) (DeCelles, et al., 2001). These formations mentioned above are discussed with more detail in the following paragraphs

(Table 1).

Pre-Foreland Basin Rocks

During late Paleozoic to Mesozoic, the Gondwana sequence was deposited unconformably on top of the Proterozoic Lesser Himalaya sequence (Robinson and

McQuarrie, 2012). Detrital zircon U-Pb ages from a lower part of the Gondwana sequence indicate a maximum depositional age of 125±1 m.y. (DeCelles et al., 2004). On top of the Gondwana sequence is the Melpani (or Amile) formation which was interpreted to be deposited between early Cretaceous to possibly early Eocene (Sakai,

1983). And according to detrital zircon U-Pb age population data, the sedimentary source was the early Proterozoic to Archean rocks of the northern Indian craton located to the south (DeCelles et al., 2000, 2004).

research area

the the

Stratigraphic column of Cenozoic formations at research area formations of Cenozoic column Stratigraphic

Stratigraphic column of Cenozoic formations at formations of Cenozoic column Stratigraphic

Table 1 Table

Table Table

Swat (or Bhainskati) Formation

The Swat (or Bhainskati) Formation was deposited during the early stage of

Foreland Basin development (DeCelles et al., 2004). In far-western Nepal, the thickness of the formation is ~100m (DeCelles et al., 1998). The estimated age for the lower part of 9

the formation is late Paleocene-early Eocene (Frank and Fuchs, 1970), and the upper part has a Mid- Late Eocene age based on foraminifera and marine fossils (Sakai, 1983). At the top of the formation, there is a well-developed unconformity, a paleosol, spans from late Eocene to Oligocene, and has been interpreted to represent sub-aerial exposure ca. 20

Ma (DeCelles et al., 1998). The unconformity was formed by the southward migration of the foreland basin system from back-bulge environment to the fore-bulge (Table 1).

Suntar (or Dumri) Formation

The Suntar (or Dumri) formation was deposited in the fore-deep depozone of the foreland basin system (Table 1) (DeCelles et al., 1998). The youngest zircon population here shows that the base of the formation is younger than 30-32 m.y. (Najman and

Garzanti, 2000; Najman et al., 2005). In far western Nepal, the lower half of the formation is a green, strongly indurated, micaceous sandstone (DeCelles et al., 1998).

Approximately 30 m up section, a pronounced bauxite paleosol occurs with kaolinite and hematite pisolites, above which the formation returns to micaceous sandstone. The upper half of the formation is dominated by red and green shale (DeCelles et al., 1998). The source of the formation is not well determined whether derived from the Tethyan

Himalaya or Greater Himalaya (Robinson et al., 2001; DeCelles et al., 2004).

Siwalik Group

The Siwalik Group is a Middle Miocene to Pliocene belt of synorogenic foreland basin sediment at the front of the thrust belt which has been informally divided into upper, middle and lower units (Quade et al., 1995). This division is mainly lithostratigraphic

(Table 1).

Lower Siwalik Unit

Deposition of the lower Siwalik unit began by ~13.3 Ma (Ojha et al., 2000). The thickness of the formation is about 900m and the formation consists of interbedded sandstone and shale (DeCelles et al., 1998). The first appearance of high-grade metasedimentary lithic grains, kyanite and sillimanite, occurs in the upper part of the lower Siwalik unit (DeCelles et al., 1998) which is dated at ca. 11 Ma (Ojha et al., 2000).

This documents the presence of the Greater Himalaya rocks at the surface shedding sediment into foreland basin during this period of time (Robinson and McQuarrie, 2012).

Middle Siwalik Unit

The thickness of the middle Siwalik unit is ~2500m and is dominated by stacked channel sandstones punctuated by thin shale beds (DeCelles et al., 1998). The age of this unit is between ca. 10.5 to ca.4.6 m.y. (Quade et al., 1995; DeCelles et al., 1998; Ojha et al., 2000, 2009). Lesser Himalaya-derived carbonate clasts first appear at the base of the section (at 10.8 Ma) (DeCelles et al., 1998). The transition between the lower and middle

Siwalik is marked by prevalence of stacked fluvial channel sandstones (DeCelles et al.,

1998).

Upper Siwalik Unit

The transition between the middle and upper Siwalik is estimated to be younger than 4.5 Ma (Ojha et al., 2000). There is at least 1000m of predominantly fluvial conglomerate punctuated by fluvial sandstone in this unit (DeCelles et al., 1998).

1.2 STRATIGRAPHY IN THE RESEARCH AREA

Within different Himalayan tectonostratigraphic zones, the stratigraphy has been informally subdivided into several units, and different researchers use different nomenclature to describe those units. The stratigraphy in west Nepal is relatively consistent. In this research, we employ the description of the stratigraphic units describe by Robinson et al., (2006), whose research area is about 50km west of my research area.

The stratigraphic units which are related to this research will be discussed in every tectonostratigraphic zone (Table 2). The following paragraphs will introduce major characteristics of the strata and the locations where they crop out (Figure 4).

Subhimalayan Sequence

The Subhimalayan sequence consists of the lower, middle and upper Siwalik units which were introduced in detail above. The Siwalik group rocks only crop out in the

Subhimalayan zone (Figure 4).

Table 2. Stratigraphic column for the research area

)

(NDMG);

D’ see Figure D’

11; D 11;

research area research

the

C’ see Figure see C’

ic map of

Regional geolog

B’ see Figure 1 B’

Unpublished Field 2010; Unpublished Mapping

;

Geologic Map of Western Nepal 1:250,000 NDMG Geologic of Western Map

Figure Figure

1:1,000,000 map from: Nepal Department and Mines of Geology Nepal map from: 1:1,000,000

A’ see Figure18 A’

Reference:

Lesser Himalayan Sequence

In Lesser Himalayan zone, the stratigraphy has been subdivided into three stratigraphic successions by regional unconformities which are the Lesser Himalayan sequence, the Gondwana sequence and the Eocene-lower Miocene foreland basin sequence. According to the previous research (Paudel and Arita, 1998; Upreti, 1999;

DeCelles et al., 2001; Pearson, 2002), the whole thickness of the Lesser Himalayan sequence is about 10-13 km.

