Active Tectonics of the Kashmir Himalaya (NW India) and Earthquake Potential on Folds, Out-Of-Sequence Thrusts, and Duplexes

Home , Dhauladhar, Tectonostratigraphy, Western Himalaya

AN ABSTRACT OF THE DISSERTATION OF

Yann G. Gavillot for the degree of Doctor of Philosophy in Geology presented on September 29, 2014.

Title: Active Tectonics of the Kashmir Himalaya (NW India) and Earthquake Potential on Folds, Out-of-Sequence Thrusts, and Duplexes

Abstract approved:

______

Andrew J. Meigs

ABSTRACT

Active tectonics of a deformation front constrains the kinematic evolution and structural interaction between the fold-thrust belt and the most-recently accreted foreland basin. At the

Himalayan deformation front, the thrust front is blind, characterized by a broad fold (the

Suruin-Mastgarh anticline (SMA)), and displays no emergent faults cutting the southern limb.

Dated deformed terraces on the Ujh River constrain the structural style of deformation and shortening rates. Six terraces are recognized, and three yield OSL ages of 53 ka, 33 ka, and 0.4 ka. Terrace restorations through long profiles reveal a deformation pattern characterized by uniform uplift across the anticlinal axis and northern limb, and variable uplift due to rotation of the southern limb. Offset terraces occur between the fold trace and the northern limb.

Bedrock dips, stratigraphic thicknesses, and cross sections suggest that a SW-directed duplex at depth drives uniform uplift in the north, and a NE directed wedge thrust drives variable tilt in the south. Localized faulting at the fold axis introduces asymmetrical fold geometry.

Folding of the oldest dated terrace suggests rock uplift rates across the SMA range between

1.8 and 2.0 mm/yr. Assuming a 25-dip for the duplex ramp on the basis of dip data constraints, the shortening rate across the SMA ranges between 3.8-5.4 mm/yr or ~4.6 mm/yr

since ~53 ka. Of that rate, 2.7-1.1 mm/yr is likely absorbed by faulting at the fold axis. In comparison, long-term bedrock shortening rates are consistent with our data of Pleistocene shortening rates. Cross sections at the Ujh River and Chenab reentrant transects indicate 6.5 mm/yr and ~4 mm/yr, respectively, using an onset age of thrusting of ~ 3 Ma.

Within the Sub-Himalaya deformation belt, new geomorphic mapping demonstrates that active emergent thrust faulting occurs north of the deformation thrust front in the Kashmir

Himalaya. The Riasi thrust (RT) comprises the southeastern strand of a ~250 km-long seismically active fault system in Pakistan and Indian Kashmir. Multiple fault strands with

Quaternary activity characterize the fault zone near the Chenab River. Vertical separation of

~272 m of Pleistocene fluvial deposits marks the main strand of the RT, or the Main Riasi thrust (MRT). A shortening rate of 10.8-2.8 mm/yr characterizes a ~91-39 ka time interval for the MRT at this location. A fault scarp and offset Holocene terrace mark the southernmost and frontal splay of the RT fault zone, called here the Frontal Riasi thrust (FRT). Differential uplift

(21.6±1m) of a Holocene terrace yields a preferred shortening rate of 8.8-4.4 mm/yr. Contact relationships in a trench across the FRT date the last surface rupture of the RT fault zone at

~4,500 yrs ago. Average shortening rates since ~100 ka across the MRT and the FRT range from 10.8 mm/yr to 4.4 mm/yr (7.6±3.3 mm/yr), consistent with long-term bedrock shortening of ~10-14 mm/yr since 5 Ma.

Given that Himalayan-Indian convergence is ~11-18 mm/yr, the sum of the intermediate-term shortening rates for the RT (10.8-4.4 mm/yr) and the deformation front (5.4-

3.8 mm/yr), accounts for most of the total geological shortening in Kashmir. Using average rates, internal faults (RT) absorb 50% of the Himalayan shortening, comparatively more than the deformation front (~30%) in Kashmir. Similarities in the structural setting between the

Riasi thrust and the Balakot-Bagh fault, including millennia recurrence, suggest the potential of Mw 7 to 8 earthquakes on the RT fault zone, similar to the source of the Mw 7.6 2005

Kashmir earthquake on the Balakot-Bagh fault segment. Lower rate at the deformation front

may suggest an even longer recurrence interval, but nonetheless potentially devastating with predicted Mw of 7.5-7.6 along the 180-250 km-long structure.

Apatite and zircon (U-Th)/He cooling ages are used to quantify the pattern of distributed deformation and overall thrust-belt kinematics at longer timescales across the

Kashmir Himalayas. Apatite (U-Th)/He (AHe) cooling ages for the foreland molasse sediments are consistently younger than the sediment age, indicating that ages of Sub-

Himalayan belt samples are reset. We interpret regional pattern of young AHe cooling ages

(<3 Ma) in the foreland with peak cooling at 1.9-3.2 Ma to represent rapid coeval fault-related exhumation on multiple structures across the Sub-Himalaya. In the hinterland front ranges of the Pir Panjal, the MBT and MCT thrusts are characterized by older cooling (>4.7 Ma). For zircon (U-Th)/He (ZHe), probability density plots of samples from both detrital cooling ages in the foreland and reset cooling ages in the hinterland show a pronounced spike in cooling between ~16 and 20 Ma, a period where MCT motion is well documented throughout the

Himalaya. Estimates of exhumation rates for the sum of Sub-Himalayan structures are higher

(2.8-2.2 mm/yr) than across the MCT/MBT (<1 mm/yr) in the Pir Panjal Range. Cooling patterns across the Kashmir Himalayas do not correlate with monsoon precipitation gradients, suggesting climate forcing is decoupled from erosion and exhumation. Distributed rather than localized forward-propagating deformation characterizes fault-related exhumation for the orogenic wedge development in the Sub-Himalayan belt since at least ~2-3 Ma. In the hinterland, coeval young cooling ages (< 3 Ma) and high exhumation rates (2.8-1.3 mm/yr) across the Kishtwar Window, >100 km north of the deformation front, suggest tectonic-driven rapid exhumation across the orogenic wedge coincides with localities of predicted changes in ramp geometry and/or active orogenic growth. We attribute a pattern of distributed deformation and coeval faulting across the Sub-Himalaya to the effects of pre-existing basin architecture in Kashmir.

©Copyright by Yann G. Gavillot

September 29, 2014

Active Tectonics of the Kashmir Himalaya (NW India) and Earthquake Potential on

Folds, Out-of-Sequence Thrusts, and Duplexes

Yann G. Gavillot

A DISSERTATION

Submitted to

Oregon State University

in partial fulfillment of

the requirements for the

degree of

Doctor of Philosophy

Presented September 29, 2014

Commencement June 2015

Doctor of Philosophy dissertation of Yann G. Gavillot presented on September 29, 2014.

APPROVED:

Major Professor, representing Geology

Dean of the College of Earth, Ocean, and Atmospheric Sciences

Dean of the Graduate School

I understand that my dissertation will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my dissertation to any reader upon request.

Yann G. Gavillot, Author

ACKNOWLEDGEMENTS

I owe a great deal of thanks to my advisor, Andrew Meigs, who has taken me in for

PhD studies at Oregon State University. I feel very fortunate to be able to work with him together for the many years I have been here, and I learned a tremendous amount from him.

Andrew Meigs has been a great mentor and collaborator, but also a great friend, as he would often say “I always have time for you Yann”. He has invested hugely in refining model interpretations, and reviewing my writing, which I am most grateful for. I would like to also give sincere gratitude to all other members of my committee: Robert Yeats, Jay Noller, John

Nabelek, and my GCR, Ben Mason. Robert Yeats has been a huge source of wisdom in the research project. His collaboration and extensive research network in India were instrumental to the success in getting the project off the ground. I have also greatly enjoyed interacting with both John Nabelek and Jay Noller during graduate courses as outside conversations. They have provided me with many constructive feedbacks and improvements to my thesis work.

I would like to thank NSF for funding and making this research possible. Although we have struggled to obtain renewed funding from NSF, I am thankful for many supplementary grants and fellowships that were instrumental in paying for analytical lab costs. To name a few, funding and support came from GSA, AAPG, Daniel Stockli and the IGL laboratory at

Kansas University, Marc Caffee and PRIME LAB, Department of Forestry at OSU, CEOAS, and Andrew Meigs.

I would like to thank the many collaborators and field assistants that have contributed to the research in the field. Professor Manzoor Malik at Jammu University for making the project run smoothly and making it possible to access field sites, which were at times logistically complicated with security issues across Kashmir. During the first field season,

Chris Madden, Michael Karicher, and Aaron Hebeler assisted in data collection, and provided much needed camaraderie in the field. I have great memories of the many days spend in our hotel, gossiping about hotel staff, and exploring the streets of Riasi. During my second field season, I had received help from two field assistants Vikram Sharma and Idrees Bhat. Both

provided much needed help in data collection and logistics, and I am thankful they were there to rescue me when I was repeatedly “interrogated” or “even arrested” by the police and army.

Who would blame them, a strange looking person, walking around with a giant GSP antenna, within a few kilometers from the Pakistan border? I also, thank Prof. Bhat for logistical support in Srinagar, and for the great hospitability I received from Shabir and his family.

I would also like to thank the many people involved in helping in the laboratory analyses: Tammy Rittenour and Michelle Suma at Utah State OSL age analysis lab, Daniel

Stockli and his army of students, including Charlie Verdel for running apatite and zircon (U-

Th)/He age analyses, and the OSU crew in the cosmogenic lab, with first Shawn Marcott, followed by Josh Cuzzone and Nilo Bill.

During the many years of my PhD studies, I especially enjoyed interacting with the many talented and patient graduate students either while teaching classes, at field camps, or just talking geology. I would like to thank Chris Madden and Lalo Guerrero, for being awesome office mates, as we were for a while the only students in neotectonics, who both were great colleagues, and have helped me in so many ways. I would like to single out Chris

Madden, with whom I interacted closely during our research in Kashmir, and who became a great friend.

Last but not least, I would like to thank my family. My wife Joanna who has given me unbelievable support and patience, in order for me to finish my PhD. Saying thank you does not describe how much she has helped me in making all of this happen during the many years of my studies and especially in the last couple of months of intense thesis writing. Finally, I would like to thank my mom; without whom my graduate studies would not be possible. Her love and support gave me direction in growing up and reach my goals. I only wish my father, who passed away many years ago, could be here to see me succeed.

TABLE OF CONTENTS

Page

1 Active Tectonics of the Kashmir Himalaya (NW India) and Earthquake Potential on Folds, Out-of-Sequence Thrusts, and Duplexes: Introduction...... 1

1.1 Motivation...... 2

1.2 Research questions...... 2

1.3 Synopsis of thesis work...... 4

1.4 References...... 6

2 Late Cenozoic – Recent exhumation history of the Kashmir Himalaya from the Siwalik Murree belt to the High Himalaya using (U-Th/He) thermochronometry...... 9

2.1 Abstract...... 10

2.2 Introduction...... 11

2.3 Tectonic setting...... 13

2.4 Structure of the SMA...... 16

2.4.1 SMA geometry at the surface...... 16 2.4.2 Depth to décollement...... 18 2.4.3 SMA deep structure...... 19 2.4.4 Structural restoration and long-term shortening rate (105 yr timescale)...... 21

2.5 River terraces...... 22

2.5.1 Ujh River...... 22 2.5.2 Terrace mapping methodology...... 23 2.5.3 Geomorphic map relations of Ujh River terraces...... 24 2.5.4 Terraces...... 24 2.5.5 Terrace geochronology...... 25 2.5.6 Terrace ages...... 26 2.5.7 Deformation pattern – SMA...... 28 2.5.8 Terrace uplift rates...... 31

2.6 Discussion…………………………………………………………….…...…32

2.6.1 Terraces constraints on bedrock structural interpretation…………….….32 2.6.2 Shortening rates using terraces (103 yr timescale) – Focus on T1.95...... 34 2.6.3 Comparison with long-term and geodetic convergence rates...... 35 2.6.4 Earthquake potential for the deformation front in Kashmir...... 36 2.6.5 What controls an active blind thrust front in the Kashmir...... 37

TABLE OF CONTENTS (Continued) Page

2.7 Conclusion ...... 39

2.8 References...... 41

2.9 Appendix A...... 50

3 Shortening rate and Holocene surface rupture on the Riasi thrust in the Kashmir Himalaya: Active thrusting within the Northwest Himalayan orogenic wedge...... 70

3.1 Abstract...... 71

3.2 Introduction...... 72

3.3 Geological settings and long term distribution of Himalayan shortening...... 73

3.4 Geochronologic Methods...... 76

3.5 Riasi thrust fault zone- nomenclature and structure...... 77

3.6 Quaternary stratigraphic units and geomorphic map relations...... 79

3.6.1 Chenab River conglomerate (Cgm) – Hinterland-sourced conglomerate...... 80 3.6.2 Quaternary conglomerate (Qgm) – Sirban Limestone-sourced conglomerate...... 80 3.6.3 Terraces...... 81

3.7 Geomorphologic and stratigraphic units – age constraints...... 83

3.7.1 Age of hinterland-sourced (Cgm) and Sirban Limestone-sourced (Qgm) conglomerate...... 83 3.7.2 Terrace ages...... 85

3.8 Riasi fault zone structural relationships...... 89

3.8.1 Main Riasi thrust (MRT)...... 90 3.8.2 Frontal Riasi thrust (FRT)...... 91 3.8.3 Structural summary...... 93

3.9 Quaternary shortening and paleoseismology...... 93

3.9.1 MRT shortening rate...... 94 3.9.2 FRT shortening rate...... 95 3.9.3 FRT paleoseismology ………………...... 95 3.9.4 Riasi thrust fault zone (MRT + FRT)...... 96

3.10 Regional tectonics and discussion...... 97

3.10.1 Long-term versus Pleistocene-Holocene shortening rates – How high must they be?...... 99

TABLE OF CONTENTS (Continued) Page

3.10.2 Comparisons of plate, geodetic, and geologic convergence rates...... 101 3.10.3 Earthquake potential on the RT...... 103 3.10.4 Constraints of the seismogenic zone in Kashmir Himalaya...... 104

3.11 Conclusion...... 104

3.12 References...... 107

4 Late Cenozoic – Recent exhumation history of the Kashmir Himalaya from the Siwalik-Murree belt to the High Himalaya using (U-Th/He) thermochronometry...... 137

4.1 Abstract ...... 138

4.2 Introduction...... 139

4.3 Geologic setting...... 141

4.4 Architecture of the Kashmir Himalaya...... 143

4.5 Apatite and zircon (U-Th)He thermochronometry...... 145

4.5.1 Analytical Methods...... 145 4.5.2 Sampling Methods...... 147 4.5.3 Paleo-temperature estimates...... 149

4.6 Results...... 150

4.6.1 AHe cooling ages...... 150 4.6.2 ZHe cooling ages...... 153 4.6.3 AFT cooling ages comparison...... 154

4.7 Exhumation rates...... 155

4.8 Discussion...... 156

4.8.1 Exhumation pattern in the Sub-Himalaya: evidence of distributed deformation at 105 yrs timescales ...... 157 4.8.2 Constraints on age of thrusting for Sub-Himalayan structures...... 157 4.8.3 Exhumation pattern in the hinterland...... 158 4.8.4 Kinematics of the Kashmir Himalaya orogen ...... 160 4.8.5 Climate forcing?...... 161 4.8.6 Structural control and wedge behavior in the Kashmir Himalaya orogeny...... 163

4.9 Conclusion...... 165

4.10 References...... 167

5 Conclusion...... 195

6 References...... 198

LIST OF FIGURES

Figure Page

2.1 Regional structure map of the Himalayan thrust front in northwest India...... 53

2.2 Regional geologic map of the Kashmir Himalayas...... 54

2.3 Structural cross sections of three transects across the SMA and Sub- Himalayan belt in Kashmir at the Chenab reentrant (line A-A’), Tawi River (line B-B’), and the Ujh River (line C-C’)...... 55

2.4 Geoligcal map of the study area (see box for location in Figure 2.2)...... 56

2.5 Balanced (top) and restored (bottom) structural cross section along a NE- SW transect, in the surrounding area of the Ujh RIver (line C-C’; see Figures 2.2 and 2.4 for location)...... 57

2.6 DEM image of the same area as shown in Figure 2.5 for the Ujh River. DEM imagery is 90X90 m resolution...... 58

2.7a Topographic and stream gradient profiles along the Ujh River transect (line C-C’ in Figure (see Figure 2.2 for location) at matching horizontal and vertical scales. Traces of the MKT and SMA fold axis are indicated...... 59