Kushma Formation (Ks)

The Kushma (Ks) Formation with an age of ~1.85Ga (DeCelles et al., 2000;

Martin et al., 2005) is the oldest unit in the Lesser Himalayan package. The Kushma formation is structurally competent, 0.8 – 1.5 km thick, medium to coarse grained and with some green ortho-quartzite and local muscovite-rich parting (Robinson et al., 2006).

The outcrop location of the formation will be introduced in the following paragraph.

Ranimata Formation (Rm)

The Ranimata Formation is 1.5-3.0 km thick and mainly consists of green chloritic phyllite with scattered 10-100m thick intervals of thinly bedded white to green quartzite, gritty phyllite and rare thinly bedded carbonates. Dark green chloritic zone and quartz augen are common in the formation (Robinson et al., 2006). The Ranimata and

Kushma formations belong to the Ramgarh thrust hanging wall. They always crop out together at the southern and northern portions of the research area (figure 4), at two flanks of the Jajarkot klippe and crop out in repetition map pattern in the Lesser

Himalayan imbricate zone (Figure 4).

The Sangram Formation (Sg)

The Sangram Formation is about 500 m thick in western Nepal, and fine-grained quartzite intervals are observed at lower and upper part of the formation respectively

(Robinson et al., 2006). The age of the Sangram Formation is ~1.68 Ga based on detrital zircons ages from the Sangram Formation (DeCelles et al., 2000). The formation only crops out in a limited area north of the MFT surface trace along the southern portion of the Lesser Himalayan zone (Figure 4).

Galyang Formation (Gl)

The Galyang Formation is ~500-1000m thick and consists of thinly bedded, strongly foliated, olive-green, brown, and gray phyllite and black slate with scattered quartz augen (Robinson et al., 2006).The formation only crops out in a limited area near the MBT surface trace at southern portion of Lesser Himalayan zone in the research area

(Figure 4).

Syangia Formation (Sy)

The Syangia Formation is ~500m thick in far western Nepal. It contains a wide variety of rocks including green phyllite; fine-grained, reddish, purple, and green slate; pink, white, and maroon thinly bedded quartzite; and thinly bedded blue and white dolostone and limestone. Cross-stratification, mud cracks, and oscillatory current ripples are present in the Syangia Formation (Robinson et al., 2006). The formation crops out north of the MBT surface trace and at the area near a structural window (north of the

Jajarkot klippe) (Figure 4).

Lakaharpata Group (Lk)

This is a mainly carbonate unit. Within the formation, there are laminated

microcrystalline dolostone and stromatolitic limestones, thinly bedded white quartzite and minor gray phyllite with scattered limestone beds (Robinson et al., 2006). The thickness of the formation is around 3 km. This formation crops out at the Lesser

Himalaya zone together with the Ranimata and Kushma formations. It also crops out in a large area along the MBT surface trace and at the structural window with other MBT hanging wall strata like Sangram and Syangia (Figure 4).

Gondwana Sequence

In far-western Nepal, a several hundred meters thick succession of clastic sedimentary rocks of probable Late Cretaceous to Paleocene age rests unconformably on top of the Lakharpata Group. These rocks are assigned to the upper part of the Gondwana sequence (Robinson et al., 2006). The Gondawana sequence is poorly exposed in the research area.

Foreland Basin Sequence

The characteristics of the foreland basin formations were discussed in detail above. The Swat formation crops out in a structural window along northern portion of the

Lesser Himalayan zone in the research area. The Suntar and Swat formations together crop out north of the MBT surface trace along southern portion of the Lesser Himalayan zone. The Siwalik group only crops out south of the MBT (Figure 4).

Greater Himalayan Zone

The rocks in this zone are composed of upper amphilolite-grade metasedimentary and meta-igneous rock (Pecher, 1989) that have been formally subdivided into Formation

I, II, and III (LeFort, 1975; Colchen et al., 1986) and recently referred to more informally as units I, II, and III (Searle and Godin, 2003). For the purpose of simplification, we

treated these rock units in Greater Himalayan zone as a single package. The Greater

Himalayan rocks crop out north of the MCT and in the Dadeldhura and Jajarkot klippe.

Tibetan (Tethyan) Himalayan Zone

The Tibetan Himalayan Sequence is composed of Proterozoic to Eocene silici- clastic and carbonate sedimentary rocks inter-bedded with Paleozoic and Mesozoic volcanic rocks (Baud et al., 1984; Gaetani and Garzanti, 1991; Liu and Einsele, 1994,

1999). In this study, we didn’t take the Tibetan Himalayan sequence into consideration.

1.3 MOTIVATION AND METHODS

Erosion of a thrust sheet can result in the formation of klippe and windows (Figure

5). An isolated remnant is called a klippe. And a hole formed by erosion on the thrust sheet is called a window (Figure 5).

A structural window is located in the Lesser Himalaya of western Nepal and exposes rocks which lie structurally beneath the Main Central thrust (MCT) and Ramgarh thrust (RT) sheets. The thrust sheet consists of Proterozoic metamorphic rocks. Below the thrust sheet, the footwall rocks exposed in the window belong to MBT hanging wall.

These rocks are unmetamorphosed sedimentary rocks which consist of the early foreland basin strata (Suntar, Swat, formations) and pre-foreland basin strata including the

Melpani and Lakharpata formations. These unmetamorphosed rocks and metasedimentary rocks are present in the foreland basin beneath the Siwalik group, ~100km to the south (towards the foreland).

Figure 5. The concepts of window and klippe.

Presently, structural interpretations of the thrust belt do not explain in detail how these strata were incorporated into the thrust belt and why they are not widely exposed throughout the Himalaya. This study addresses these two questions.

Field mapping was conducted by Dr. Murphy in 2010 in western Nepal along the

Bheri and Thuli Bheri rivers from the Jajarkot to the Dunai region (Figure 6), the length of the mapping area is about 100 km., and the polygon with light gray color shows the area mapped (Figure 6). The regional geologic map (Figure 4) covering the Himalayan thrust belt from the Main Boundary thrust to the high Himalaya was compiled from field maps, the geologic map of Nepal (scale 1:1,000,000, from: Nepal Department of Geology and Mines) and the geologic map of west Nepal (scale 1:250,000 from Nepal Department of Geology and Mines). The geologic map covering the window area is mainly compiled from the unpublished field mapping. The field mapping data are compiled and shown in the Appendix of this thesis. Several cross-sections were constructed from both maps.