2.7b Same profiles with a vertical exaggeration X10, showing variation in stream gradients slopes...... 59

2.8 Map of terraces along the Ujh River focused on the forelimb section and anticlinal axis of the SMA...... 60

2.9 Map of terraces along the Ujh River focused for the backlimb section of the SMA...... 61

2.10 Field photographs of various terrace expsoures and stratigraphy along the Ujh RIver...... 62

2.11a Terrace long profiles and stream profile along the Ujh River crossing the SMA...... 63

2.11b Structural cross section with the same horizontal scale is shown for comparison...... 63

2.12 Progressive vector restoration models at different time intervals (T1-T6) for surveyed terraces and river profiles along the Ujh River...... 64

2.13 Summary of deformed terraces T1 and T2c at three different sections across the SMA, in the forelimb, the zone of maximum uplift at the fold axis, and the backlimb...... 65

2.14 Different cross section interpretations of the non-emergent thrusts beneath the folds based on four proposed structural models, with their associated surface uplift pattern...... 66

LIST OF FIGURES (Continued)

Figure Page

3.1 Regional structure map of the Himalayan thrust front in northwest India...... 115

3.2 Regional geologic map of the Kashmir Himalayas...... 116

3.3 Stratigraphic succession of study area for the Riasi thrust, in the state of Jammu & Kashmir, compiled from nearby studies of Karunakaran and Ranga Rao...... 117

3.4 Simplified geologic map near the town of Riasi, showing major structures and stratigraphy...... 118

3.5a Panoramic photograph looking west of the MRT and RFT...... 119

3.5b Photograph looking northwest at the MRT dipping 45-50°NE that places Sirban Limestone Formation over ~25°NE tilted Cgm (person for scale)...... 119

3.5c View looking southeast at the RFT that deforms Qgm and offsets the overlying Dugalla terrace (T3)...... 119

3.6 Quaternary mapping of MRT near the Bidda Terrace and Joytipuram Canyon sites, and of the FRT near the Dugalla Terrace site (see box in Figure 3.4 for location)...... 120

3.7a Cross section through the Main Riasi thrust (RT) and the Frontal Riasi thrust (FRT), along X-X’ transect from Figures 3.4 and 3.6...... 121

3.7b Detailed view of the cross section from Figure 8a of the Qgm and terraces chronological sequences at the FRT...... 121

3.8 Stratigraphic section of the Bidda Terrace (T7) underlain by ~20 m of Chenab River Conglomerate (Cgm)...... 122

3.9 Depth profile samples from T7 (Bidda Terrace) soil pit. The uppermost sample has a lower 10Be concentration than the sample at 50 cm...... 123

3.10a Schematic NE-SW cross-section diagram simplified from Figure 3.8...... 124

3.10b Age correlation diagram for the same NE-SW cross section depicting sequential ages of sediment deposits (Cgm, Qgm) and terraces (T7, T5-T3)...... 124

3.11 Soil profiles diagram across FRT of surface T3 (Dugalla Terrace) for pit 1- 4 (see Figure 3.6 for soil pit locations)…………………………………………...…125

3.12a Topographic profile across geomorphic surface T3 (Dugalla Terrace) offset by the Frontal Riasi thrust (FRT)...... 126

3.12b Photograph of the same surfaces shown in Figure 3.12a looking SE of scarp above FRT...... 126

3.13 Simplified paleosiesmic trench log for the Riasi Frontal thrust………………….…127

3.14 Regional cross section of the Kashmir Himalayas. See Figure 3.1 for transect location...... 128

LIST OF FIGURES (Continued)

Figure Page

3.15 Interpretative model for the sequences of deformation for the offset Bidda Terrace and underlying Cgm unit...... 129

Chapter 3: Appendix B: Supplemental info on cross section interpretation………...... 136

4.1 Regional fault map in Kashmir and northwest Himalaya...... 177

4.2 Regional geologic map of the Kashmir Himalayas...... 178

4.3 SW-NE regional cross sections across the Kashmir Himalya along the lines A-A’ and B-B”...... 179

4.4 AHe and ZHe regional age distribution map...... 180

4.5 Foreland samples apatite (U-Th)/He ages distribution and relative probability plots using both mean sample and single grain ages collected on local transects...... 181

4.6 Cooling ages versus stratigraphic age of detrital foreland samples for (a) AHe mean sample age data (b) ZHe single grain age data...... 182

4.7 AHe foreland sample ages plotted against distance in the direction perpendicular to respective faults, to examine pattern of age gradients and thrust displacements...... 183

4.8 Hinterland samples apatite (U-Th)/He age distribution and relative probability plots using both (a) mean sample ages and (b) single grain ages collected on the MBT-MCT transect in the Pir Panjal Range...... 184

4.9 Zircon (U-Th)/He ages distribution and relative probability plots for both mean sample ages with standard deviations and single grain ages with analytical errors of (a) hinterland samples across the MBT and MCT thrust sheet in the Pir Panjal Range, (b) foreland samples in the Sub-Himalayan belt...... 185

4.10 AHe ages plotted against elevations for foreland samples collected on vertical transects: (a) SMA, (b) MKT, and (c) combined thrust sheet samples of the RT, MKT, and MT...... 186

4.11 AHe and AFT ages plotted against elevations for hinterland samples collected on vertical transects: (a) MBT/MCT thrust sheets in Pir Panjal Range, (b) Kishtwar Window, (c) High Himalya Range, and (d) Zanskar Shear Zone...... 187

4.12 Regional distribution of calculated AHe and AFT exhumation rates, derived from average estimates between the most recent exhumed samples (Table 4.3) and/or direct vertical transects measurements for selected locations (Figs. 4.11-4.12)...... 188

LIST OF FIGURES (Continued)

Figure Page

4.13 Data compilation of cooling ages, relief, precipitation, and simplified regional cross section from distance north of the mountain front (SMA) to ZSZ. (a) Plot of AHe, ZHe ages (this study), and cited AFT and ZFT ages for the Kishtwar Window and High Himalaya (Kumar et al.,1995). (b) Releif and precipitation swath (100x350 km) along the same transect location as regional cross section available...... 189

LIST OF TABLES

Table Page

2.1 Optically Stimulated Luminescence Age Information...... 67

2.2 Calculating uplift and shortening rates using terraces...... 68

2.3 Shortening rates summary since T1 (~53 ka)...... 69

3.1 OSL age report...... 130

3.2 Shortening rates calculation for the MRT and RFT...... 131

Chapter 3 Appendix A: OSL does information age model interpretations……………...... 132

4.1 Stratigraphic thickness and paleotemperature for Kashmir foreland strata...... 190

4.2 Kashmir Himalaya apatite (U-Th)/He data...... 191

4.3 Kashmir Himalaya Zircon (U-Th)/He data...... 192

4.4 Exhumation rates for AHe and AFT data from transect locations across the Kashmir Himalaya...... 194

CHAPTER 1:

Active Tectonics of the Kashmir Himalaya (NW India) and Earthquake Potential on

Folds, Out-of-Sequence Thrusts, and Duplexes: Introduction

1.1 Motivation

This thesis describes fold-thrust belt deformation across different timescales in the

Kashmir Himalaya, northwest India. The research project, funded by the National Science

Foundation and supplemented by multiple smaller grants (GSA, AAPG, OSU Department of

Forestry, Kansas University Isotopic Geochronologic Lab, PRIME LAB), is part of an international collaboration between Oregon State University, California State University

Northridge, Jammu University, University of Kashmir, and University of Peshawar. The collaborations aim to improve understanding of the active tectonics in the Pakistan and Kashmir

Himalaya; work provides some of the first documentation of patterns of active faulting, deformation rates, and earthquake records. My thesis specifically addresses active strain partitioning and orogenic wedge kinematics in the Indian-disputed territory of Jammu and

Kashmir.

1.2 Research questions

In active fold and thrust belts, the most frontal thrust at the deformation front marks a localized boundary where sediment is accreted into a deforming orogenic wedge (e.g. Thompson,

1981). Thrusting at the deformation front is intimately linked to the overall convergence absorbed by an orogenic wedge because the structure represents the up-dip surface expression of the plate boundary basal décollement (Dahlstrom, 1969; Price, 1981; Boyer and Elliot, 1982; Suppe,

1983). In a setting like the Nepalese Himalaya, it is well known that the deformation front represents the primary active thrust where earthquakes release most of the interseismic strain

(Bilham et al., 1997; Lavé and Avouac, 2000; Mugnier et al., 2004). However, for some fold- thrust belts (i.e. Taiwan, Eastern Precordillera, Kashmir Himalaya), strain can be distributed

3 within the orogenic wedge as documented by surface rupturing earthquakes behind the active deformation front in the wedge (e.g. Ji et al., 2001; Lee et al., 2001; Kaneda et al., 2008;

Rockwell et al., 2013). Thus an emerging image of fold-thrust belts is one where the pattern of active thrusting varies temporally and spatially and can be distributed on two or more active structures at any given time across an orogenic wedge.

Critical taper theory (Davis et al., 1983; Dahlen, 1984) provides an unifying model to explain this pattern of thrusting and associated earthquakes that range from localized to distributed deformation. Thus, information of the structural geometry, deformation rates, and thrust belt kinematics provides insight into the overall orogenic wedge behavior and earthquake potential.

In the Himalaya, major debates exist concerning the pattern of orogen-scale deformation across the width of the fold-thrust belt. For instance, competing interpretations argue that exhumation within the High Himalaya is either a result of out-of-sequence thrusting (e.g. Wobus et al., 2003; 2005; Hodges et al., 2004; Blythe et al., 2007) or, alternatively, a reflection of the

Main Himalayan thrust (MHT) overthrusting a mid-crustal ramp (e.g. Bollinger et al., 2006;

Brewer and Burbank, 2006; Robert et al., 2009; Herman et al., 2010). However, most studies agree that the thrust front along most of the entire length of the Himalaya is a significant structure that accommodates the majority of the active shortening and is the site of large surface-rupturing earthquakes (Bilham et al., 1997; Lavé and Avouac, 2000; Mugnier et al., 2004).

Although the Kashmir Himalaya shares many similarities with the rest of the Himalaya, the work presented here demonstrates that the thrust belt kinematics are clearly different, given evidence of distributed shortening on scales between 103 and 105 yrs.

Some of the key questions I consider within this thesis include: 1) How is the shortening distributed between the deformation front and internal faults recorded from earthquake to

4 geomorphic timescales? Does this pattern of distributed deformation characterize longer timescales (5-10 Ma)? If so, what are the implications for the behavior of the orogenic wedge front? 2) Unlike much of the rest of the Himalaya, the deformation front is not an emergent thrust. Is the blind thrust front indicative of low shortening rates, young age of the fault system, a weak detachment, and/or thickness of the sedimentary pile being deformed? Discriminating between these alternatives has implications for the earthquake potential of the frontal structure of the active thrust belt. 3) Given that thrust belt kinematics are clearly different in Kashmir, what is the primary control of the deformation pattern across the orogenic wedge in Kashmir? Variation in plate motions, surface processes, and pre-existing architecture of the accreted basin provide possible variables to understand the differences.

1.3 Synopsis of thesis work

One of my major goals was to constrain how much Quaternary deformation is partitioned on the different active structures in the Sub-Himalaya. The major structures investigated include the deformation front along the Suruin-Mastgarh anticline (SMA; Ranga Rao and Datta, 1976) presented in Chapter 2, and the Riasi thrust (RT) fault system within the thrust belt presented in

Chapter 3. Utilizing fluvial terraces and Pleistocene-Holocene deposits as strain markers spanning multiple earthquake cycles, I provide shortening rates at timescales of 103 to 104 yrs. These data constrain the earthquake potential and seismic slip deficit for the Jammu&Kashmir sector of the

Himalaya.

Finally, fault activity and thrust belt kinematics at timescales of 105 and longer are investigated using exhumed bedrock cooling ages across the width of the Kashmir Himalayan orogen (Chapter 4). Low-temperature thermochronology provide a bridge to our understanding of the deformation pattern at longer timescales than recorded by active folds and faults. Exhumation

5 and cooling age data samples came from NE-SW transects from the deformation front to the High

Himalaya. Data suggest that the structural geometry and deformation pattern are the primary forcing mechanism of exhumation.

Combining structural interpretation, shortening rates, ages of fault activity, and time- space fault-related exhumation pattern, I provide a new understanding of the thrust belt kinematics in Kashmir. The work presented here demonstrates that distributed deformation related to surface faulting occurs on structures north of the thrust front at multiple timescales, including the earthquake cycle (0-4.5 ka), geomorphology (0-100 ka), and bedrock exhumation

(0-20 Ma). Internal thrusts such as the RT and along-strike fault systems represent the primary active emergent faults accommodating Himalayan shortening in Kashmir. Potential for surface- rupturing earthquakes are greatest for the RT with evidence of Holocene fault scarp. Although the

SMA absorbs less convergence than the RT, cross section and terrace deformation pattern constrain an equal or greater earthquake potential along the >100 km-long structure by blind thrusting, similar to the earthquakes beneath the Coalinga anticline or Wheeler ridge in southern

California (Guzofski et al., 2007; Medwedeff, 1992; Mueller and Suppe, 1997; Keller et al.

1998). Growth of the SMA is interpreted to result from a combination of wedge thrusting, growth on a duplex, and layer-parallel shortening. Together, coeval thrusting on the RT and at the deformation front accounts for most of the overall plate convergence between southern Tibet and

India. Young cooling ages and rapid fault-related exhumation on multiple active thrusts document a pattern of distributed deformation that has been ongoing for the last 2-3 m.y. This persisting long-term pattern of orogenic wedge behavior, as well as the short-term pattern in the earthquake record, may ultimately be controlled by a highly-variable pre-existing Precambrian basin architecture, as has been proposed for the Zagros and Appalachians, involving accreted passive margins, indentations, and reactivation of failed rift structures that play a key role in the long term orogenic development (Thomas 1977; Talbot and Alavi, 1996; McQuarrie, 2004).

1.4 References

Bilham, R., K. Larson, J. Freymueller (1997), GPS measurements of present day convergence across the Nepal Himalaya, Nature, 386, 61–64.

Blythe, A. E., D. W. Burbank, A. Carter, K. Schmidt, and J. Putkonen (2007), Plio-Quaternary exhumation history of the central Nepalese Himalaya: 1. Apatite and zircon fission-track and apatite [U-Th]/He analyses, Tectonics, 26, TC3002, doi:10.1029/2006TC001990.

Bollinger, L., P. Henry, and J. P. Avouac (2006), Mountain building in the Himalaya: Thermal and kinematic model from 20 Ma to present, Earth and Planetary Science Letters, 224, p. 58-71.

Boyer, S. E. and D. Elliott (1982), Thrust systems., AAPG Bulletin, 66, 9, p. 1196-1230.

Brewer, I. D., and D. W. Burbank, 2006, Thermal and kinematic modeling of bedrock and detrital cooling ages in the central Himalaya, Journal of Geophysical Research, 111, p. 1-23.

Dahlstrom, C. D. A. (1969), Balanced cross sections, Canadian Journal of Earth Sciences, 6, p. 743–757, doi:10.1139/e69-069.

Davis, D., J. Suppe, and F.A. Dahlen (1983), Mechanics of fold-and thrust belts and accretionary wedges, Journal of Geophysical Research, 88, p. 1153-1172.

Guzofski, C. A., J. H. Shaw, G. Lin, and P. M. Shearer (2007), Seismically active wedge structure beneath the Coalinga anticline, San Joaquin basin, California, Journal of Geophysical Research, 112, B03S05, doi:10.1029/2006JB004465.

Herman, F., P. Copeland, J. P. Avouac, L. Bollinger, G. Maheo, P. Le Fort, S. Rai, D. Foster, A. Pecher, K. Stuwe, and P. Henry (2010), Exhumation, crustal deformation, and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography, J. Geophys. Res., 115, B06407, doi:10.1029/2008JB006126.

Hodges, K., C. W. Wobus, K. Ruhl, T. Schildgen, and K. Whipple (2004), Quaternary deformation, river steepening, and heavy precipitation at the front of the Higher Himalayan ranges, Earth and Planetary Science Letters, 220, p. 379-389.

Ji, C., D. V. Helmberger, A. T.-R. Song, K.-F. Ma, and D. J. Wald (2001), Slip distribution and tectonic implications of the 1999 Chi-Chi, Taiwan earthquake, Geophysical Research Letters, 28 (23), p. 4379-4382.