Structural reconstructions were performed to address the regional structural evolution.

(Gray polygon between Jajarkot and Dunai are the are the Jajarkot and Dunai (Gray polygon between

location of field maps)field location of

. The location of fieldThe location maps .

Figure Figure

2. STRUCTURAL GEOLOGY OF THE RESEARCH AREA

2.1 FAULT ANALYSIS

Three sets of measurements (at station R-7, L-4 and L5 stations) were taken on fault surfaces (Figures 7 and 8). The measurements were taken within the middle part of the research area where foreland basin formations crop out in the structural window. The goal of the fault analysis is to determine the kinematics of faults which are related to the evolution of the structural window. Great circles in the stereoplot (Figure 9) are fault surfaces. The black points within the stereonet represent the striations on the fault surfaces. Points on the outline of stereonet are striations which have a plunge of 0º.

At station R-7, a fault is located within the Ramgarh thrust sheet imbricate zone along the northern portion of the research area (Figure 7). The fault plane dips to west, and the mean trend and plunge of the striations on the fault surface are 346º and 33º respectively. The fault slip direction is SSE (Figure 9).

At station L-4, a fault found at the structural window area was measured (Figure

8). The fault surface dips steeply (~65 degrees) to the north and the fault carries the hanging wall thrust in the SSE direction (Figure 9).

At station L-5, 2 kilometers south of the station L-4, a fault within the Swat formation was measured (Figure 8). The fault surface dips~50 degrees to the NNE with a mean slip direction of WSW. It is interpreted as a minor thrust because it has a small amount of slip (Figure 9).

section

of cross

(NDMG);

location

are

Dashed line Dashed

mapping stations

Geologic Map of Western Nepal 1:250,000 NDMG Nepal 1:250,000 of Western Map Geologic

Tectonic map of the research area showing major of the area showing map research Tectonic

Field 2010; Mapping

Figure Figure

1:1,000,000 map from: Nepal Department and Mines of Geology Nepal map from: 1:1,000,000

Reference:

Unpublished

A’ (Figure 18), red frame is the coverage of the geologic map of structural window (Figures and 14) 8 window map of the geologic structural theframe of (Figure is coverage 18), red A’

faults, structural features discussed in text and field textfield faults, discussed and structural in features

C see Figure 16).C see

showing major faults and stations and showing major faults

B see Figure 1 B see Figure

ocation of this figureocation map see 7. of

sections trace lines, A lines, trace sections

(red numbers indicate location of stations; indicate location (red numbers

cross

Structural map of the window Structural structural map

Figure Figure

red dashed lines arered dashed

Figure 9. Stereoplot of the fault surface at station L-4, L-5 and R-7.

2.2 SHEAR ZONE ANALYSIS

Shear zones displaying ductile deformation in crystalline rocks were found in both the northern and southern part of research area (Station L-1, L-6 and L-7, Figure 8).

At station L-1, the rock type is calcareous sandstone interlayered with gray and green phyllite in the Ranimata formation. S/C foliations were observed in the phyllite layers. The sense of shear is top-to-west indicating a shortening direction of ENE-WSW

(Figure 10).

At station L-6, S/C fabric foliation showed in the metamorphic rock within the

Kushma formation (Ks) shows a north-south shortening direction (Figure 10).

At station L-7, a shear zone has been observed within the gneiss of the Surbang formation which belongs to the MCT thrust sheet. The sense of shear is top-to-south. The characteristic mineral assemblage of the shear zone rock is biotite, quartz, feldspar.

Asymmetric sigma-type of feldspar porphyroclasts indicate simple shear deformation.

Shear surfaces dip approximately north with stretching lineations that are nearly orthogonal to strike. Stretched quartz defines the lineation. The mean vector of the stretching lineation is 195º/49º (Figure 10). The shortening direction is north-south.

Figure 10. Steoreoplot of the S-C fabric at station L-1, L-6 and L-7. (The fabrics were found in Ranimata (Rm), Ranimata (Rm) and Surbang (Sb) formations respectively)

2.3 MAJOR STRUCTURAL FEATURES

The following paragraphs analyze the major structural features in the research area. These features include, from south to north, the Subhimlayan thrust system, the

Lesser Himalayan imbricate zone, the Ramgarh thrust system, Jajarkot klippe, the Lesser

Himalayan duplex zone, and the Ramgarh thrust sheet imbricate zone (Robinson et al.,

2006) (Figure 7). Several zoomed-in maps from the regional geologic map and cross- section which were constructed from the maps are shown below. The structural features are discussed from south to north.

(1) Subhimalayan thrust system

This zone lies at the southernmost part of the research area and delineates the boundary between the Himalaya thrust belt and the foreland basin in front of it. It is bounded by the Main Boundary Thrust to the north and the Main Frontal thrust to the south (Mugnier et al., 1993 and 1999) The Main Frontal thrusts (MFT) in this area consist of three separate, partially overlapping faults and the thrust sheets entirely consist of the Siwalik group. These thrust faults emplaced MFT hanging wall rock at the surface and show repeated lower, middle and upper Siwaliks sequences in map view (Figure 11).

section

C’, Location of the crossC’, Location of the

Boundary thrust.

section C section

Cross

Figure

see regional geologic map (Figure 4). MFT= Main Frontal thrust, MBT= Main thrust, MBT= (Figure MFT= Main Frontal see regional geologicmap 4).

(2) Lesser Himalayan Imbricate zone

North of the Subhimalayan Thrust system is the Lesser Himalayan Imbricate zone.

It’s bounded by the Main Boundary Thrust (MBT) to the south and Ramgarh thrust (RT) to the north. In the southern portion of this zone, the MBT thrust sheet carries the RT sheet folded into a broad syncline (Figure 12). North of the syncline is a narrow anticline which is formed by the ramp beneath it (Figure 12). The map pattern of the northern portion of this zone is interpreted as a MBT branch which brought the Suntar, Swat,

Melpani and Lakharpata formations to surface (shown by Figure 12). The Ranimata (Rm) and Kushma (Ks) formations show a repetition map pattern which is formed the by the imbrication of Ramgarh thrust sheet (Figure 12).