Kaneda, H., T. Nakata, H. Tsutsumi, H. Kondo, N. Sugito, Y. Awata, S. S. Akhtar, A. Majid, W. Khattak, A. A. Awan, R. S. Yeats, A. Hussain, M. Ashraf, S. G. Wesnousky, A. B. Kausar (2008), Surface rupture of the 2005 Kashmir, Pakistan, earthquake and its active tectonic implications, Bulletin of the Seismological Society of America, 98, 2, p. 521- 557, doi:10.1785/0120070073.

Keller, E. A., R. L. Zepeda, T. K. Rockwell, T. L. Ku and W. S Dinklage (1998), Active tectonics

at Wheeler Ridge, southern San Joaquin Valley, California, Geological Society of America Bulletin, 110, 3, p. 298-310.

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

Lee, J.-C., Y.-G. Chen, K. Sieh, K. Mueller, W.-S. Chen, H.-T. Chu, Y.-C. Chan, C. M. Rubin, and R. Yeats (2001), A vertical exposure of the 1999 surface rupture of the Chelungpu Fault at Wufeng, western Taiwan: structural and paleoseismic implications for an active thrust fault, Bulletin of the Seismological Society of America, 91, 5, p. 914-929.

McQuarrie, N. (2004), Crustal scale geometry of the Zagros fold-thrust belt, Iran, J. Struct. Geol., 26, p. 519–535, doi:10.1016/j.jsg.2003.08.009.

Medwedeff, D. A. (1992), Geometry and kinematics of an active, laterally propagating wedge thrust, Wheeler Ridge, California, in Structural Geology of Fold and Thrust Belts, edited by S. Mitra and G. Fisher, p. 3 – 28, Johns Hopkins Univ. Press, Baltimore, Md

Mueller, K., and J. Suppe (1997), Growth of Wheeler Ridge anticline, California: Implications for short-term folding behavior during earthquakes, Journal of Structural Geology, 19, p. 383-396.

Mugnier, J.-L., P. Huyghe, P. Leturmy, and F. Jouanne (2004), Episodicity and Rates of Thrust- sheet Motion in the Himalayas (Western Nepal), in McClay, K.R., ed., Thrust tectonics and hydrocarbon systems, AAPG Memoir 82, p. 91-114.

Price, R.A. (1981), The Cordilleran Foreland Thrust and Fold Belt in the Southern Canadian Rocky Mountains, in K. R. McLay and N.J. Price (editors), Thrust and Napper Tectonics, Geological Society of London, Special Publication No. 9, p. 427-448.

Robert, X., P. van der Beek, J. Braun, C. Perry, M. Dubille and J.-L. Mugnier (2009), Assessing Quaternary reactivation of the Main Central thrust zone (central Nepal Himalaya): New thermochronologic data and numerical modeling, Geology, 37, 8, p. 731–734; doi:10.1130/G25736A.1.

Rockwell, T. K., D. E. Ragona, A. J. Meigs, L. A. Owen, C. H. Costa, and E. A. Ahumada (2013), Inferring a thrust‐related earthquake history from secondary faulting: A long rupture record of La Laja Fault, San Juan, Argentina, Bulletin of the Seismological Society of America, 104, 1 p. 269-284.

Suppe, J. (1983), Geometry and kinematics of fault-propagayion folding, American Journal of Science, 283, p. 684-721.

Talbot, C. J., and M. Alavi (1996), The past of a future syntaxis across the Zagros, in Alsop, G. I., Blundell, D. J., Davidson, I. (Eds.), Salt Tectonics, Geological Society Special Publication, 100, p. 89–109.

Thomas, W. A. (1977) Evolution of the Appalachian–Ouachita salients and recesses from reentrants and promontories in the continental margin, American Journal of Science, 277, p. 1233–1278.

Thompson, R. J. (1981), The nature and significance of large 'blind' thrusts within the northern Rocky Mountains of Canada, in McClay\, K. R. & Price, N. J. (eds.) Thrust and Nappe Tectonics, Special Publication of the Geological Society of London, 9, p. 449-62, Black- well Scientific Publications, Oxford

Wobus, C., A. Heimsath, K. Whipple, and K. Hodges (2005), Active out-of-sequence thrust faulting in the central Nepalese Himalaya, Nature, 434, p. 1008-1011.

Wobus, C., K. Hodges, and K. Whipple (2003), Has focused denudation sustained active thrusting at the Himalayan topographic front?, Geology, 31, p. 861-864.

CHAPTER 2: Neotectonics and structure of the Himalayan deformation front in Kashmir, India:

Implications in defining what controls a blind thrust front in an active fold-thrust

belt

Y.Gavillot, A. Meigs, D. Yule, T. Ritteneour, M. Malik, R.K. Ganjoo

2.1 ABSTRACT

Active tectonics of a deformation front constrains the kinematic evolution and structural interaction between the fold-thrust belt and the most-recently accreted foreland basin.

Discontinuous segments of south-verging emergent thrusts and fault-related folds mark the

Himalayan Frontal thrust (HFT). In contrast, the deformation front of the Kashmir Himalaya is blind, characterized by a broad fold (the Suruin-Mastgarh anticline (SMA)), and displays no surface faults cutting the southern limb. A lack of knowledge of the rate of shortening and structural framework of the SMA hampers quantifying the earthquake potential for the deformation front. We utilized the geomorphic expression of dated deformed terraces on the Ujh

River in Kashmir to constrain the structural style of deformation and shortening rates. Six terraces are recognized, and three yield OSL ages of 53 ka, 33 ka, and 0.4 ka. Terrace restorations through long profiles reveal a deformation pattern characterized by uniform uplift across the anticlinal axis and northern limb and variable uplift due to rotation of the southern limb. Offset terraces occur between the fold trace and the northern limb. Bedrock dips and stratigraphic thicknesses favor the interpretation that an active duplex at depth drives uniform uplift in the north and variable tilt in the south. Faulting at the fold axis introduces asymmetrical fold geometry. Folding of the oldest dated terrace suggests that rock uplift rates across the SMA range between 1.8 and

2.0 mm/yr. Assuming a 25 dip for the duplex ramp on the basis of dip data constraints, the shortening rate across the SMA ranges between 3.8 and 5.4 mm/yr since ~53 ka. Of that rate, 1.1-

2.4 mm/yr is likely absorbed by localized faulting in the near field of the fold axis. Given that

Himalaya-India convergence is ~11-18 mm/yr (Jade et al., 2004; Schiffman et al., 2013), internal faults north of the deformation front, such as the Riasi thrust, absorb more Himalayan shortening than does the HFT in Kashmir. We attribute a non-emergent frontal fault to primarily reflect deformation controlled by pre-existing basin architecture in Kashmir, characterized by an enormously thick succession of foreland basin deposits (~8 km), a relatively deep basal

11 décollement, and likely basement faults. Blind thrusting reflects some combination of layer- parallel shortening, high stratigraphic overburden, relative youth of the HFT, and/or sustained low shortening rate on 105 yrs at longer timescales.

2.2 INTRODUCTION

In active fold-thrust belts, the deformation front or thrust front defines the transition between accreted and undeformed foreland basin sediments (Thompson, 1981). The deformation front characterizes the most foreland thrust that is the up-dip continuation of a plate boundary basal décollement, intimately linked to the overall convergence absorbed by an orogenic wedge

(Davis et al., 1983; Dahlen and Suppe, 1988; Boyer and Elliott, 1982). Many structural variations exist for any given deformation front (Morley, 1986). Emergent or near-surface thrusts occur in many large-scale contractional orogens (e.g. Himalaya, Zagros, Andes), and commonly form as thrust splays of fault-propagation folds or fault-bend folds (e.g. Suppe, 1983). Alternatively, some deformation fronts are blind or buried (Morley, 1986, and references therein), such as detachment folds (e.g. Appalachian Plateau), buried wedge thrusts into triangle zones (e.g. Canadian

Rockies), frontal monocline (Pyrenees, western Alps), or complex fold tips lines (Atlas

Mountains). In a setting like the Nepalese Himalaya, a large earthquake generated on the basal décollement is thought to release most of the interseismic strain through surface ruptures concentrated at the deformation front (Bilham et al., 1997; Lavé and Avouac, 2000; Mugnier et al., 2004). However, in some thrust settings, if the thrust front is blind, earthquakes do not result in surface ruptures (e.g. 1983 Coalinga earthquake; Guzofski et al., 2007). Hence tectono- geomorphic studies are ideally suited to constrain the structural style and displacement rates of a deformation front, providing insights into the earthquake potential and pattern of strain release for a seismically active orogen (e.g. Lavé and Avouac, 2000).

The Himalayan Frontal thrust (HFT) defines the thrust front or deformation front of the south-verging Himalayan fold-thrust belt (Hagen, 1956; Hodges, 2000; Yeats et al., 1992; Lavé and Avouac, 2000, DiPietro and Pogue, 2004). From central to northwest Himalaya, the deformation front is a well-documented discontinuous segment of fault-related folds, which are bounded by an emergent south-verging thrust on the southern limb (Champel et al., 2002;

Delcaillau et al., 2006; Lavé and Avouac, 2000; Mugnier et al., 2004; Valdiya, 2003; Yeats et al.,

1992; Yeats and Thakur, 2008). Geodetic data demonstrate that the HFT at the deformation front is the primary active structure within the Himalayan fold trust belt (e.g. Bilham et al., 1997;

Powers et al., 1998; Lavé and Avouac, 2000; Mugnier et al., 2004). Historical and paleoseismic data reveal that moderate to large earthquakes rupture to the surface along the HFT (Seeber and

Armbruster, 1981; Wesnousky et al., 1999; Upreti et al. 2000; Bilham et al., 2001; Kumar et al.,

2001; Thakur, 2004; Valdiya, 2003; Lavé et al., 2005; Kumar et al., 2006; Yule et al., 2006;

Kumar et al., 2010, Malik et al., 2010; Sapkota et al., 2013).

The deformation front in the Kashmir Himalaya, the region of northwest Himalaya between the Kangra reentrant and the Pakistan Himalaya (Fig. 2.1), differs fundamentally from neighboring areas to the east and west. The Salt Range thrust in Pakistan to the west is emergent and the HFT from Kangra eastward is also emergent. In Kashmir, a ~250 km-long, NW-SE striking, broad fold system, the Suruin-Mastgarh anticline (SMA; Ranga Rao and Datta, 1976) and correlative folds along strike (Fig. 2.1), represent the deformation front. No emergent thrust occurs in either limb of the fold system. We exclude the Pabbi Hills at the eastern edge of the

Pakistan Himalaya from our study of the regional frontal fold system in Kashmir Himalaya, given the Pabbi Hills contrastingly differs with its NE-SW trend, structurally relating to thrust splays of the SRT (Pennock et al., 1989). Structural architecture and displacement rates of the SMA are poorly constrained. It is not known, for example whether the lack of an emergent thrust results from a simple buried thrust, potentially more complex structure (i.e. triangle zone wedge thrusts,

13 passive roof thrust, detachment fold), youthful age, or low shortening rate. In Pakistan, many fold structures bounded by blind thrusts were interpreted to be cored by either detachment folds or duplexes depending on distribution of salt at depth (Baker, 1987; Leathers, 1987; Jaumé and

Lillie, 1988; Pennock et al., 1989. Lack of constraints on the structural geometry and shortening across the SMA inhibit understanding what fraction of the India-Eurasia convergence is absorbed at the thrust front in Kashmir and whether earthquakes rupture to the surface, similar the deformation front to the southeast in Nepal and India. Hence, a fundamental lack of knowledge of the structural geometry and shortening rates for the SMA hampers quantifying the earthquake potential and seismic hazard in Kashmir Himalaya.

In this paper, we focus on constraining the structural style of deformation and shortening rates utilizing bedrock data and fluvial terraces, to characterize the thrust front in Kashmir. We present 3 new cross-sections across the front and new geomorphic mapping of terraces along the

Ujh River parallel to one of the sections (Fig. 2.2). At this location, multiple flights of strath terraces are well preserved in a longitudinal direction perpendicular to the regional structural trend of the SMA. Terrace geometry and elevation were surveyed in detail. Bedrock relations combined with structural information from fluvial terraces form the basis for characterizing the kinematic evolution of the SMA at the deformation front. We use OSL and cosmogenic surface dating to constrain terrace ages and to quantify intermediate-timescale shortening rates (>103 yr).

Our results indicate progressive long-wavelength tilting occurs on the southern limb and uniform uplift across the northern fold limb. Estimates of displacement rates suggest that the deformation front is one structure among other active faults accommodating shortening in the Kashmir

Himalaya.

2.3 TECTONIC SETTING

India-Eurasia plate collision during the Cenozoic produced an extensive zone of crustal

14 deformation extending from the northern edge of the Indo-Gangetic plain northward into central

Asia (DeMets et al., 1990; Banerjee and Bürgmann, 2002; Bettinelli et al., 2006). In the

Himalaya, a continuous fold-thrust belt developed by a series of generally range-parallel, southward-verging thrusts (e.g. DeCelles et al., 1998b; DiPietro and Pogue, 2004; Yin 2006). All major Himalayan thrusts root northward into the Main Himalayan thrust (MHT), the plate- boundary basal décollement accommodating India-Eurasia plate convergence for the Himalayan orogen (Schelling and Arita, 1991; Schelling, 1992, Pandey et al., 1995; DeCelles et al., 1998a;

Powers et al., 1998; Lavé and Avouac, 2000; Hodges et al., 2000).

At the deformation front, the Main Frontal thrust or Himalayan Frontal thrust (HFT) represents to the southern-most emergent thrust splay of the MHT (DiPietro and Pogue, 2004;

Hodges, 2000; Yeats et al., 1992). Low-amplitude structures 15 km south of the HFT have been suggested to represent active faulting stepping out further into the foreland (Piedmont fault; Yeats and Thakur, 2008). Along the length of the Himalayan orogen, the HFT consists of discontinuous segments of fault-related folds, with nearly every fold now recognized to have a south-verging thrust cutting the southern limb, which typically cuts all but the youngest surficial sediments

(Wesnousky et al., 1999; Kumar et al., 2006; Kumar et al., 2001; Lavé et al., 2005; Mugnier et al., 2004; Thakur, 2004; Valdiya, 2003). The HFT bounds the southern limit of the Sub-

Himalaya, a thrust belt of dominantly Tertiary and Quaternary deformed foreland strata, including the Siwalik and Murree formations (Hodges, 2000; DiPietro and Pogue, 2004). Although the deformation front and Sub-Himalaya consist of a number of fault splays younger than the hinterland thrusts, the HFT generally represents the locus of Holocene shortening (Powers et al.,

1998; Mugnier et al., 1999; 2004; Lavé and Avouac, 2000).

The one notable exception is the Suruin-Mastgarh anticline (SMA), equivalent to the

HFT in the Kashmir Himalaya, characterized only by a broad fold and no emergent thrust cutting either fold limbs (Figs. 2.1 and 2.2). Magnetostratigraphic, sedimentological, and

15 thermochronologic studies define an onset of thrust-related uplift during the Upper Siwalik deposition associated with folding at the deformation front between 2.1-1.6 Ma in Pakistan

(Burbank et al., 1986), ~2.0-1.5 Ma in India and Nepal (Burbank et al., 1996a, Valdiya, 1998;

Sangode and Kumar, 2003; Mugnier et al., 2004; Kumar et al., 2006; Van der Beek, et al., 2006), and ~3-1.7 Ma in Kashmir (Ranga Rao et al., 1988; Gavillot et al., 2013; see Chapter 4).

In the central and eastern Himalaya, Holocene shortening rates derived from folded fluvial terraces above the HFT range between 18 to 11 mm/yr (Mugnier et al. 2004) and 22 to 19 mm/yr (Lavé and Avouac, 2000) in Nepal, 29 and 12 mm/yr (Berthet et al., 2014) in Bhutan, and

29.2 to 16.8 mm/yr (Burgess et al., 2012) in northeastern India, from west to east, respectively.

Measurements of geodetic convergence between southern Tibet and India, representing the short- term interseismic strain accumulation along the basal décollement of the Himalaya orogenic wedge, show that in the central and eastern Himalaya, the MHT creeps at 20.52 to 192.5 mm/yr, and is locked in the High Himalaya, ~150-100 km north of the deformation front (Bilham et al., 1997; Larson et al., 1999; Banerjee and Bürgmann 2002; Bettinelli et al., 2006). Given comparable rates between the short-term geodetic and Holocene geological rates, the HFT is presumed to accommodate most of the India-Eurasia convergence absorbed in the Himalaya (e.g.

Lavé and Avouac, 2000; Mugnier et al., 2004).

Along strike to the west in India, geodetic convergence rate measurements range from

18.83 to 141 mm/yr in the western and northwest Himalaya (Banerjee and Burgmann, 2002;

Jade et al., 2004). Here, estimates of Quaternary shortening for the HFT are lower between 16 to

4 mm/yr (Powers et al., 1998; Kumar et al., 2001; 2006; Malik and Nakata, 2003; Malik et al.,

2010; Sapkota et al., 2013). As it does to the east, the deformation front is the primary active structure and MHT earthquakes potentially rupture to the surface along the HFT (Yeats and

Lillie, 1991; Kumar et al., 2001; Kumar et al. 2006, Malik et al., 2010). In the Kashmir Himalaya, estimates of geodetic convergence range between 18.83 and 111 mm/yr (Jade et al., 2004;

Schiffman et al., 2013). Holocene and active shortening across the deformation front (SMA) in

Kashmir is unknown, but suggested to be low (<6 mm/yr; Gavillot et al., 2010b; Vignon 2011; see Chapter 2). An estimate of long-term Pliocene shortening rate is available for the SMA at the

Chenab reentrant (Figs. 2.1 and 3.2), ranging between ~5 to 3 mm/yr using restored line-length cross section measurements, suggesting the remainder of convergence absorbed in the Kashmir

Himalaya is accommodated by other faults via distributed shortening (Gavillot et al., 2010b; see

Chapter 3).

2.4 STRUCTURE OF THE SMA

The Suruin-Mastgarh anticline (SMA) is a NW-SE to E-W trending fold that extends

~250 km across Pakistan and northwest India (Fig. 2.1). Bedrock mapping, dip measurements, and cross section transects across the SMA constrain the upper crustal-scale structural architecture above the basal décollement. Our study area focuses on the structural style of deformation along a nearly continuous +100-km long, NW-SE trending segment of the SMA, between the Chenab River and Ravi River (Figs. 2.2 and 2.3). Topographic data for the cross section profiles were extracted from an ASTER digital elevation model (DEM).

SMA geometry at the surface

Three transect cross section lines (A-A’, B-B’, and C-C’; Figs 2.1-2.3), provide information of structural fold style and along strike variation across the deformation front.

Erosion exhumed tilted and deformed Tertiary-Quaternary Lower-Upper Siwalik strata into a broad fold. Resistant sandstone lithology in the Middle Siwalik Formation and not the fold axis controls the topographic crest across the SMA, which ranges between 1200 and 800 m in elevation. The Lower Siwalik Formation represents the deepest stratigraphic levels exhumed along the fold axis. No significant fault-related offset is observed along any of the limbs or fold

17 axis. Small intra-formational faults occur locally the northern limb. These minor faults account for no more than a few hundred meters of separation as shown in the cross sections (Fig. 2.3).

At the Chenab reentrant transect on the northwest, the SMA is at its narrowest (line A-A’;

Fig. 2.3). The southern limb exposes an entire stratigraphic sequence of Siwalik units. Bedrock dips to the southwest and gradually steepens towards the core of the fold, ranging between 5-

10(Boulder Conglomerate), 15-30(Upper Siwalik), 30-75 (Middle Siwalik), and 75 to vertical

(Lower Siwalik). In comparison, the northern limb is steeply dipping. Attitudes decrease to the northeast from vertical (Lower Siwalik), to 60-45 (Middle and Upper Siwalik). Although dips within the core are vertical, the overall fold geometry is asymmetrical as reflected by the broad

(10 km) southern limb and narrow (<2 km) north limb. To the north of the SMA, the northern limb forms a series of folds and footwall syncline to the RT (Fig. 3.2).

At the Tawi River transect to the southeast, the SMA attains its greatest width (line B-B’;

Fig. 2.3). Stratigraphic units exhumed across the southern limb of the SMA are nearly identical to the Chenab section. The Boulder Conglomerate and Upper Siwalik units are notably thicker and more extensive along the Tawi River transect. Dip gradually increases from the southwest towards the core of the fold, ranging between 5-10(Boulder Conglomerate), 15-30(Upper

Siwalik), 30-60 (Middle Siwalik), and 60-75 (Lower Siwalik). Angular intra- and inter- formational unconformities in the uppermost deposits of the Upper Siwalik (<5 Ma) and Boulder

Conglomerate (1.7-0.4 Ma) are interpreted to represent synorogenic growth strata related to

SMA. Paleomagnetostratigraphic and sedimentological data also indicates that fold growth initiated after Middle Siwalik deposition (Ranga Rao et al., 1988). More steeply northeast- dipping bedrock characterizes the northern limb (75 (Lower Siwalik) to 45 (Middle and Upper

Siwalik from the axis to the northeast)). Given the 17 km-wide southern limb, and <5 km wide

18 more steeply dipping northern limb, the fold geometry is asymmetrical. North of the SMA, the northern limb forms part of an overturned footwall syncline bounded to the MKT (Fig. 2.2).

The Ujh River transect is the southeastern-most transect. River terraces preserved along the channel are used in this study to demonstrate intermediate term deformation pattern across the

SMA (line C-C’; Figs. 2.1-2.4). The stratigraphy across the southern and northern limbs match closely the other transects along strike (Figs. 2.2 and 2.3). Within the southern limb, southwest bedrock dip gradually increases from ~10 (Boulder Conglomerate), 10-30 (Upper Siwalik), 30-

60 (Middle Siwalik), and 60 to locally rolling over 30 (Lower Siwalik) at the fold axis (Fig.

2.4). Angular intra- and inter-formational unconformities are also observed in 10-20 dipping

Boulder Conglomerate and Upper Siwalik along the Ujh River transect. In the northern limb, dip shallows to the northeast from 75 (Lower Siwalik) to 45 (Middle-Upper Siwalik). An asymmetric fold geometry is indicated by the 16 km-wide southern limb and <5 km-wide northern limb. North of the SMA, the northern limb forms part of an overturned footwall syncline to the MKT (Figs. 2.1-2.4).

Depth to décollement

Deep structural geometry and depth of the basement-cover are unknown because no subsurface data exist except for a poor-resolution and shallow subsurface seismic surveys in the area of the Tawi River transect (Raiverman et al., 1983). Depth to décollement estimated using map relations of stratigraphic units and nearby seismic studies places constraints on the deep structural geometry beneath the SMA. A detailed reflection, well-log, and surface study in neighboring Kangra reentrant provided by the Oil Natural and Gas Commission (Fig. 2.1)

(Powers et al., 1988), along with a cross section and some low resolution seismic data Pakistan

Azad Kashmir (Burbank et al., 1986), suggest that a ~2.5 regional dip characterizes the

19 basement-cover interface and basal décollement. Thickness of the Siwalik group is ~6 km given dip and map relations (Fig. 2.2). Addition of ~2 km of the Murree group projected at depth beneath the deformation front, indicates ~8 km of total foreland stratigraphic thickness (Fig. 2.3), consistent with published maximum stratigraphic thickness in Kashmir and Pakistan

(Karunakaran and Ranga Rao, 1976; Johnson et al., 1979; 1982; 1985; Raynolds, 1980; Burbank et al., 1988; Thakur and Rawat, 1992; Powers et al., 1998, and references there in; Shah, 2009.

Our estimate of basement depth is 4 km deeper than in the Kangra reentrant (Powers et al., 1998;

Webb, 2013), 4-6 km deeper than in Pakistan (Pennock et al., 1989; Baker, 1987; Leathers, 1987;

Jaumé and Lillie, 1988), and 2.5-3.5 km deeper than in Nepal (Schelling 1992; Harrison et al.,

1993; DeCelles et al., 1998a; Lavé and Avouac, 2000).

Basement depth on the cross sections reflects the assumptions that 1) both Lower and

Upper Murree exist at depth, 2) units thickens to the NE at gradients of (0.2 to 1.5) consistent with a 2.5 regional basement dip, and 3) units do not significantly thin or pinch out beneath the

SMA. Whereas the Lower and Upper Murree units are not exposed in the SMA, they are present and thick in hanging walls of the MKT and RT (Fig. 2.2). Both units appear at depth in well-log data in the correlative fold of the SMA in the Kangra reentrant (Powers et al., 1989). The assumption that the section thickens northward without abrupt pinch out is consistent with published balanced cross sections, seismic and well-log data in Pakistan and northwest Himalaya

(Powers et al., 1998; Pennock et al., 1989, Baker, 1987, Leathers, 1987; Webb, 2013).

SMA deep structure

Cross section relations and a décollement depth of ~8 km imply a space problem below the fold axis of the SMA, from observations and measurements that 1) the oldest and deepest exhumed unit of the Lower Siwalik exposed at the core of the SMA is stratigraphically thin

(~1000 m); 2) stratigraphic thickness of the Murree group beneath the core of the fold is not sufficient to fill the structural space; 3) constraints on the depth of the basal décollement requires that base of the Murree group is elevated ~ 4km relative to expected depth of at ~ 8 km.,

The structural hole can be accounted for with several alternative interpretation of a deep structure. No geological evidence exists for the presence of significant source of salt along the basal décollement in the Kashmir Himalaya (e.g. Thakur and Rawat, 1992) that might act as weak material to flow into the core folds, as occurs in the folds in Salt Range and Potwar Plateau in

Pakistan (Fig. 2.1) (Pennock et al., 1989, Baker, 1987, Leathers, 1987; Jaumé and Lille, 1988). A duplex structure (Boyer and Elliott, 1982) and associated multi-bend fault-bend folding above the roof thrust (Medwedeff and Suppe, 1997), is a preferred interpretation that provides the required structural thickening to fill the space (Fig. 2.3).

The latter structural interpretation, infers repetition of the Lower and Upper Murree

Formations between the MHT basement duplex and a roof thrust at the Upper Murree and Lower

Siwalik contact (Fig. 2.3). Any structural thickening must occur below the Lower Siwalik, which cores and is continuous across the SMA (Fig. 2.2). Wedge thrusting (Boyer and Elliot, 1982), along the roof thrust is assumed to partly transfer to duplex displacement via this upper detachment level. Wedge thrusting explains well NE-directed faults within the fold axis, and the asymmetrical detachment fold-like geometry (line C-C’; Fig. 2.3).

A duplex and associated fault-bend folding suggests that triangle zone geometry characterizes the Kashmir Himalayan thrust front (Price, 1981; Thompson, 1981; Boyer and

Elliot, 1982; Morley, 1986). Active thrusting at the SMA remains blind due to a combination of displacement transfer to southwest and to the northeast via the southwest-directed wedge emplacement beneath the foreland, which may be accompanied by layer-parallel shortening as well. Our structural interpretation is comparable to the duplex and wedge thrust interpreted for

21 structures along strike to southeast in the Kangra reentrant (Powers et al., 1988). The interpretation is consistent with both active thrust systems the Coalinga anticline or Wheeler

Ridge (Medwedeff, 1992; Mueller and Suppe, 1997; Keller et al. 1998 Guzofski et al., 2007) and ancient thrust belts such as the Canadian Rockies thrust front (Thompson, 1981; Price 1981).

Structural restoration and long-term shortening rate (105 yr timescale)

We utilize a restored cross sections along the transect line C-C’ to measure the horizontal displacement absorbed by the SMA (Fig. 2.5). Map relations and balanced cross sections provide information of fault cut-offs associated with duplex and wedge thrusting at depth. Shortening south of the MKT is controlled by interpretation of two duplexes at the core of folds and displacement along the roof thrust. For the SMA, duplex emplacement as an antiformal stack and displacement on the roof thrust yields 19 km of shortening (Fig. 2.5). Total deformation south of the MKT, including an interpreted footwall duplex at the core of an overturned fold yields 23 km of shortening for the frontal folds.

Several possible ages exist for the onset of fold-related thrusting on the SMA. Ranga Rao et al. [1988], delineates a decrease in sedimentation rates and appearance of thick conglomerate at

1.7 Ma in the Upper Siwalik exposed within the fold structure. However, a recent thermochronologic study across the Sub-Himalayan belt in Kashmir, indicates onset of rapid cooling and fault-related exhumation at ~3 Ma for Lower Siwalik samples exhumed within the core of the fold at the deformation front (Gavillot et al., 2013; see Chapter 4), similar to a published result of a minimum of ~2 Ma for the onset for the HFT in Nepal (Van der Beek et al.,

2006).

Age of initiation of thrusting and measurements of shortening restricted to the SMA provide estimates of long-term shortening. Using younger onset age (1.7 Ma), predicts 11.4

22 mm/yr of shortening. Using an older onset age (~3 Ma), predicts 6.5 mm/yr of shortening. Rates differ nearly by a factor of two, purely dependent on the age interpretation. Although we cannot refute the possibility of a 1.7 Ma age, comparative studies on the RT with rates as high as 10 mm/yr, favor for low shortening rate absorbed by the non-emergent deformation front (Gavillot et al., 2010b; Vignon, 2010; see Chapter 3). In the following section, we evaluate our result of long- term bedrock shortening with intermediate-term shortening rates recorded by river terraces.

2.5 RIVER TERRACES

Mapping and dating fluvial terraces has emerged as a leading geophysical tool to constrain deformation pattern and rates of tectonic processes on 103 yr and longer timescales

(Rockwell et al., 1988; Burbank et al., 1996b; Lavé and Avouac, 2000; 2001; Pazzaglia and

Brandon, 2001; Thompson et al., 2002; Amos et al., 2007; Hubert-Ferrari, et al., 2007; Simoes et al., 2007). River down cutting that accompanies fold-related uplift causes strath in bedrock terrace abandonment and elevates the surface relative to the modern river. All terraces are bedrock straths capped by a relatively thin alluvial cover. Assuming that the initial geometry of abandoned and elevated surfaces mimics the modern stream profile, dated terraces are markers that record active folding in geomorphic timescales (103 yrs) (e.g. Lavé and Avouac, 2000;

Thompson et al., 2002; Amos et al., 2007).

Ujh River

The Ujh River is a transverse drainage across the SMA with a drainage area of 990 km2,

(Jain et al, 2007). The Ujh River flows from the southwestern face of Mount Kailash at ~4000m across the Lesser Himalaya, Siwalik, and Murree fold belt within the Sub-Himalaya (Fig. 2.2).

Fluvial drainage pattern varies notably across the main structures (Figs. 2.6). The Ujh River is characterized by deep, narrow gorges north of the MKT and within the core of the SMA fold.

Along both limbs of the SMA away from the anticlinal axis, the river forms a wide braided to meandering channel in which multiple flights of strath terraces are preserved (Fig. 2.6). Across the fold, the channel bed intermittently switches between a bedrock channel and one buried by ~3 m thick depositional gravel bars. Channel variation is mimicked by variations in river gradient along our 30 km-long river and terrace profile (Fig. 2.7). Channel gradient gradually steepens from 0.1 at the edge of the foreland basin, 0.4 along the southern section of the southern limb, and 0.9 across the anticlinal axis and northern limb. North of the SMA, channel gradient increases to 1.4 across the MKT.

Terrace mapping methodology

Terraces were mapped using CORONA satellite images as base maps. Field mapping of the different terraces consisted of correlating discontinuous terrace remnants from reach to reach using elevation above the modern channel, terrace stratigraphy, thickness of the alluvial cover, weathering color, and soil development. Soil thickness did not permit direct correlation of terrace age, except for the youngest terrace, which was no thicker than a few tens of centimeters.

Weathering color of soils provided a clear means to differentiate the oldest terrace (yellow- brown) from younger terraces (dark-brown to red-brown). Deformed terraces were surveyed at high resolution using differential GPS with a vertical accuracy of 1 m or less. The modern channel provides a marker for measuring long-wavelength deformation (Burbank et al., 1996b).

Measurement of elevations of the terrace treads and active channel were conducted via a 30 km- long survey along the Ujh River. Where terrace stratigraphy was well exposed, we measured the thickness of the terrace gravels above the strath level using laser distance measurements. Lower

resolution (90x90m) DEMs provided further calibration of elevation to correlate the

discontinuous terrace treads and the river channel (Figs. 2.6 and 2.8). Surface topographic data

and bedrock structural relations constrain the location of the active structural hinge lines to model

progressive fold evolution (Thompson et al., 2002; Amos et al., 2007; Hubert-Ferrari, et al., 2007;

Simoes et al., 2007).

Geomorphic map relations of Ujh River terraces

Along the Ujh River valley, a series of elevated erosional strath terraces formed on tilted

Siwalik bedrock units cross both limbs of the SMA (Figs. 2.8 and 2.9). Location of the various

terraces is herein referred to their relative position on the “southern” and “northern” limb. Many

of the terraces form large mappable geomorphic surfaces, characterized by kilometer-long treads,

gravel caps (<7m) and well-exposed bedrock straths (Fig. 2.10a-e).