(3) The Ramgarh thrust system

The Ramgarh Thrust (RT) is a major thrust fault which extends throughout Nepal and northern India. The Ramgarh thrust places greenschist-grade rocks of the Kushma and Ranimata Formations on top of less metamorphosed Lesser Himalayan rocks

(Valdiya, 1980). In western Nepal the Ramgarh thrust sheet ranges from ~1.5 to 2.5 km thick (Robinson et al., 2006). The cross-section of the Lesser Himalayan imbricate zone

(Figure 13) shows the RT thrust sheet resting on top of the MBT thrust sheet. In map view (Figure 13), the Ranimata and Kushama formations crop out along both sides of the

Jajarkot klippe which indicates that the RT thrust sheet lies structurally beneath the klippe which belongs to the MCT thrust sheet (Figure 13). South of the klippe, imbrication of the RT sheet causes a repetition of the Ranimata and Kushma formations in map view

(Figure 13).

B’

section B section

thrust, MBT= Main Boundary thrust, thrust, Main Boundary MBT= thrust,

Cross

section see regional geologic map(Figue 4) regional section geologic see

Figure

RT= Ramgarh thrust, MCT= Main Centralthrust. Main Ramgarh MCT= thrust, RT=

Location of the cross Location of

Abbreviations: MFT= Main MFT= Frontal Abbreviations:

(4) Jajarkot klippe

The Jajarkot klippe is located south of the structural window and north of the

Lesser Himalayan imbricate zone (Figure 7). The klippe is floored by the Main Central

Thrust and is folded into a regional-scale syncline (Figure 13). The Ramgarh thrust sheet crops out to the north and south of the Jarjarkot klippe. Because of this, a long Ramgarh thrust sheet folded into a syncline is interpreted to lie beneath the klippe. The Jarjarkot klippe and the northwestern Dadeldhura klippe are commonly known as the “Lesser

Himalayan crystalline nappes” (Location of klippe see figure 7). One of the differences between these two klippe is their dimension in north-south direction. The Dadeldhura klippe has a width of about 50 km. In contrast, the Jajarkot klippe is narrower with a width of about 15 km (Figure 13). More differences between these two klippe are discussed in a later chapter.

Figure 13. Cross-section D-D’. Location see regional geologic map (Figure 4).

(5) Lesser Himalayan duplex zone

The Lesser Himalayan duplex crops out in a structural window through the

Ramgarh thrust between the Jajarkot klippe and the Main Central thrust (Srivastava and

Mitra, 1994; DeCelles et al., 1998, 2001) (Figure 7). The regional geologic map (Figure

4) shows that the Swat, Melpani and Lakhapata formations crop out among large areas of

Ranimata and Kushma formations. In the geologic map of the structural window (Figure

14), a strip of repeated Swat and Melpani formations with a width of about 10 km, is shown among large outcrops of the Lakhapata formation. We interpret this as Lesser

Himalayan units are imbricated into a hinterland-dipping duplex. The horses of the duplex consist of pre-foreland formations which include the Sangram, Galyang, Syangia,

Lakharpata and Melpani formations, and foreland basin strata (the Swat and Suntar

formations) (Figures 15 and 16).

Because the stacking of duplex horses, the Swat and Melpani formations which ride on top of the duplex horses were moved to a higher structure level. This explains why these foreland basin strata are exposed in the thrust belt. Because they need structure like duplexes to move them to a higher structural level. Moreover, we interpret the top of the duplex to be almost flat and this explains why the thin Swat formation crops out in large areas in map view (Figure 14).

)

trace lines

ions

C see Figure 16 C see Figure

sect

;

C are cross

Geologic of the structural window map

B see Figure 15

B, B

Figure Figure

Figure 15. Cross-section A-B south of the structural window. Location see geologic map of the window area (Figure 8). (Sb and Sr belong to Main Central thrust sheet, Rm= Ranimata Fm., Ks=Kushma Fm. Lk= Lakhapata Fm. Sg= Sangram Fm. Gl= Galyang Fm.)

alyang Fm.) alyang

C through the structural window

section B section

Cross

Sg= Sangram Fm., and Gl= G Sg= Sangram Fm., and

ocation see geologic map of the window area of (Figure the window 8). geologicmap ocation see

(Sw= Swat Fm., Mp= Melpani Fm.,Lk= Lakhapata Fm., Fm.,Lk= Fm.,Melpani (Sw= Swat Mp=

Figure Figure

(6) Imbricated Ramgarh thrust sheet

North of the Lesser Himalayan duplex zone is a zone composed of imbricated

Ramgarh thrust sheet (Figure 7). Repetition of gray silver phyllite and sheared quartzite is observed in the field with imbricates bounded by several north dipping thrusts (Figure

17). The northern part of this zone is bonded by the Main Central thrust.

Figure 17. Characteristics the

Ramgarh imbricate thrust sheet

(RT= Ramgarh thrust;

Rm= Ranimata Formaiton,

Ks= Kushma formation)

2.4 REGIONAL GEOLOGIC MAPS AND CROSS-SECTIONS

Based on all the structural analysis above, we constructed regional cross-sections.

The cross-sections show all the major faults and structural zones. Figure 18 is the regional cross-section which extends from the Subhimalaya in the south to the Main

Central Thrust in the north. The following discussion includes, in detail, of the key points of the regional cross-section.

igure 7, igure

A’

4 and F

section A section

gure

section see Fi section see

Regional cross

Figure

location of the crossthe location of

numbers are detail parts which are discussed in the thesis the are in numbers detail parts which discussed are

MFT= Main Frontal thrust, MBT= Main Boundary thrust, RT= Ramgarh thrust, MCT= Main Central thrust Main Central MCT= thrust, Ramgarh Boundary thrust, MBT= RT= FrontalMFT= Main thrust, Main

Explanation of the Regional cross-section

① The Main Boundary Thrust carries the thrust sheet which includes early foreland basin deposits (the Suntar and Swat formations) onto the Siwalik group in the south

(Figure 18). The Suntar (Sn) formation is documented in the regional geologic map

(Figure 4).

② A broad syncline ~20km wide has been identified based on map pattern (Figure 12).