Terrace deposits are dominated by boulder to pebble-sized clasts (Fig. 2.10d),

intercalated locally with 20-30 cm thick laminated and cross-bedded sand layers. Clasts from the

Siwalik and Murree formations dominate the terrace gravels. An uppermost 1-1.5 m thick sand

deposit that overlies the gravel beds characterizes the lowest terrace. A thin silt-rich soil horizon

ranging from ~1 m to predominantly <30 cm typically caps the terrace gravels (Fig. 2.10d).

Terraces

Fluvial terraces consist of six mappable surfaces (T1-T6; Figs. 2.8, 2.9, 2.11). T1

represents the highest and oldest terrace, which reaches a maximum elevation of 69-77 m above

the active channel near the axis of the SMA (Fig. 2.11). T1 is distinguished from the other

terraces by its characteristic yellow-brown soil in horizons <1 m thick. Soil color may reflect the

25 weathering of clasts of yellow sandstone in the Upper Murree Formation. Thickness at the capping gravel varies between 3 and 7 m in both limbs. Further north, limb deposit thickness increases locally to ~46 m (Fig. 2.10c) between a mapped normal fault and the MKT (Fig. 2.4).

The strath, however, is low, only ~4 m above the active channel.

Sequentially younger terraces (T2, T3, T4, T5, and T6) occur incrementally at elevations below T1 and are characterized by dark-brown- to red-brown-weathering soils. Soil color likely results from the higher proportions of clasts sourced from red-colored Lower Siwalik and/or

Lower Murree Formation siltstone and sandstone relative to T1. T2 is subdivided as T2a, T2b, and T2c, on the southern limb based on step-like exposures elevated to 45 m, 35 m, and 25 m, respectively above the active channel. On the northern limb, only the lowest terrace (T2c) is preserved as a prominent surface. T3 has a maximum elevation of 20 m above the active channel and is the most prominent, continuous, and well-preserved terrace surface on the southern limb.

Multiple discontinuous treads 1000 m long and 400-1000 m wide extend across the width of the southern limb (Figs. 2.6 and 2.8a). Capping gravels on both T2 and T3 have similar thicknesses

(3-6 m). T4 and T5 are 10 m and 5 m above the active channel, respectively, and are capped by 3-

4 m of gravel. The lowest terrace T6 is 2-3 m above the bedrock strath of the active channel.

Although mapped as a separate terrace, T6 is likely infrequently active during large monsoon flood events, given its low elevation above the modern channel (Fig. 2.10a). T6 is unique relative to the other terraces because its age includes up to 1.5 m thick sand-rich layers. T6 is not observed on the northern limb (Fig. 2.9).

Terrace geochronology

In our study, we utilize a combination of terrestrial cosmogenic nuclides (TCN) using

10Be depth profiles and optically-stimulated luminescence (OSL) dating techniques to date

26 deformed fluvial terraces and establish shortening rates associated with thrust-related activity of the SMA. A brief summary of the dating techniques is described below. A more detailed account of the dating and sampling methodology is explained in Appendix A.

Exposure history of grains and clasts includes both the post-abandonment period and the period of erosion and transport prior to deposition. To account for both the post-abandonment and pre-depositional inheritance contribution of nuclides of a terrace deposit, we used depth profile surface-subsurface sampling methodology to establish terrace ages and to isolate the inheritance signal (Anderson et al., 1996; Hancock et al., 1999; Repka et al., 1997). Given that the parent material consists of predominantly sandstone and siltstone, we utilized the 10Be isotope for these quartz-rich sediments (Gosse and Phillips, 2001; Frankel et al., 2007).

The optically-stimulated luminescence (OSL) measurement records the time elapsed since a deposit was last exposed to sunlight at the surface (Prescott and Hutton, 1994; Aitken,

1998). OSL is used to target thin sedimentary layers rich in silt and sand that retain the accumulation of charge within minerals created due to the interaction of ionizing sunlight radiation with electrons. In comparison to cosmogenic surface dating, an OSL date records the depositional age of sub-surface terrace deposits and is not an abandonment age of a geomorphic surface. Hence, age results determined from 10Be surface dating and OSL allows a multiple, complementary dating strategies to compare and constrain terrace ages.

Terrace ages

No existing geochronological constraints existed for the terraces along the transverse drainages to the SMA. Recent work on the Riasi thrust along the Chenab River, however, provides a chronology for terraces deformed by the thrust system to the northwest (Figs. 2.1 and

2.2) (~100-7 ka; Gavillot et al., 2010; Vignon, 2011; see Chapter 3).

OSL ages from the T1, T2, and T6 terraces currently provide at this moment our only geochronological constraints of terrace deposits. 10Be depth profile surface age samples have been processed, and I am currently awaiting results. The OSL samples came from subsurface sand lenses and hence provide minimum ages of surface abandonment. Although these dates represent minimum ages for abandonment, they allow calculation of incremental and cumulative uplift rates.

We sampled terrace T1 on the northern limb of the SMA, which is one of the best exposures of T1 stratigraphy (sample UJH-T1; Fig 2. 9) The sample came from a 30 cm-thick yellow-colored sand rich interval consisting of multiple laminar and cross-bedded primary structures. Stratigraphic position is ~7 m below the terrace tread and ~1m above the erosional bedrock strath. The sample yielded an OSL age of 53.48.5 ka (Fig. 2.9; Table 2.1). On both the southern limb and northern limb, T1 is characterized by yellow-colored soil weathering and we thus assign a ~53 ka age for all mapped T1 terrace deposits. Given its stratigraphic position, our age represents a maximum possible age for T1 as surface abandonment occurred after ~53ka.

A 20-30 cm sand rich layer was sampled within the T2c terrace deposits on the southern limb of the SMA (sample UJH-T2c; Fig. 2.8). The sample was collected 1.5 m below the surface and ~4 m above the bedrock strath. The OSL sample yields an age of 32.8  6.3 ka for T2c terrace deposits (Fig. 2.8; Table 2.1), consistent with its geomorphic position as a terrace younger than T1. Given the shallow sample depth (1.5 m), the ~32 ka OSL age likely closely approximates the T2c surface abandonment age.

For T6, ~1 m thick sand-rich layers dominated by laminations and meter-wide cross-beds were sampled in the southern limb. Exposed along a river cut bank, this sample’s stratigraphic position is ~60 cm below the surface and ~1.3-2.3 m above the bedrock strath and river level. The sample (UJH-T6) yielded an age of 0.40.2 ka for T6 (Fig. 2.8; Table 2.1). This young age is

28 consistent with a soil development, which does not exceed 30-40 cm, the low position with respect to the active channel, and a strath essentially at the elevation of the active channel.

In summary, OSL provides ages for terraces T1, T2, and T6 of ~53 ka, ~32 ka, and 0.4 ka respectively. These results are consistent with geological constraints such as terrace elevation above the active channel, soil color, and extent of soil development.

Deformation pattern – SMA

Terrace profiles are plotted as surface data points along a SW-NE line perpendicular to the trend of the SMA structure (Fig. 2.11). Profiles along the 30 km-long section along the Ujh

River extend from the Indian foreland (-4 to 0 km), the SMA fold structure (4 to 23 km), and continue to the MKT (23 to 25 km).

Southern limb

Terraces are characterized by similar thin gravel fill (<10m), with 3-7 m for T1, 3-6 m for

T2-T3, 3-4 m for T4-T5, and 2-3 m for T6, which indicates that terraces formed due to continuous bedrock uplift and relatively similar depositional lag times between terrace formation and abandonment (Wegmann and Pazzaglia, 2002). At the lowest and most downstream section of the southern limb, terraces are difficult to differentiate because of the nearly identical lithology in the Boulder Conglomerate unit of the Upper Siwalik Formation. There is some indication that terrace capping gravel thickness (T3) increases to ~10m, likely resulting from increased effect of basin aggradation and decreased tectonic uplift.

Terrace surfaces indicate a clear pattern of progressive tilting and uplift. Along the southern limb, all terraces slope to the SW, progressively steepening towards the SMA anticlinal axis and increases in slopes with terrace elevation (Fig. 2.11). Terraces surface and strath

29 elevations converge towards the modern channel in the downstream direction from the axis.

Apparent terrace fanning merging with the active channel (Fig. 2.11), suggests that base level remained relatively constant or is lowered at a very low rate downstream of the southern limb.

We assume here that the modern channel gradient approximates that of the abandoned river terraces (Burbank et al., 1996b).

Terrace restorations of T1-T6 constrain limb dip evolution, consistent with our terrace projection and assumption of constant base level south of the fold structure (Fig. 2.10; Appendix

A). Progressive restoration modeling further depicts a fanning terrace deformation pattern characterized by progressively tilting of the southern limb around a relatively fixed hinge line in the foreland. Maximum deformation projects at the anticlinal axis (Figs. 2.11 and 2.12; Appendix

A). Southern limb terraces show no geomorphic expression or deformation pattern that can be associated with a NE-dipping blind thrust (Fig. 2.9). A small step in stream profile and T6 data points, at horizontal markers between 0 and 2 km, deviates from our terrace projection.

Anthropogenic activity likely induced local hydrological changes and reorganized channel bed geometry, given that the small step in stream profile is located between the downstream section of a hydrological dam, and highway and train transportation bridges. Alternatively, this apparent step in the stream and youngest terrace profiles could represent the location of a minor gravitational fault within the bedrock at the southern edge of the southern limb.

Anticlinal axis

In the vicinity of the fold trace, little to no terrace preservation coincides with the area characterized with maximum relief and gradient change along the stream profile (Fig. 2. 7).

Fewer preserved terraces suggest that an increase in maximum erosion and uplift near the SMA

30 fold axis. Correlation of southern limb terraces across the fold axis and to the northern limb shows an apparent offset or discontinuity in elevation, especially for T1 (Fig. 2.11).

Two end-member models of terrace correlation across the anticlinal axis and fault- related offset are possible (Fig. 2.12). In one model, terrace slope increases towards the fold axis, which implies a higher instantaneous and total uplift for T1-T6 (Fig. 2.12a). Alternatively, a simple projection utilizing the shape of the modern river profile across the fold to the fault yields lower and minimum uplift for T1-T6 (Fig. 2.12b). These two models also have different implications for the evolution of base level. In model A, terraces are uplifted from a constant river level, as well as along the southern limb. Meanwhile, on model B, terrace uplift has a component of base river level increase. Given map relation of an asymmetrical fold (Fig. 2.2), steep bedrock dips within the fold axis (Fig. 2.4), and asymmetric terrace preservation across the fold trace, we favor the high uplift model as more consistent with interpretation of a local fault involved in the overall growth of the SMA at time scales longer than T1.

Northern limb

On the northern limb, the Bhini River and Ujh River join (Fig.2.6). Downstream from the northern limb, terraces remain SW-sloping, but are all characterized by parallel treads in between terraces and the stream profile (Fig. 2.11). In contrast with the southern limb, northern limb terraces show no long wavelength tilting of surface slope inclinations away from the anticlinal axis. Local tilting with NE dipping slopes does occur only within <1 km of the SMA fold axis

(Fig. 2.11). Terrace-capping gravels at the southern section of the northern limb are 3-7 m thick.

One notable exception is between a local extensional fault and the MKT (markers 19-23 km;

Figs. 2.9, 2.11). At that locality, T1 terrace fill thickness increases from 7 to 54 m, and its strath is close to river level adjacent to the MKT (Figs. 3.10d and 2.11). Although T1 is not apparently

31 uplifted in that reach of the Ujh River, further downstream of the northern limb where terraces are parallel to the stream profile, the same terrace T1 and its strath climb to higher elevations relative to lower and younger terraces (T2-T5) downstream. We interpret thick T1 fill and a down- dropped strath to result from tectonically-controlled local fault-related subsidence in the footwall of the MKT.

Summary of terrace deformation pattern across SMA

Long-profile terrace elevation data (Fig. 2.11), and our terrace restoration models (Fig.

2.12; Appendix A), constrain an overall deformation pattern controlled by vertical incision

(uplift), tilting, and faulting. Differential tilt and uplift characterize southern limb terraces, which suggest rotation of the treads with respect to the modern channel. Maximum relief between the terraces and the channel occurs across the anticlinal axis. Terraces on the northern limb on the other hand, are uplifted relative to the channel and show no titling. Adjacent to the SMA fold axis, terraces are locally tilted and show apparent fault-related offset consistent with local faulting by a NE-directed structure.

Terrace uplift rates

Measurements of uplift recorded by deformed terraces include both defining the incremental vertical displacements between individual terraces (Ui) and cumulative uplift (Uc) defining the total terrace vertical displacement with respect to the original river level. Differential elevation between straths is derived from the tread surface elevation by correcting the elevation for capping gravel thickness above a bedrock strath. Ui and Uc measurements are made at three different structural zones across the SMA, (Fig. 2.13).

Combining the geochronological data with the terrace restorations, which constrain the cumulative (U(c)) and instantaneous (U(i)) uplift, rates were calculated for three different time intervals (T1-T2c: 53-33 ka; T2c-river: 33-0 ka; T1-river: 53-0 ka) (see Figure 2.13 and Table

2.2). All uplift rates range between 1.3 to 2.5 mm/yr for U(c) and 0.7 to 4.4 mm/yr for U (i).

Larger rate variation in U (i) than U(c) reflects a highly variable rates for incremental periods of vertical displacement. Hence using the differential uplift between the oldest terrace (T1) and the river level, U(c) approximates the average long-term uplift rate for the SMA.

Cumulative uplift of the southern limb and northern limb yield nearly identical results of

1.4 mm/yr and 1.3 mm/yr respectively. Similar uplift rates for the southern limb and northern limb, favor the interpretation that the fold limbs are deformed by a common deep structure. Using alternative uplift models for the fold axis yield rates that are higher than the limbs and range between 2.5 mm/yr (high uplift model; Fig. 2.12a) and 1.8 mm/yr (low uplift model; Fig. 2.12b).

Differential uplift of 1.2-0.4 mm/yr between the limbs and fold axis is thus implied by the data

(see summary rates in Table 2.3). Localized deformation at the fold axis (2.5-1.8 mm/yr) is therefore higher than the long wavelength deformation across the SMA (Table 2.3), consistent with our structural interpretation of faulting at the fold trace.

2.6 DISCUSSION

Terraces constraints on bedrock structural interpretation

Terrace deformation patterns can be used to constrain alternative structural models for the structural geometry of the thrust front in Kashmir. Three competing models of the structures at depth are explored to evaluate our balanced cross section of the SMA. Each structural model predicts unique displacement fields at the surface, and is evaluated with the associated structural style of deformation using our earlier concept of terrace restoration. One model, as presented in

33 our along strike cross sections of the SMA (Fig. 2.3), is a southwest-directed duplex and northeast-directed wedge thrust model. In this model, vertical uplift is concentrated north of the anticlinal axis, while progressive fold tilting characterize the southern limb, consistent with our terrace data in long wavelength tilting on the southern limb and parallel uplifted treads on the northern limb (Fig. 2.14 a-b; Fig. 2.11). A dominantly northeast directed fault-bend fold and duplex interpretation is a second alternative model, representing a mirror image structural interpretation to the previous model (Fig. 2.14c). Here the displacement fields predict a long wavelength tilting and/or parallel surface vertical uplift on the southern limb, and progressive tilting on the northern limb. Although this model can equally explain our terrace data on the southern limb, displacements field are in contradicts to the vertical surface uplift observed on the northern limb (Fig. 2.11). A third structural interpretation as a detachment fold model reconciles the space problem at depth and geometry with presence of weak ductile material in the core of fold (Fig. 2.14d). Displacement fields for both fold limbs are characterized by long wavelength progressive tilting, predicts little to no horizontal displacement or vergence. Two main issues with this structural model is an inconsistent deformation pattern compared to our northern limb terrace data (parallel surface uplift rather than long wavelength tilting); Fig. 2.11), and mechanically requires presence of an unknown ductile material, such as salt at the core the SMA.

Although salt has been documented for the basal décollement for the Salt Range thrust, equivalent of the HFT in Pakistan (Pennock et al., 1989, Baker, 1987, Leathers, 1987), no salt has been reported anywhere both at the surface or seismically in the Kashmir Himalaya (Raiverman et al, 1983; Thakur and Rawat, 1992; Burbank et al., 1996; Powers et al., 1998).

In summary, although the structural style of deformation observed along the SMA is non- unique, utilizing prediction in the terrace deformation pattern and our terrace data along the Ujh

River, we are to able to invalidate competing structural models. Using terraces displacement fields across the SMA constrains our cross section as a viable interpretation of the orogenic

34 growth with a southwest-directed deep duplex structure emplaced along the MHT beneath a folded fault-bend fold roof thrust, explaining the regional uplift at the anticlinal axis and northern limb. At higher structural levels, displacement on the duplex is interpreted to transfer most of the slip by tilting the southern limb partly by a NE-directed wedge thrust, consistent with the progressive terrace tilts on the southern limb, localized faulting within the fold trace, and asymmetry in the fold geometry. Similar uplift rates in the southern limb and northern limb suggests deformation on both limbs is intimately linked to same displacement at depth on a duplex ramp (Figs. 2.5 and 2.14).

Therefore, if we assume a simplistic model that most fault-related deformation on both limbs relates to displacement on the duplex, any displacement causing uplift for either limb result from horizontal displacement along a duplex ramp (Figs. 2.5 and 2.14). A 25 NE-dipping thrust ramp defines the SW-verging duplex ramp, based from cross section and dip data constraints

(Figs. 2.4 and 2.5). Although growth of the SMA is likely characterized by some component of southwestward translation on the southern limb, we are unable to resolve distinctive hinge line migrations, indicative that tilting is the dominant deformation pattern. Combination of opposing verging thrusts between the wedge thrust and the duplex may essentially create little net horizontal propagation of the fold structure to the southwest.

Shortening rates using terraces (103 yr timescale)

For all calculation of instantaneous (Sh (i)) and cumulative (Sh(c)) shortening rates, fault ramp geometry of 25º is used to characterize a first-order approximation of the fault-related vertical displacement of terraces. In the northern limb, terrace uplift is interpreted to parallel fault dip vector. In the southern limb and fold axis, terrace uplift is interpreted to represent the same displacement vectors translated either vertically (fold axis) or in the tilt direction (southern limb).

All shortening rates range between 2.7-5.4 mm/yr for Sh(c) and 1.5-9.5 mm/yr for Sh(i). Again, larger spread in instantaneous shortening rates suggests variable uplift and displacement activity of the SMA at incremental time intervals. Measurement of displacement using the oldest geomorphic marker (T1) hence approximates the average shortening rate for the SMA.

Cumulative shortening rates for the southern limb and northern limb are 2.7 mm/yr and 3.0 mm/yr respectively (Table 2.2). Similar rates for both limbs are interpreted to characterize SW- directed displacement of deep structure since ~53 ka. At the fold axis, Sh(c) of 3.8-5.4 mm/yr are higher than the shortening recorded on the limb sections closest to the core of the fold. A shortening rate differential of 2.4-1.8 mm/yr between limbs and the fold axis (Table 2.3) may reflect a better representation of duplex displacement, or alternatively, localized deformation associated with faulting at the fold trace may absorb part of that differential. Overall, total deformation across the SMA, indicates an average shortening of ~4.6 mm/y since ~53 ka (Table

2.3). Broad deformation related to a duplex at depth attributes most of the shortening across the

SMA (~3 mm/yr). Localized faulting at the fold axis may absorb up to 1.1-2.4 mm/yr of shortening.

Comparison with long-term and geodetic convergence rates

Using an older thrust-related onset age of ~3 Ma for the SMA (Gavillot et al., 2013; see

Chapter 4), our intermediate-term shortening rates of ~4.6 mm/yr using a Pleistocene river terrace, match fairly well with derived long-term shortening rates recorded in bedrock deformation at the SMA of 6.5 mm/yr using a balanced cross section (Fig. 2.5). This comparison suggests deformation rates recorded in geomorphic timescales (103 yrs) are representative estimates of long-term bedrock deformation (105 yrs) for the SMA.

At the short-term deformation rate, geodetic measurements predict that the total convergence absorbed across the Himalaya in Kashmir range between 11.1 and 18.8 mm/yr (Jade

36 et al., 2004; Schiffman et al., 2013). In comparison to the HFT in the central and western

Himalaya where nearly 100% of geodetic convergence is absorbed (e.g. Lave and Avouac, 2000), our low shortening rates of ~4.6 mm/yr suggest the deformation front in the Kashmir Himalaya accounts on average ~30 % of the convergence. Pleistocene-Holocene shortening rates of 10.8-

4.4 mm/yr and long-term shortening rate of >10 mm/yr on the Riasi thrust (RT), an emergent active fault system ~40 km to the north of the deformation front (Gavillot et al., 2010b; see

Chapter 3), indicates that internal faults absorbs an equal or greater percent of the convergence in the Kashmir, Himalaya.

Earthquake potential for the deformation front in Kashmir

A low shortening rate on the SMA is consistent with the apparent observation of long earthquake recurrence interval in Kashmir, supported by the simple observation of no historical earthquakes in the vicinity of the deformation front in Kashmir. In the surrounding region, active thrusting occurs on internal faults north of the deformation front, on the Riasi thrust (RT),

Balakot-Bagh fault (BBF), and the Balapora fault (BF) (Fig. 2.1). The BBF was the source for the

2005 Mw 7.6 earthquake in Pakistan Azad Kashmir, and represents the only historic earthquake uniquely tied to any of the active structures in the Kashmir Himalaya. Historical documents of a

1555 A.D event in the Kashmir basin is inferred to have been a great earthquake (Ambraseys and

Douglas, 2004; Bilham and Ambraseys, 2005), but its source remains. Existing paleoseismic data for the RT, BBF, and BF all document surface rupture earthquake events with apparent millennia scale recurrence intervals (Kondo et al., 2008; Hebeler et al. 2010; Madden et al., 2010; see

Chapter 3).

Although the SMA is a blind structure accommodating only part of the overall

Himalayan-India shortening, nonetheless, it is a sink for shortening and slip in large earthquakes on the MHT at depth. Apparent pattern of a long recurrence interval may preclude large events sourced along the MHT in the Kashmir Himalaya. Assuming a blind thrust ramp characterizes the

37 subsurface structure throughout the SMA along strike, as demonstrated with along strike structural cross sections (Fig. 2.3), suggests a potential seismogenic fault length mimicking the

~180 km-long continuous NW-SE to E-W trending fold, extending as much as 250 km with correlative folds in the Kashmir region of Pakistan. Constraints of a geodetically modeled seismogenic locking line of the MHT beneath the Zanskar Range (Schiffman et al. 2013), defines a rupture area >30 000 km2, with a predicted Mw earthquake of 7.5-7.6 (Wells and Coppersmith,

1994; Wesnousky, 2008). Given a lack of historic earthquakes that can be attributed to the deformation front at the SMA, the RT, or the BF, significant seismic deficit may exist for most potentially active structures in the Kashmir Himalaya (Bilham and Ambraseys, 2005; Bilham et al., 2001). Strain release at the SMA would be accommodated by blind thrust earthquakes, similar to a seismically more active structural analog on the Coalingua anticline in southern California,

(Guzofski et al., 2007)

What controls an active blind thrust front in the Kashmir?

Interpreting the thrust front as a blind non-emergent duplex in Kashmir Himalaya clearly differs from the rest of the Himalaya east of the Salt Range and Potwar Plateau. From central

Himalaya to Kangra reentrant, to the southeast, the deformation front is consistently characterized by a southwest-verging fold bounded by an emergent thrust (HFT) (Nakata, 1989; Powers et al,

1998; Mugnier et al., 1999; Lavé and Avouac, 2000; Champel et al., 2003; Mugnier et al., 2004;

Delcaillau et al., 2006). In Pakistan, to the west, the deformation front is again characterized by a prominent emergent thrust, the Salt Range thrust (SRT), cored by a salt detachment (Pennock et al., 1989, Baker, 1987; Leathers, 1987; Baker et al., 1988).

In the absence of salt, active blind structures at the deformation front can result from a variety of factors in a modern fold-thrust belt setting, such as a young fault system, uplift rate less

38 than foreland sedimentation rate, or deep basal décollement overlain by thick stratigraphic overburden. Hence, we can use comparative approach of known geological constraints along the

Himalaya to identify what controls the different structural style of deformation in Kashmir

Himalaya. Given a presumed age of ~3 Ma using thermochronologic data (Gavillot et al., 2013, see Chapter 4), suggest the SMA is no younger than other fault-related folds along strike with ages of 2.1 and 1.6 Ma in Pakistan (Burbank et al., 1986), and 2 and 1.5 Ma for the HFT in Nepal and India (Burbank et al., 1996a; Valdiya, 1988; Sangode and Kumar, 2003; Kumar et al., 2006;

Van der Beek et al., 2006).

Our terrace study on the Ujh River documents low to moderate shortening of ~4.6 mm/yr for the SMA. We cannot refute the possibility for synorogenic burial of a potential near-surface blind thrust along the southern limb of the SMA. Synorogenic strata associated with growth of the

SMA in the Upper Siwalik and Boulder Conglomerate units, are known to account for at least

~1000 m of sediments at the surface (Ranga Rao et al., 1988). Low-resolution seismic data for the area along strike, primarily resolves shallow structural architecture beneath the core of the SMA

(Raiverman et al., 1983), with no definitive evidence for any stratigraphic horizons affected by a thrust telescoping to the near surface. Efforts were made to obtain potentially higher resolution seismic data across the SMA acquired from the Oil Natural and Gas Commission, however our requests were unsuccessful due to security concerns of the data in proximity of the Pakistan-

Indian border.

Utilizing our map relations of stratigraphic thickness (Fig. 2.3), and known units at depth constrained by well-log data in a nearby study in the Kangra reentrant (Powers et al, 1998), our cross section projects a basement or basal décollement to depth of up to ~8 km beneath SMA, indicating one of the thickest stratigraphic overburden above the basal décollement along the

Himalayan deformation front. In comparison, basement or basal décollement depths beneath the same correlative structure at the deformation front, from northwest to central Himalaya range

39 between 2 km and 5.5 km (Powers et al. 1988; Jaumé and Lillie, 1988; Schelling 1992; Harrison et al., 1993; DeCelles et al., 1998a; Lavé and Avouac, 2000; Webb et al., 2013). Our depth constraint for the basal décollement is consistent with known thick successions of foreland strata

(Murree and Siwalik) in the Kashmir Himalaya (Karunakaran and Ranga Rao, 1976; Johnson et al., 1979, 1982, 1985; Raynolds, 1980; Burbank et al., 1988; Thakur and Rawat, 1992; Powers et al., 1998, and references there in; Shah, 2009).

Given the different parameters of fault age, fault rates, and depth of the basal décollement, we interpret that a combination of pre-existing Tertiary foreland basin geometry and thick stratigraphic overburden overlying the basal décollement exerts a primary structural control on the development of blind and non-emergent structure at the deformation front in Kashmir

Himalaya. Thick (~5 km) Precambrian limestone strata thrusted by the RT, originated from deep stratigraphic levels (see Chapter 3); suggest basement faults may occur elsewhere in Kashmir, consistent with an observation of basement high in the vicinity of the deformation front

(Raiverman et al., 1983; see Chapter 3). Mechanical response of stratigraphic basin geometry has been proposed in other thrust belts reflecting differences in maturity and resulting thickness of the sedimentary pile encountered at the deformation front (e.g. Davis and Lillie, 1994). Similar structural patterns are observed with triangle-zone thrusts in the Canadian Rockies (Thompson,

1981; Price, 1981), however the SMA differs in which SW-directed shortening is likely translated both to the south and north by layer-parallel shortening. We do not refute the possibility of emergent thrusting further south of the SMA, as MHT slip could telescope large displacement further into the foreland on a coeval thrust system, similar to the proposed Piedmont fault (Yeats and Thakur, 1998) south of the HFT in the western Himalaya.

2.7 CONCLUSION

Utilizing geomorphic expression of fluvial terraces and bedrock stratal relations, our study constrains the structural geometry, deformation pattern, and shortening rates across the frontal fold structure, the Suruin-Mastgarh anticline (SMA), defining the deformation front in

Kashmir (Fig. 2.1). In the Kashmir Himalaya, the deformation front is blind, and displays no emergent faults cutting either limb. Map relations and multiple cross-sections constrain the structural geometry at depth of a SW-directed duplex of Murree units along the basal décollement, folding an overlying roof thrust that joins a NE-directed wedge thrust rooting into upper detachment horizon. Total bedrock long-term shortening rate predict ~6.5 mm/yr of shortening using a balanced cross section, and an onset of thrusting at ~3 Ma. River terraces are used to evaluate our structural interpretation and constrain intermediate deformation rates. Six terraces are recognized, and three yield OSL ages of 53 ka, 33 ka, and 0.4 ka. Vector fold restoration of long terrace profiles indicates a deformation pattern characterized by regional uplift across the anticlinal axis and northern limb, and by fold limb rotation on the southern limb. Dip data and stratigraphic thicknesses suggest that a duplex structure is emplaced at depth along the basal décollement, folding an overlying passive roof thrust carrying Siwalik-Murree strata into a detachment-like fold. Differential uplift across the fold trace suggest localized faulting, explaining an asymmetrical fold geometry.

Competing structural models for the SMA are tested using predicted terrace deformation patterns associated with a given structural interpretation (SW-directed duplex, NE-directed duplex, detachment fold; Fig. 2.14). Long wavelength tilting terraces on the southern limb, and vertical uplift in the northern limb is most consistent with a SW-directed duplex, folding an overlying roof thrust into a broad fold. At depth, the duplex join a wedge thrust that correlates with surface faulting within the fold axis, explaining no emergent thrust on the southern limb of the SMA, and overall asymmetric fold geometry (Figs. 2.2-2.3).

Fault-related displacement at depth is assumed to relate to a 25-dipping ramp for the duplex on the basis of dip data constraints. Since ~53 ka, the duplex structure absorbs 2.7-3.0 mm/yr of shortening. Fold axis faulting absorbs an additional 1.1-2.4 mm/yr of shortening.

Summing the average shortening rates accounting for all deformation partitioned across the SMA yields a total of 3.8-5.4 mm/yr, averaging ~4.6 mm/yr.

Shortening rate of ~4.6 mm/yr recorded in geomorphic timescales ate consistent with estimates of bedrock long-term shortening for the SMA (6.5 mm/yr). Given that India-Tibet convergence is ~11.1-18.8 mm/yr in the Kashmir Himalaya, the SMA absorbs an average of 30% of the Himalayan shortening. Internal faults north of the deformation front, such as the Riasi thrust absorbs an equal proportion or more of the Himalayan shortening than does the HFT in

Kashmir. We attribute a blind thrust front in Kashmir to reflect deformation controlled by pre- existing basin architecture, in which a ~8 km thick succession of foreland Siwalik-Murree strata overlies a deepened basal décollement. Blind thrusting reflects some combination of layer- parallel shortening, high stratigraphic overburden, relative youth of the HFT, and/or sustained low shortening rate on 105 yrs to longer timescales. Assuming a blind duplex characterized the

180 to 250 km long fold structure of SMA, interseismic strain release could be released at the deformation front in Kashmir by moderate to large infrequent earthquakes.

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Thompson, S. C., R. J. Weldon, C. M. Rubin, K. Abdrakhmatov, P. Molnar, and G. W. Berger (2002), Late Quaternary slip rates across the central Tien Shan, Kyrgyzstan, Central Asia, Journal of Geophysical Research, 107, p. 7-1 - 7-32.

Upreti, B.N., T. Nakata, Y. Kumahara, H. Yagi, K. Okumura, T. K. Rockwell, N. S. Virdi, and H. Maemoku (2000), The latest active faulting in southeast Nepal, in Okumura, K., K. Takada, and H. Goto, eds., Active Fault Research for the New Millennium, Proceedings of the Hokudan International Symposium and School on Active Faulting, January 17th- 26th, 2000: Letter Press Co., Hokudan, Japan, p. 533-536.

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Valdiya, K. S. (2003), Reactivation of the Himalayan frontal fault: Implications, Current Science, 85, p. 1031-1040.

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Vignon, V. (2011), Activité hors séquence des chevauchements dans la syntaxe nord-ouest himalayenne: apports de la modélisation analogique et quantification quaternaire par analyse morphotectonique, PhD thesis, Université de Grenoble, France.

Webb, A. A. G. (2013), Preliminary palinspastic reconstruction of Cenozoic deformation across the Himachal Himalaya (northwestern India), Geosphere, 9, p. 572-587.

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Wells, D. L., and K. J. Coppersmith (1994), New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, Bulletin of the Seismological Society of America, 84, p. 974-1002.

Wesnousky, S. G. (2008), Displacement and Geometrical Characteristics of Earthquake Surface Ruptures: Issues and Implications for Seismic Hazard Analysis and the Process of Earthquakes Rupture, Bulletin of the Seismological Society of America, 98, 4, p. 1609- 1632, doi:10.1785/0120070111.