The core is made up of the Rarmgarh thrust sheet formations (Ranimata and Kushma formations), with the Lakharpata formation cropping out at the two flanks (Figure 18).

③ A ramp has been interpreted to fold overlying MBT and RT thrust sheets (Figure 18).

④ A very limited area of the Lakharpata (Lk) formation crops out among massive

Ranimata and Kushma formations in map view. And we interpreted a MBT branch which emplaced the Lk formation at the surface (Figure 18).

⑤ A wide exposure area of the Kushma formation in map view indicates it has an almost flat geometry (Figure 18). The Rarmgarh thrust sheet lies beneath the Jajakot klippe and crops out along two flanks of the klippe (Figure 18). South of the klippe are imbricates of the Rarmgarh thrust sheet which repeat the Ranimata and Kushma formations as seen in map view south of the klippe.

⑥ A structurally lower ramp in front of the Jajarkot klippe beneath the Main Boundary and Rarmgarh thrust sheets is interpreted to fold the overlying rocks. This ramp together with the Lesser Himalayan duplex folded the MBT thrust sheet into a narrow klippe

(Figure 18).

⑦ A long narrow klippe located in the middle part of the research area has a narrow synformal geometry with MCT and RT thrust sheets in the core. This occurs because the ramp to the south and the duplex to the north are located near each other causing the thrust sheet to be folded into a narrow syncline (Figure 18). Moreover, we interpreted that the klippe is deep as it rides on a single package of MBT hanging wall rocks (Figure 18).

⑧ A large area of the Lakharpata formation crops out north of the Jajarkot klippe (Figure

4) and is interpreted to lie along a flat (Figure 18). Several south vergent thrusts have been observed in the field (Figure 18).

⑨ The area with out-crops of the thin Swat and the Melpani formations (Figure 18).

Because the Lesser Himalayan duplexes carry foreland basin rocks to structurally higher levels and based on the flat geometry at the top of duplex horses, the thin Swat and

Melpani formations can crop out over a large area. North of this area is a large outcrop of the Lakharpata formation; the younger Swat and Melpani formations have been removed by erosion (Figure 18).

⑩ The number of the duplex horses is determined by the map pattern and thickness of the package of rock in duplex horses (Figure 18).

⑪ at the northern most part of the research area, a repetitive occurrence of phyllite and quartzite was observed in the field. This is interpreted as imbricates of the Ramgarh thrust sheet overlying a lower duplex horse (Figure 18).

3. DISCUSSION AND MODELS

3.1 TIMING DATA IN RESEARCH AREA

From 25 to 21 Ma, the Main Central thrust sheet was emplaced onto a regional flat of Lesser Himalayan rock which is the Ramgarh thrust sheet. The hanging wall and footwall of the MCT have a flat on flat relationship (Robinson et al., 2003). The Ramgarh thrust initiated at ca. 15 Ma (DeCelles et al., 2001) and was emplaced over Lesser

Himalayan rocks which would later become the Lesser Himalayan duplex (Robinson et al., 2003). Displacement on the Ramgarh thrust is interpreted to have occured from 17 to

7 Ma (Robinson et al.,2006) with uplift and exhumation of the Ramgarh thrust sheet is interpreted to occur 9-11 Ma (Bollinger and Janots, 2006). The Lesser Himalayan duplex began to grow ca. 10-11 Ma (Quade et al., 1997; DeCelles et al., 1998; Robinson et al.,

2001). The Lesser Himalayan imbricate zone formed in early Pliocene and includes the southernmost thrust (DeCelles et al., 1998). The Subhimalayan thrust system was active from Mid Pliocene-Holocene (Lavé and Avouac, 2000).

Table 3 is from Robinson and McQuarrie (2012) and shows the timing of fault motions in far western Nepal as well as the timing of deposition of the foreland basin sequence. These timing data were used to construct the regional cross-section.

Table 3. Timing data of major faulting and deposition of foreland basin strata MFT= Main Frontal thrust, MBT= Main Boundary thrust

3.2 RECONSTRUCTION OF THE REGIONAL CROSS-SECTION

Constrained by the timing data (Table 3) (Robinson and McQuarrie., 2012) for the deposition of the foreland basin formations and major faults development, a sequential reconstruction of the regional cross-section (Figure 18) was made to better understand the regional structural evolution of the research area (Figure 19 and 20).

Several simplifications were made during the reconstruction. (1) I didn’t consider topography through time; (2) I didn’t consider South Tibetan Detachment (STD) and

Tethyan Himalaya rocks; (3) I used a ~4º N as the dip angle of Main Himalayan thrust

(MHT).

Several time steps were chosen to show the major events through time. Figures 19 and 20 show reconstruction of the regional cross-section from 25 to 0 Ma.

From 20 Ma to 15 Ma, the Suntar (or Dumri) formation was deposited on top of the earlier-formed foreland basin strata Swat and pre-foreland basin deposit (the Melpani formation) (Table 3); Ramgarh thrust imbricate formed early during this period and then a long Ramgarh thrust sheet (~125km) thrust on to the footwall through a ramp whose location is unknown (Figure 19). Approximately 100km of shortening is accommodated by the Ramgarh thrust sheet imbricate. A total 225 km shortening is estimated to be accommodated by the imbricating and sliding of the Ramgarh thrust during this time

(Figure 19).

Figure 19. Sequential reconstruction of regional cross- section from 20Ma to 10Ma.

Figure 20. Sequential reconstruction of regional cross- section from 6Ma to Recent.

From 15 Ma to 10 Ma, the lower Siwalik formation was deposited as the Lesser

Himalaya duplex developed (Table 3). The shortening during this period is accommodated through development of the Lesser Himalayan duplex. We estimate a shortening of about 125 km. Due to the building of the duplex, the top of the duplex horse moved to a higher structural level, and the Ramgarh and MCT thrust sheets were folded into a broad anticline (Figure 19).

From 10 Ma to 6 Ma, the Middle Siwaliks were deposited as the Lesser

Himalayan duplex and MBT developed (Table 3). The Main Boundary thrust emplaced its thrust sheet (~75km long) on top of the Lesser Himalaya strata, the Ramgarh and

MCT thrust sheets rode on top of the MBT thrust sheet and were then passively moved toward the foreland (Figure 20). The last horse of the Lesser Himalayan duplex began to form at the beginning of this period. Due to the movement over a ramp in the MBT, the

Ramgarh and MCT thrust sheet were folded into a narrow syncline. About 75 km of shortening is estimated to have been caused by the MBT sliding and ~15km by movement of the last duplex horse (Figure 20).

From ~6Ma to ~5Ma, the middle Siwalik formation was deposited as the Lesser

Himalayan imbricate zone developed (Table 3). The southern portion of the Lesser

Himalaya was deformed into a syncline in the south and an anticline in the north because of a ramp developed beneath this region (number 3 of the Figure 18). ~40 km shortening is estimated to occur in the Lesser Himalayan imbricate zone (Figure 20).

From 5 Ma to recent, the upper Siwalik unit was deposited as the Subhimalyan thrust system developed at ~5Ma; the MBT started emplacing the hanging wall rocks on top of the Siwalik group. The Main Frontal thrust had developed since ~4 Ma. According

to the restoration, ~35 km shortening has been absorbed by displacement on the MFT

(Figure 20).

According to the restoration of the regional cross-section, about 515km of shortening was accommodated by the major faults (RT, MBT and MFT) in the research area (Figure 19 and 20). This number does not include the shortening absorbed by the

MCT. The Lesser Himalayan duplex plays an important role in moving early foreland basin strata to structural higher levels. The shortening estimation of the duplex is 125km which is 35 km greater than the shortening estimated in the Simikot cross-section

(Robinson,2008) (location of the cross-section see figure 2).

3.3 COMPARISON WITH PUBLISHED CROSS-SECTIONS

To better understand the variation of the structural patterns along strike of the thrust belt and find the differences of shortening accommodated by major faults between my study and previous ones, it is necessary to compare the cross-sections in this study with published cross-sections (Figures 21, 22 and 23). We use four cross-sections from this and previous research. The DeCelles et al. (2001) cross-section is near the Nepal and

India political boundary. The Robinson (2008) cross-section is east of DeCelles et al.

(2001) and runs through Simikot. The cross-section in this research is east to Robinson

(2008) and runs through Jarjakot. The Yin (2006) cross-section is in east Nepal and runs through the Kathmandu area. The locations of these cross-sections can be seen in Figure

The differences and similarities between the regional cross-section developed in this study and previous studies will be introduced from south (left in figures) to north

(right in figures) (Figures 18, 21, 22 and 23). 45

(1) North of the MBT surface trace, cross-sections in this study and Robinson (2008)

interpret a footwall ramp beneath the MBT hanging wall which controls the geometry

of the syncline and anticline in the southern Lesser Himalaya. In the DeCelles et al.

(2001) and Yin (2006) cross-sections, there is no ramp in that structural position

(Figures 18, 21 and 23);

Figure 21. Talkot cross-section of DeCelles et al., (2001), location see Fiugre 2

Figure 22. Simikot cross-section of Robinson (2008), location see Figure 2.

Figure 23. Kathmandu cross-section of Yin (2006), location see Figure 2. (2) In all cross-sections, the klippe are carried by Ramgarh thrust sheet and all the klippe

have a synformal geometry. Folding of the RT and MCT strata is caused by MBT

hanging wall displacement over a ramp in the south and the stacking of Lesser

Himalayan duplex horses in the north;

(3) The widths of the klippe are different in these cross-sections, the klippe in this study

is the narrowest of four cross-sections and this may be due to more shortening along

the MBT in this region. The depth of the klippe in my study is deeper than that of

Robinson (2008). We interpret this to be caused by the difference between the

location of the MBT ramp (Number 6 in Figure 18). In this study the ramp is south of

the klippe, however, in Robinson (2008), the ramp is beneath the klippe (Figures 18

and 22).

(4) In all of these cross-sections, except Robinson (2008), duplex horses developed north

of the klippe. In Robinson’s (2008) cross-section, three duplex horses lie beneath the

klippe .

(5) Outcrops of the early foreland basin strata can be seen in this study, DeCelles et al.

(2001) and possibly in Yin’s (2006) cross-sections but not in Robinson’s (2008). This

may be the result of differences in shortening accommodated by the duplex and

vertical stacking of the duplex horses between these study areas.

3.4 PREDICTION

Figure 24 is a tectonostratigraphic map of Nepal showing the major klippe in

Nepal including the Dadeldhura/Karnali klippe (DKK) in far west, the Jajarkot klippe (JK) in west and the Kathmandu klippe (KK) in the east. Foreland basin strata were documented to crop out north of the DKK (DeCelles et al., 2001) and north of the JK

(this study). According to the model proposed in this study, the forming of klippe is partially caused by the development of the duplex in the north and the duplex horses bring the foreland basin strata to a structurally higher position (Figure 18). We predict that foreland basin strata may crop out north of the Kathmandu klippe where the foreland basin strata were not observed.

crops of the foreland basin strata foreland basin of the crops

Tectonostratigraphic map of Nepal (Robinson, 2008) map of Nepal (Robinson, Tectonostratigraphic

icate and possible out observed

Figure

Red square ind

4. CONCLUSIONS

1. A ~ 75 km-long MBT hanging wall flat extends northward from its surface trace

to the southern margin of the Lesser Himalayan duplex; about 125 km of

shortening is accommodated by the duplex and a total ~225km shortening is

accommodated by the RT.

2. The geometry of the Jarjarkot klippe is narrower and structurally deeper than

klippe to the west. The geometry of the northern flank of the klippe results from

stacking of the duplex horses, while the geometry of the southern flank results

from slip over a ramp in the footwall of the MBT;

3. The early foreland basin strata in the window are modeled to have originated

craton-wards of the thrust belt, and subsequently overridden by the Ramgarh and

MCT thrust sheets;

4. Exposure of the early foreland basin strata results from growth of the duplex

which brought them to a structural level higher than most parts of the thrust belt;

5. The model predicts that foreland basin strata are likely to be exposed wherever

duplexes exist throughout the Himalayan thrust belt, e.g. the region north of the

Kathmandu klippe.

5. APPENDIX

Several abbreviations were used in this study. They appear in the text and figures.

Below is a table shows all the abbreviations used in this study.