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2.9 APPENDIX A

Geochronologic and sampling methodology

We employed a combination of terrestrial cosmogenic nuclides (TCN), optically stimulated luminescence (OSL), and radiocarbon methods to date geomorphic surfaces and

51 associated deposits to establish the absolute timing of thrust activity and shortening rates of active structures

Cosmogenic nuclide concentrations accumulated as the result of the interaction of cosmic rays with material at the Earth’s surface provide ages of stable geomorphic surfaces (Lal, 1 1;

Cerling and Craig, 1994; Gosse and Phillips, 2001). A key assumption of the TCN method is that geomorphic surfaces remain stable and neither accumulate nor loose material from the surface after abandonment. The concentration and production of in-situ cosmogenic nuclides is therefore a function of the time the surface has been exposed to cosmic rays up until the point that the system reaches secular equilibrium (ratio of parent to daughter isotopes remain constant because of production decay rate). TCN production for a stable geomorphic surface is greatest at the surface and decreases exponentially with depth. Nuances of this dating technique include 1) ability to constrain the pre-depositional nuclide concentration (so-called ‘inheritance’), and 2) ability to establish that the geomorphic stability of a surface to determinate the magnitude of post- abandonment changes (inflation, deflation). Dating geomorphic surfaces underlain by unconsolidated alluvium (e.g. a fluvial terrace) typically involves recovering a suite of samples from the surface to ~2 m depth (Anderson et al., 1996; Hancock et al., 1999; Repka et al., 1997).

Whereas the TCN concentration at or near the surface reflects the total exposure history of the sediment, the subsurface concentration reflects the inherited concentration because of the asymptotic decay of production as a function of depth. Thus, a depth profile constrains the surface stability and the inheritance history of material comprising a fluvial terrace.

The inheritance and surface modification can be deduced from the change in concentration of in-situ cosmogenic nuclides with depth. A non-zero concentration at the bottom of the depth profile is a measure of the pre-deposition and burial acquisition of nuclides, which is normally taken to represent the inheritance. Post-depositional modification in soil thickness by inflation due to aeolian input or deflation due to erosion yields an older- or younger-than-

52 expected age, respectively. Depending of the lithology of the parent material, measurements of in-situ concentration of cosmogenic nuclides typically utilize 10Be for quartz-rich sediments and

36Cl for carbonate-rich sediments (Gosse and Phillips, 2001; Frankel et al., 2007). Exposure ages are calculated using local production rates, and are corrected for effects on cosmic ray flux as the result of latitude, elevation, topographic shielding, erosion or the extent of overlying material.

Independent data such as soil profile descriptions, constrain whether a surface has remained stable after abandonment.

Optically stimulated luminescence (OSL) dates the time elapsed since a deposit was last exposed to sunlight at the surface (Prescott and Hutton, 1994; Aitken, 1998). OSL targets sedimentary layers comprised of silt to fine sand, which provides an alternative technique for dating fine-grained deposits. Sand-rich deposits retain the accumulation of charge within minerals due to the interaction of ionizing sunlight radiation with electrons. Illuminating the minerals will detrap the charge, which results in the measurable emissions of photons (luminescence).

Artificially dosing sub-samples and comparing the luminescence emitted with the natural luminescence can determine the relationship between radiation flux and luminescence. The equivalent dose (DE) experienced by the grains during burial therefore can be determined. The other quantity needed to calculate the age is the ionizing radiation and cosmic dose rate, which can be derived from direct measurements or measured concentrations of radioisotopes. Age uncertainty, reported as 2-sigma in this study, is influenced by the systematic and random errors in the DE values and the possible temporal changes in the radiation flux. Age results determined from TCN and OSL allow a multiple dating method strategy to compare ages.

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CHAPTER 3: Shortening rate and Holocene surface rupture on the Riasi thrust in the Kashmir

Himalaya: Active thrusting within the Northwest Himalayan orogenic wedge.

Gavillot Y., A. Meigs, D. Yule, A. Hebeler, C. Madden, T. Rittenour, M. Malik and R.

Heermance

3.1 ABSTRACT

New mapping demonstrates that active emergent thrust faulting occurs within the fold-thrust belt north of the deformation thrust front in the NW Himalaya. At least 60 km-long, the Riasi thrust

(RT) comprises the southeastern strand of a seismically-active fault system, which extends stepwise from the Balakot-Bagh fault in Pakistan, more than 200 km southeast into NW India.

Field relationships near the Chenab River indicate that the RT fault zone consists locally of multiple strands with Quaternary activity. The main strand of the RT, described in our study as the Main Riasi thrust (MRT), places Precambrian Sirban Limestone over folded unconsolidated

Pleistocene conglomerate. Vertical separation between the 95.8±14.1 ka Chenab River fluvial deposits beneath the Bidda Terrace in the MRT hanging wall and correlative 87.0±14.2 ka fluvial deposits in the MRT footwall is 272±5 m. Undeformed 39.2±8.0 ka alluvial deposits overlie the

MRT. A shortening rate of 11-2.8 mm/yr characterizes a ~91-39 ka time interval on the MRT at this location. A fault scarp and offset Holocene terrace mark the southernmost and frontal splay of the RT fault zone, called here the Frontal Riasi thrust (FRT). Differential uplift (21.6±1m) of a

~7-14 ka terrace yields a shortening rate of 8.8-2.2 mm/yr for the FRT, with a preferred rate of

8.8-4.4 mm/yr using the younger and shallowest age constraint of 7.1±1.1 mm/yr. Contact relationships in a trench across the FRT reveal an angular unconformity that separates ~25° south-dipping, polylithic cobble and boulder alluvium from undeformed limestone-clast conglomerate and colluvial wedge, implying surface rupture of the RT fault zone ~4,500 yrs ago.

Average shortening rates since ~100 ka across the MRT and the FRT, range from 10.8 mm/yr to

4.4 mm/yr (7.6±3.3 mm/yr). A regional cross section provides a long-term shortening of

~10mm/yr for the RT, suggesting the higher rates (MRT: 10.8 mm/yr; FRT: 8.8 mm/yr) are likely more representative for the average RT Pleistocene-Holocene shortening rates. Given an estimate of 18.8-11.1 mm/yr for the India-Tibet convergence rate, internal deformation within the orogenic belt on the RT accounts on average 50% of the total shortening absorbed in the Kashmir

Himalaya. Similarities between the Riasi thrust and the Balakot-Bagh fault suggest the potential of Mw 7 to 8 earthquakes on the RT fault zone, similar to the source of the Mw 7.6 2005 Kashmir earthquake on the Balakot-Bagh fault segment. Discovery of surface-rupturing reverse faults within the thrust belt indicates that seismic sources within the NW Himalayas include the Main

Himalayan thrust (the basal décollement) as well as upper plate faults capable of generating moderate but nonetheless devastating earthquakes.

3.2 INTRODUCTION

Spatial and temporal patterns of active deformation in orogenic belts constrain orogenic structural development and the associated seismic hazards. At the southern topographic front of the Himalayan range, the Himalayan Frontal thrust (HFT) defines the deformation front of the fold-thrust belt and is the locus of active shortening (Hagen, 1956; Hodges, 2000; Yeats et al.,

1992; Powers et al., 1998; Mugnier et al., 1999; Lavé and Avouac, 2000, DiPietro and Pogue,

2004). In Nepal and parts of the western Himalaya, the HFT and folded belt of Siwalik foreland basin strata define the narrow Sub-Himalayan structural belt (Hodges, 2000; DiPietro and Pogue,

2004). Linked fault-related folds bounded by emergent thrust faults verge toward the foreland in the Sub-Himalaya (Nakata, 1989; Mugnier et al., 1999; Lavé and Avouac, 2000; Champel et al.,

2002; Mugnier et al., 2004; Kumar et al., 2006). In the western and northwestern Himalaya, from the Kangra Reentrant westward into the Pakistan Himalaya, the Sub-Himalayan units form a wider zone of imbricate thrusting (Fig. 3.1) (Baker, 1987; Leathers, 1987; Pennock et al., 1989;

Thakur and Rawat, 1992; Powers et al., 1998; DiPietro and Pogue, 2004). Although a series of emergent thrusts and fault-related folds define the Sub-Himalayan belt, the southern most thrust is geomorphically the most active structure associated with surface rupture earthquakes in the

Kangra reentrant in India (Fig. 3.1) (Yeast et al., 1992; Delcaillau et al., 2006; Kumar et al., 2006.

Along the Salt Range in the Pakistan Himalaya (Fig. 3.1), the southernmost thrust of the Salt

Range thrust (SRT) is also documented as the most active structure (Baker et al., 1988; Yeats et al., 1992), but information of surface rupture earthquakes is unknown.

In the Kashmir Himalaya, the region of northwest India between the Kangra reentrant and the Pakistan Himalaya, the deformation front is not characterized by an emergent thrust (Nakata,

1989; Mugnier et al., 1999; 2004; Lavé and Avouac, 2000; Champel et al., 2002; Kumar et al.,

2006). A broad fold, the Suruin-Mastgarh anticline (SMA), defines the deformation front between the Ravi River and the India-Pakistan border (Figs. 3.1 and 3.2). A foreland-directed emergent thrust fault similar to the HFT does not bound the SMA and neither limb is cut by a fault at the surface. Active emergent thrust faulting within the thrust belt, however, occurs roughly 40 km north of the deformation front.

In this paper, we present new mapping, age data, and structural analysis of the Riasi thrust (RT) fault zone within the Kashmir Himalaya (Figs. 3.1 and 3.2). The RT fault zone represents one strand of a seismically-active emergent fault system that extends stepwise more than 200 km northwestward to the Balakot-Bagh fault (source of the Mw 7.6 2005 Kashmir earthquake in Pakistan Azad Kashmir; A. Hussain et al., 2009; MonaLisa et al., 2009). Whereas earlier studies recognize the Riasi thrust to have Quaternary fault activity (Awasthi et al., 1984;

Ram, 1984; Nakata, 1989; Ganjoo, 1990; S.H. Hussain et al., 2004; A. Hussain et al., 2009;

Thakur et al., 2010), little is known about the fault geometry, timing of thrusting, displacement rates, and paleoearthquake history. Understanding of the activity of this fault system constrains the degree to which India-Eurasia convergence is absorbed within the orogen that partitions shortening between the deformation front and internal faults such as the RT, and constrains the relative seismic hazards in the Jammu & Kashmir sector of the Himalaya.

3.3 GEOLOGIC SETTING AND LONG-TERM DISTRIBUTION OF HIMALAYAN

SHORTENING

High topography of the Himalaya, the Tibetan Plateau, and mountain ranges as far as north central Asia represent a broad zone of deformation resulting from the collision of the Indian plate with Eurasia (DeMets et al., 1990; Banerjee and Bürgmann, 2002; Bettinelli et al., 2006).

Continuous continental collision since the Eocene (e.g. Le Fort, 1975; Molnar and Tapponnier,

1977; Besse et al., 1984), has now created a curved plate boundary over ~2000 km in length from

Assam to Pakistan. The principal thrust faults root northward into the Main Himalayan thrust

(MHT), the plate boundary décollement between the Indian and Eurasian plates (Schelling and

Arita, 1991; Schelling, 1992, Pandey et al., 1995; DeCelles et al., 1998a; Powers et al., 1998;

Lavé and Avouac, 2000). A break-forward thrust sequence generally characterizes the pattern of thrusting in space and time for the major thrust sheets emplaced along the Himalayan plate boundary. In detail, however, distributed deformation where two or more thrusts are active in any given interval of time defines the thrust sequence. In the hinterland, the Main Central thrust

(MCT) or the Panjal thrust in the Kashmir Himalaya, places high-grade metamorphic rocks that underlie the High Himalaya against medium- to low-grade rocks of the Lesser Himalaya (Figs.

3.1 and 2.2) (Wadia, 1934; Le Fort, 1975; Thakur and Rawat, 1992; DiPietro and Pogue, 2004).

Age of major displacement for the MCT is constrained to be 22-20 Ma (DeCelles et al., 1998b), with evidence of reactivation during the Miocene-Pliocene (Nakata, 1989; Macfarlane, 1993;

Hodges et al., 1996; Harrison et al., 1997). Thrust sheets emplaced by the MCT are bounded to the north by the South Tibetan Detachment system (STD) or the Zanskar Shear Zone (ZSZ) in the

Kashmir Himalaya (DiPietro and Pogue, 2004; Yin 2006), similarly active during the early

Miocene with evidence of even earlier fault activity in the Eocene (e.g. Vannay and Hodges,

1996). South of the MCT, the Main Boundary thrust (MBT) or the Murree thrust in the Kashmir

Himalaya, marks the contact between Lesser Himalaya rocks and Tertiary to Quaternary strata of the Sub-Himalaya (Wadia, 1934; Thakur and Rawat, 1992; Valdiya, 1992; Hodges, 2000;

DiPietro and Pogue, 2004). Motion on the MBT developed by ~11-10 Ma (Meigs et al., 1995,

Burbank et al., 1996; Sangode and Kumar, 2003), and persisted until ~5 Ma (Kumar et al., 2003).

The southernmost and youngest splay from the MHT is the Main Frontal thrust or the

Himalayan Frontal thrust (HFT), and its western equivalent the Salt Range thrust (SRT) in

Pakistan (Hagen, 1956; Hodges, 2000; Yeats et al., 1992; Lavé and Avouac, 2000, DiPietro and

Pogue, 2004). A low-amplitude fold in the footwall of the HFT suggests that active faulting occurs 15 km farther south of the HFT locally (Piedmont fault of Yeats and Thakur, 2008). The

HFT juxtaposes Middle Miocene to Pleistocene Siwalik molasse deposits and Quaternary sediments of the modern Indo-Gangetic foreland basin (DeCelles et al., 1998; Valdiya, 2003;

Mugnier et al., 2004). Magnetostratigraphic and sedimentologic studies indicate that thrust- related folding associated with the HFT initiated during the Upper Siwalik deposition sometime between 1.77 and 1.5 Ma in Nepal and India (Burbank et al., 1996, Valdiya, 1998; Sangode and

Kumar, 2003 Kumar et al., 2006), and between ~6 and 1.6 Ma on the SRT of Pakistan (Johnson et al., 1986; Burbank and Raynolds, 1988; Baker et al., 1988). In detail, the HFT at the deformation front is not continuous along strike, but is marked by isolated emergent fault segments with evidence of surface-rupturing earthquakes during the Holocene (e.g. Powers et al.,

1998; Mugnier et al., 1999; Lavé and Avouac, 2000 Kumar et al., 2006; 2010).

Data demonstrate that larger proportion of India-Asia convergence absorbed in the

Himalaya is accommodated at the deformation front or thrust front from the Kangra reentrant southeastward into Nepal (Nakata, 1989; Powers et al., 1998; Lavé and Avouac, 2000; Valdiya,

2003; Yeats and Thakur, 2008). Active thrust activity may continue locally on the MCT (Nakata,

1 ; Wobus et al., 2003; 2005), however would represent ≤1 mm/yr of convergence given that most of shortening in the Himalaya can be accounted by the thrust front (Lavé and Avouac, 2000;

Mugnier et al., 2004). The Kashmir Himalaya is clearly different, as the thrust front is blind marked by frontal folds including the Suruin-Mastgarh anticline (SMA; Figs. 3.1 and 3.2), and because potentially a large percentage of the convergence is taken up by the Riasi thrust and related faults within the orogenic belt.

3.4 GEOCHRONOLOGIC METHODS

We employed a combination of terrestrial cosmogenic nuclides (TCN), optically stimulated luminescence (OSL), and radiocarbon methods to date geomorphic surfaces and associated deposits to establish the absolute timing of thrust activity and shortening rates of active structures.

Cosmogenic nuclide concentrations accumulated as the result of the interaction of cosmic rays with material at the Earth’s surface provide ages of stable geomorphic surfaces (Lal, 1 1;

Cerling and Craig, 1994; Gosse and Phillips, 2001). A key assumption of the TCN method is that geomorphic surfaces remain stable and neither accumulate nor lose material from the surface after abandonment. The concentration and production of in-situ cosmogenic nuclides is therefore a function of the time the surface has been exposed to cosmic rays up until the point that the system reaches secular equilibrium (ratio of parent to daughter isotopes remain constant because of production decay rate). TCN production for a stable geomorphic surface is greatest at the surface and decreases exponentially with depth. Nuances of this dating technique include 1) ability to constrain the pre-depositional nuclide concentration (so-called ‘inheritance’), and 2) ability to establish that the geomorphic stability of a surface to determinate the magnitude of post- abandonment changes (inflation, deflation). Dating geomorphic surfaces underlain by unconsolidated alluvium (e.g. a fluvial terrace) typically involves recovering a suite of samples from the surface to ~2 m depth (Anderson et al., 1996; Hancock et al., 1999; Repka et al., 1997).