Abbreviations in this study Number Abbreviation Definition Type 1 MFT Main Frontal thrust Fault 2 MBT Main Boundary thrust Fault 3 RT Ramgarh thrust Fault 4 MCT Main Central thrust Fault 5 STD(S) South Tibetan detachment (system) Fault 6 DT Dadeldhura thrust Fault 7 TH Tethyan (or Tibetan) Himalaya Mega-sequence 8 GH Greater Himalaya Mega-sequence 9 LH Lesser Himalaya Mega-sequence 10 SH Subhimalaya Mega-sequence 11 DKK Dadeldhura/Karnali klippe Klippe 12 JK Jajarkot klippe klippe

The following data are from Dr. Michael Murphy’s 2010 field notebook. The following table shows the field measurement data, which include station name, station location and elevation, description note, Plane data (including bedding, foliation, fault surface, shear zone, S-C foliation) and Line data (including lineation, striation, fold axis)

Here are the abbreviations in the table:

Fol=Foliation, FA=Fold Axis, L=Lineation, FR=Fracture, FS=Fault Surface, S

Fol=S Foliation, C Fol=C Foliation.

6. REFERENCES

Baud, A., Gaetani, M., Garzanti, E., Fois, E., Nicora, A., Tintori, A., 1984, Geological observations in southeastern Zanskar and adjacent Lahul area (northwestern Himalaya): Eclogae Geologicae Helvetiae, v. 77, p. 171–197.

Bilham, R., Larson, K., Freymueller, J., Jouanne, F., Le Fort, P., Leturmy, J. L. Mugnier et al., 1997, GPS measurements of present-day convergence across the Nepal Himalaya: Nature, v. 386, p. 61-64.

Bollinger, L., and Janots, E., 2006, Evidence for Mio-Pliocene retrograde monazite in the Lesser Himalaya, far western Nepal: European Journal of Mineralogy, v. 18, p. 289–297.

Burbank, D.W., Beck, R.A. and Mulder, T., 1996, The Himalayan foreland basin, In: Yin, A., Harrison, T.M.: The Tectonics of Asia. Cambridge University Press, New York, p. 149– 188.

Burbank, D. W., Blythe, A. E., Putkonen, J., Pratt-Sitaula, B. G., 2003, Decoupling of erosion and precipitation in the Himalayas: Nature, v. 426, p. 652-655.

Colchen, M.,Mascle, G. and Van Haver, T., 1986, Some aspects of collision tectonics in the Indus Suture Zone, Ladakh, Geological Society, London, Special Publications, v. 19, p. 173-184.

DeCelles, P. G., and Giles, K. A., 1996, Foreland basin systems: Basin Research, v.8, p. 105-123.

DeCelles,P.G., Gehrels, G.E., Quade, J., Kapp, P.A., Ojha, T.P., and Upreti, B.N., 1998, Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal: Geological Society of America Bulletin, v.110, p.2-21.

DeCelles, P. G., Gehrels, G. E., Quade, J., LaReau, B., and Spurlin, M., 2000, Tectonic implications of U-Pb zircon ages of the Himalayan orogenic belt in Nepal: Science, v. 288, p. 497-499.

DeCelles, P. G., Robinson, D. M., Quade, T. P., Ojha, Carmala N. Garzione, C. N., Copland, P., and Upreti, B. N., 2001, Stratigraphy, structure and tectonic evolustion of the Himalayan fold-thrust belt in western Nepal: Tectonics, v.20, p.487-509.

DeCelles, P.G., Gehrels, G.E., Najman, Y., Martin, A.J., Carter, A. and Garzanti, E., 2004, Detrital geochronology and geochemistry of Cretaceous–Early Miocene strata of Nepal: implications for timing and diachroneity of initial Himalayan orogenesis: Earth and Planetary Science Letters, v. 227, p. 313– 330.

Demets, C., Gordon, R. G., Argus, D. F. and Stein, S., 1994, Effect of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions: Geophysical Research Letters, v. 21, p. 2191–2194.

Dewey, J. F., Cande, S. and Pitman, W. C., 1989, Tectonic evolution of the India–Eurasia collision zone: Eclogae Geologicae Helvetiae, v. 82, p. 717– 734.

Frank, W., and Fuchs, G. R., 1970, Geological investigations in west Nepal and their significance for the geology of the Himalayas, Geologische Rundschau, v. 59, p. 552-580.

Gaetani, M., and Garzanti, E., 1991, Multicyclic history of the northern India continental margin (northwestern Himalaya): American Association of Petroleum of Geologists Bulletin, v. 75, p. 1427– 1446.

Gansser, A., 1964, Geology of the Himalayas: Interscience Pubisher (London), 289 pp.

Heim, A. and Gansser, A., 1939, Central Himalaya; geological observations of the Swiss expedition 1936, Schweizer. Naturf. Ges., Denksch., p. 1 –246.

Larson, K.M., Bürgmann, R., Bilham, R. and Freymueller, J.T., 1999.Kinematics of the India–Eurasia collision zone from GPS measurements: Journal of Geophysical Research, v. 104, p. 1077– 1093.

Lavé, J. and Avouac, J.P., 2000, Active folding of fluvial terraces across the Siwaliks Hills, Himalayas of central Nepal: Journal of Geophysical Research, v. 105, p. 5735– 5770.

LeFort, P., 1975, Himalayas: The collided range. Present knowledge of the continental arc: American Journal of Science, v. 275, p. 1– 44.

Liu, G. and Einsele, G., 1994. Sedimentary history of the Tethyan basin in the Tibetan Himalayas: Geologische Rundschau, v. 82, p. 32– 61.

Liu, G. and Einsele, G., 1999, Jurassic sedimentary facies and paleogeography of the former Indian passive margin in southern Tibet, In: Macfarlane, A., Sorkhabi, R.B., Quade, J. (Eds.), Himalaya and Tibet: Mountain Roots to Mountain Tops: Geological Society of America Special Papers, v. 328, p. 75–108.

Long, S., McQuarrie, N., Tobgay, T., and Grujic, D., 2011, Geometry and crustal shortening of the Himalaya fold-thrust belt, eastern and central Bhutan, Geological Society of America Bulletin, v. 123, p. 1427- 1447.

Martin, A.J., DeCelles, P.G., Gehrels, P.G., Patchett, P.J.,and Isachsen, C., 2005, Isotopic and structural constraints on the location of the Main Central thrust in the Annapurna Range, central Nepal Himalaya: Geological Society of America Bulletin, v. 117, p. 926– 944.

Mugnier, J.L., Mascle, G., and Faucher, T., 1993, Structure of the Siwaliks of western Nepal: An intracontinental accretionary prism: International Geological Review, v.35, p. 32-47.

Mugnier, J.L., Leturmy, P., Mascle, G., Huyghe, P., Chalaron, E., Vidal, G., Husson, L., and Delcaillau, B., 1999, The Siwaliks of western Nepal I, geometry and kinematics: Journal of Asian Earth Sciences, v.17, p.629-642.

Murphy, M.A. and Yin, A., 2003, Structural evolution and sequence of thrusting in the Tethyan fold-thrust belt and Indus-Yalu suture zone, southwest Tibet: Geological Society of America Bulletin, v.115, p. 21– 34.

Najman, Y. and Garzanti, E., 2000, Reconstructing early Himalayan tectonic evolution and paleogeography from Tertiary foreland basin sedimentary rocks, northern India: Geological Society of America Bulletin, v. 112, p. 435–449.

Najman, Y., Carter, A., Oliver, G., and Garzanti, E., 2005, Provenance of Eocene foreland basin sediments, Nepal: Constraints to the timing and diachroneity of early Himalayan orogenesis: Geology, v. 33, p. 309-312.

Ojha, T. P., Butler, R. F., Quade, J., DeCelles, P. G., Richards, D. and Upreti, B. N., 2000, Magnetic polarity stratigraphy of the Neogene Siwalik Group at Khutia Khola far western Nepal, Geological Society of America Bulletin, v. 112, p. 424-434.

Ojha, T. P., Butler, R. F., DeCelles, P. G., and Quade, J., 2009, Magnetic polarity stratigraphy of the Neogene foreland basin deposits of Nepal: Basin Research, v. 21, p. 61–90.

Parrish, R. R., and Hodges, K. V., 1996, Isotopic constraints on the age and provenance of the Lesser and Greater Himalayan sequences, Nepalese Himalaya: Geological Society of America Bulletin, v. 108, p. 904–91.

Patriat, P. and Achache, J., 1984, India–Eurasia collision chronology has implications for crustal shortening and driving mechanism of plates: Nature v. 311, p. 615– 621.

Paudel, L.P., Arita, K, 1998, Geolog, structure and metamorphism of the Lesser Himalayan metasedimentary sequence in Pokhara region, western Nepal: Joural of Nepal Geological Society, v.18, p. 97-112.

Pearson, O. N., 2002, Structural evolution of the central Nepal fold-thrust belt and regional tectonic and structural significance of the Ramgarh Thrust: Ph.D. thesis University of Arizona, Tucson.

Pȇcher, A., 1989, The metamorphism in the central Himalaya: Journal of Metamorphic Geology, v. 7, p. 31–41.

Powell, C. M. and Conaghan, P. J. 1973, Plate tectonics and the Himalayas: Earth and Planetary Science Letters, v. 20, p. 1-12.

Quade, J., Cater, J. M. L., Ojha, T. P., Adam, J., and Harrison, T. M., 1995, Late Miocene environmental change in Nepal and the northern Indian subcontinent: Stable isotopic evidence from paleosols: Geological Society of America Bulletin, v. 107, p. 1381–1397.

Quade, J., Roe, L., DeCelles, P.G., and Ojha, T.P., 1997, The late Neogene 87Sr/86Sr record of lowland Himalayan Rivers: Science, v. 276, p. 1828–1831.

Robinson, D. M., DeCelles, P. G., Patchett, P. J. and Garzione, C. N., 2001, The kinematic evolution of the Nepalese Himalaya interpreted from Nd isotopes: Earth and Planetary Science Letters, v. 19, p. 507-512.

Robinson, D. M., DeCelles, P. G., Garzione, C. N., Pearson, O. N., Harrison, T. M., and Catlos, E. J., 2003, Kinematic model for the Main Central thrust in Nepal, Geology, v. 31, p. 359-362.

Robinson, D. M., DeCelles P. G., and Copeland, P., 2006, Tectonic evolution of the Himalayan thrust belt in western Nepal: Implications for channel flow models: Geological Society of America Bulletin, v. 118, p. 865-885.

Robinson, D. M., 2008, Forward modeling the kinematic sequence of the central Himalayan thrust belt, western Nepal: Geosphere, v. 4(5), p. 785-801.

Robinson D. M. and McQuarrie N., 2012, Pulsed deformation and variable slip rates within the central Himalayan thrust belt: Lithosphere, v.4, p.449-464.

Sakai, H., 1983, Geology of the Tansen Group of the Lesser Himalaya in Nepal: Memoir of Faculty of Sciences Kyushu University, v. 25, p. 27– 74.

Schelling, D., 1992, The tectonostratigraphy and structure of the eastern Nepal Himalaya: Tectonics, v.11, p.925– 943.

Searle, M.P. and Godin, L., 2003, The South Tibetan Detachment system and the Manaslu leucogranite: a structural re-interpretation and restoration of the Annapurna–Manaslu Himalaya, Nepal: Journal of Geology, v. 111, p. 505– 523.

Shrestha, S. B., Shrestha, J. N., Sharma, S. R., 1987, Geological map of mid-western Nepal (scale: 1250, 000): Department of Mines and Geology, Kathmandu.

Srivastava, P., and Mitra, G., 1994, Thrust geometries and deep structure of the outer and lesser Himalaya, Kumaon and Garhwal (India): Implication for evolution of the Himalayan fold-and-thrust belt: Tectonics, v. 13, p. 89–109.

Upreti, B.N., 1999, An overview of the stratigraphy and tectonics of the Nepal Himalaya: Journal of Asian Earth Science, v. 17, p. 577–606.

Valdiya, K.S., 1980, Geology of the Kumaon Lesser Himalaya: Wadia Institute of Himalaya, Dehra Dun, India, p. 291.

Yin A., 2006, Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation: Earth-Science Reviews, v. 76, p.1– 131.