The inheritance and surface modification can be deduced from the change in concentration of in-situ cosmogenic nuclides with depth. A non-zero concentration at the bottom

77 of the depth profile is a measure of the pre-deposition and burial acquisition of nuclides, which is normally taken to represent the inheritance. Post-depositional modification in soil thickness by inflation due to aeolian input or deflation due to erosion yields an older- or younger-than- expected age, respectively. Depending of the lithology of the parent material, measurements of in-situ concentration of cosmogenic nuclides typically utilize 10Be for quartz-rich sediments and

Independent data such as soil profile descriptions, constrain whether a surface has remained stable after abandonment.

Optically stimulated luminescence (OSL) dates the time elapsed since a deposit was last exposed to sunlight at the surface (Prescott and Hutton, 1994; Aitken, 1998). OSL targets sedimentary layers composed of silt to fine sand, which provides an alternative technique for dating fine-grained deposits. Sand-rich deposits retain the accumulation of charge within minerals due to the interaction of ionizing sunlight radiation with electrons. Illuminating the minerals will detrap the charge, which results in the measurable emissions of photons (luminescence).

3.5 RIASI THRUST FAULT ZONE – NOMENCLATURE AND STRUCTURE

This study focuses on the Riasi thrust (RT) fault zone in the region of the Chenab River near Riasi, India (Figs. 3.1 and 3.2). The RT shares structural and stratigraphic similarities with an en-echelon series of faults to the northwest (Ranga Rao and Datta, 1976; Iqbal et al., 2004;

S.H. Hussain et al., 2004), which includes the Kotli thrust (KT) and the Balakot-Bagh fault (BBF)

(Fig. 3.1). Key similarities include their position within the orogenic belt behind the deformation front, a hanging wall stratigraphic succession marked by a unique Precambrian limestone at the base of the section (the Muzaffarabad Limestone in Pakistan; the Sirban or Vaishno-Devi

Limestone Formation in India), Paleocene-Eocene limestone (Patala Formation in Pakistan;

Subathu Formation in India), and limited Siwalik strata in the hanging wall (see stratigraphic description in Fig. 3.3) (Wadia, 1937; Ranga Rao and Datta, 1976; Thakur and Rawat, 1992; S.H.

Hussain et al., 2004; Iqbal et al., 2004; A. Hussain et al., 2009).

Numerous names and interpretations of the relation of the RT to other faults in the northwest Himalayas appear in the literature. The Riasi thrust was first described in the region of the Chenab River (Fig. 3.1) (e.g. Wadia, 1937; Ranga Rao and Datta, 1976). The fault has been equated with the Main Boundary thrust by Wadia [1937] and termed the Mandili-Kishanpur thrust in Raiverman et al. [1983]. Several authors consider the RT to be part of a regionally- extensive fault system, which include the Balakot-Bagh, Kotli, Riasi, Mandili-Kishanpur,

Jawalamukhi, and other faults from northwest to southeast, respectively (Fig. 3.1). Names applied to the regional fault systems in literature include the Jammu thrust system (DiPietro and

Pogue, 2004) and the Medlicott-Wadia thrust (Thakur et al., 2010). We retain the original name of the Riasi thrust as defined at the Chenab River (Ranga Rao and Datta, 1976). Moreover, we limit our regional fault structural correlation to the Balakot-Bagh fault, Kotli thrust, and Riasi thrust because the same unique hanging wall stratigraphic succession characterizes each of the three thrust sheets.

Our mapping in the Chenab River study area reveals that in details, the RT consists of a fault zone with multiple strands (Fig. 3.4). The RT in this study hence refers to the Riasi thrust fault zone as a whole. The main strand of the RT fault zone is referred in our study area as the

Main Riasi thrust (MRT). At least two other fault splays south of the MRT have been mapped; the southernmost splay is hereby termed the Frontal Riasi Thrust (FRT).

3.6 QUATERNARY STRATIGRAPHIC UNITS AND GEOMORPHIC MAP

RELATIONS

The MRT underlies the Vaishno-Devi Mountains (Fig. 3.4), a major topographic step within the Sub-Himalayas (Figs. 3.1 and 3.5a). Hanging wall units include Precambrian (Sirban

Limestone Formation) to Oligocene–Miocene (Murree formations) bedrock. Footwall bedrock units primarily include the Miocene–Pliocene (Siwalik formations) and locally the Murree formations (Figs. 3.2 and 3.4). Local Quaternary conglomerates unconformably overlie all underlying bedrock units and are truncated or folded by fault splays south of the MRT, such as the FRT (Figs. 3.4, 3.6, 3.7). Quaternary fluvial terraces have incised both the hanging wall and footwall of the MRT and the FRT (Figs. 3.5 and 3.6).

Chenab River conglomerate (Cgm) – Hinterland-sourced conglomerate

At the village of Bidda, in the hanging wall of the MRT (Figs. 3.5a, 3.6, 3.7), roughly 20 m of unconsolidated gravel and sand cap an erosional surface above the Sirban Limestone below the Bidda Terrace (Fig. 3.8). Provenance of the boulder to cobble size conglomerates includes the

Murree Formation and hinterland sources (e.g. granite, metavolcanic rocks, schist, and gneiss)

(see Bidda Terrace depth profile in Fig. 3.9). We refer to these conglomerates as the Chenab

River conglomerate (Cgm) because of their source from upstream of an ancestral Chenab River drainage that once occupied the Bidda Terrace.

Fluvial gravel and sand of identical provenance than Cgm is locally preserved above an erosional strath in the Upper Siwalik Formation in the footwall of the MRT and truncated by the fault, in the Jyotipuram Canyon (Figs. 3.5b, 3.6, 3.7). Distribution of the Cgm fluvial deposits is limited spatially, and is easily recognized because of its characteristic purple to reddish brown weathering pattern, hinterland-sourced conglomeratic clasts, and near absence of limestone clasts

(Figs. 3.8 and 3.9). Deposits in the Jyotipuram Canyon in the MRT footwall are at least ~15 m thick, but were likely thicker being incised by younger fluvial terrace deposits above an angular unconformity.

Chenab River conglomerate (Cgm) occurs above strath contacts in the hanging wall and footwall of the MRT at elevations of ~730 m and ~460 m respectively, above the modern channel of the Chenab River (Fig. 3.7). Occasional ~30-100 cm thick micaceous sand lenses are intercalated within the Cgm conglomerate sequence in both sites. These lenses were targeted for

OSL sampling (Fig. 3.8).

Quaternary conglomerate (Qgm) – Sirban Limestone-sourced conglomerate

A massive Quaternary alluvium conglomerate unit (Qgm) blankets the southern base of the Vaishno-Devi Mountains (Figs. 3.4 and 3.6). Limestone-rich conglomerates dominate the provenance of Qgm, derived locally from the Precambrian Sirban Limestone Formation exposed mainly in the Vaishno-Devi Mountains. An unconformable contact marks the base of Qgm.

Siwalik and locally Murree Formations underlie the Qgm in the footwall of the MRT and local exposures reveal that Qgm overlies the Sirban Limestone Formation in the hanging wall (Figs.

3.4, 3.6, 3.7a). Map relations also indicate that Qgm incised the Chenab River conglomerate

(Cgm) in the footwall of the MRT (Figs. 3.6). Whereas Qgm is largely a younger unit than Cgm, both units may be partially coeval in their basal sequences (Fig. 3.10).

Lithologic variations exist in detail within Qgm due to sediment mixing between different locally sourced drainages. Exposures of Qgm along the Anji River and underlying terraces (Qat)

81 surrounding the town of Riasi (Fig. 3.4), for example, contain gravel sourced from the Murree,

Siwalik, and Sirban Formations. West of the Chenab River, along the mountain front to the NW

(Fig. 3.4), Qgm is largely dominated by Sirban Limestone sourced clasts.

Field relations in the study site along the Jyotipuram Canyon walls (Fig. 3.6), south of the

MRT, near the Chenab River; indicate that Qgm locally consists of multiple units on the basis of stratigraphic variations. Limestone-clast conglomeratic layers sourced from the Sirban Limestone

Formation with subordinate polylithic sandstone and conglomerate characterize the basal sequence of Qgm. Subordinate polylithic clast provenance with similar lithologies as within Cgm, is interpreted to represent recycling and erosion of the Cgm unit within Qgm. Sirban Limestone- clast conglomerate dominate the upward succession of Qgm with multiple fining upward sequences. Uppermost Qgm consists of well-cemented limestone clast conglomerate of a few meters thick, which indicates a degree of cementation likely associated with past soil development. Qgm sub-sequences in Jyotipuram Canyon interpreted as alluvial fan sequences and synorogenic strata, have total cumulative stratigraphic thickness of locally at least ~100 m.

Terraces

A series of erosional strath terraces in the MRT hanging wall formed on Sirban

Limestone Formation substrate. The highest and oldest terrace, the Bidda Terrace or T7 is a flat and relatively well-preserved surface, with a tread up to 1000 m long and 600m wide, that sits

350 m above the present river level (Figs. 3.5a, 3.6, 3.7). The Bidda Terrace overlies ~20 m of

Cgm (Fig. 3.8). Topographic and GPS elevation data, mapping, and field relations suggest that the terrace is essentially sub-horizontal (Figs. 3.5a and 3.6).

Four local younger geomorphic surfaces (T6, T5, T4, and T3) occur to the west and south at lower elevations than the Bidda Terrace (Fig. 3.6). The two highest and oldest of these four terraces (T6 and T5) are characterized by a bedrock strath contact with the Sirban Limestone

Formation in the hanging wall of the MRT. T6 is mapped as an undifferentiated unit,

82 characterized by a series of poorly preserved composite terraces. T6 contrasts to the Bidda

Terrace (T7) and its underlying Cgm deposit with thinner (<5m) and more heterogeneous hinterland-sourced and Sirban Limestone-sourced terrace deposits. Downslope, along the MRT fault trace, T5 is the highest of a series of well-preserved surfaces near a local drainage referred as the Jyotipuram Canyon (Fig. 3.6). T5 overlies ~25 m of depositional fill rich in Sirban

Limestone clasts, including at its base recycled hinterland-sourced Cgm clasts and a sand interval

(up to 5 m thick) sampled for OSL dating. Terrace stratigraphy deposit for T5 differs also from the Cgm unit due to the presence of Sirban Limestone gravels. T5 is characterized by carbonate- rich low silt content gravel-dominated soils, similarly to T4 and uppermost Qgm unit. Dip varies between 7-6 º for T5, interpreted to reflect the primary depositional dip (Fig. 3.6). T5 deposits unconformably overlie the MRT, the Sirban Limestone Formation in the hanging wall, and

Quaternary conglomerate units (Cgm, Qgm) in the footwall (Figs. 3.5b and 3.6). These relationships suggest that no fault activity occurred on the MRT near the Chenab River following the formation of T5. T5 has been sequentially incised southward by younger terraces (T4-T0), in which most terrace risers parallel the structural trend of the MRT and the FRT.

Sequentially younger T4 and T3 are characterized by thin terrace stratigraphy (<2 m) with very similar deposits to Qgm. In contrast to T5 characterized by a thick sequence of aggradational deposits in the footwall and overlying the MRT, abandonment of T5 and formation of T4-T3 developed during an overall period of incision with erosional surfaces set into Qgm

(Fig. 3.6). Both T4 and T3 are offset across the FRT, but can be correlated between the hanging wall (T4a and T3a) and footwall (T4b and T3b) (Figs. 3.5c, 3.6, 3.11). T3 has a low surface gradient of 2-3° in the hanging wall and footwall (Fig. 3.11). Soil stratigraphy of T3 differs from those at T5, T4 and the Sirban Limestone-clast rich upper Qgm unit. Soil profiles across T3 reveal evidence of similar terrace deposits in both the hanging wall and footwall, suggesting similar surface ages for T3 across the FRT. T3 is characterized by brown-red silt and gravel upper

83 soil horizons (0 m to ~1 m depth), overlying thinly bedded carbonate and matrix-dominated gravels (see soil profiles in Figure 3.12).

The lowermost terraces (T2, T1) observed in the terrace study sites of Figure 3.6, are primarily exposed south of the FRT and at lower reaches of the local drainages, which meet the

Chenab River. Minimum stratigraphic thickness for T2 and T1 is ~20 m and ~5 m, respectively, with unknown total depth of strath contacts buried beneath the active channel (T0). These young terraces (T2-T1) show no geomorphic evidence of offset surfaces. To the south of the FRT, near the town of Riasi (Fig. 3.4), correlative terraces to T2 (included as Qt generalized map unit) with nearly identical elevations are characterized with thinner deposits and strath contacts ~50 m above the modern channel, as observed by Thakur et al. [2010]. N-S morphology of T2 and correlative Qt terraces suggest that south of the FRT, increased river incision is related to the uplift of the SMA.

3.7 GEOMORPHOLOGIC AND STRATIGRAPHIC UNITS – AGE CONSTRAINTS

Cosmogenic surface exposure (TCN) and optically-stimulated luminescence (OSL) samples were collected to constrain the age of Cgm, Qgm, and the geomorphic surfaces to aid stratigraphic and geomorphic correlations across faults.

Age of hinterland-sourced (Cgm) and Sirban Limestone-sourced (Qgm) conglomerates

Chenab River conglomerate (Cgm) – Hinterland-sourced conglomerate

A thin ~30 cm micaceous sand lens was sampled from Cgm beneath the Bidda Terrace in the hanging wall of the MRT (Figs. 3.5a, 3.6, 3.7, 3.8). The stratigraphic position of the sample was ~15 m below the surface of the Bidda Terrace and ~5 m above the erosional strath of the

Sirban Limestone. The sample yielded an OSL age of 95.8±14.1 ka (Fig. 3.8; Table 3.1). At the

Jyotipuram Canyon (Fig. 3.6), the sample came from an exposure of the Cgm unit in the footwall of the MRT (Fig. 3.5b). Stratigraphic position of the footwall sample projects ~6 m above the

84 erosional strath contact of the Upper Siwalik Formation. Lithologically, the sample was nearly identical to the Bidda Terrace sample. A ~1 m sand lens intercalated by hinterland-sourced conglomerate yielded an OSL age of 87.0±14.2 ka (Table 3.1). We use the observation that the lithology and ages of the Cgm fluvial deposits beneath the Bidda Terrace and at Jyotipuram

Canyon (Fig. 3.6) are the basis for measuring the offset and rate of slip across the MRT (Fig. 3.7).

Quaternary conglomerate (Qgm) – Sirban Limestone-sourced conglomerate

Samples to constrain the age of Qgm come from various stratigraphic positions in the hanging wall of the FRT (Fig. 3.6), and along strike in the footwall of the MRT (Fig. 3.4). Along the Jyotipuram Canyon in the hanging wall of the FRT, Qgm samples from thin isolated 3-30 cm sand lenses within coarse-grained carbonate rich massive conglomerate yield ages of 54.7±10.6 and 47.7±7.5 ka (Fig. 3.6; Table 3.1). Dated samples originate from two different stratigraphic levels, and indicate an apparent inverse age relationship with stratigraphic position. For those specific samples, thin sand lenses collected adjacent to heterogeneous layer deposits (~55 ka: pebble rich; ~48 ka: clay-rich) could have affected the dose rate age results (Kenworthy et al,

2014; see Table 3.1; Appendix A). Along strike to the SE, near the trench site (Fig. 3.4), an OSL sample from unknown stratigraphic position of Qgm in the hanging wall of the FRT yielded an age of 84.9±11.2 ka (Table 3.1). Although we never see the base of the Qgm along the FRT, a sample location near the Anji River in the footwall of the MRT (Fig. 3.4), collected at the basal conglomerate sequence of Qgm above a strath contact with the Lower Siwalik Formation, yield an age of 78.0±11.5 kyr (Table 2.1). An age bracket of upper Qgm sequence is provided by dated

T5 basal terrace deposits set into Qgm in the Jyotipuram Canyon (Fig. 3.6), with an OSL age of

39.2±8.0 ka (see Terrace ages section). Given our limited ability to correlate sample sand lenses within Qgm, age spread of various localities at different stratigraphic positions is interpreted as the range of possible ages of Qgm between ~85 to 48 ka (Table 3.1). With respect to samples within our detailed terrace study sites in Figure 6, Qgm deposition (~55-48 ka and no younger

85 than ~39 ka) postdates Cgm deposition of ~96-87 ka subsequentially offset by the MRT (see Fig.

3.10). Deposition of Qgm conglomerates would essentially represent synorogenic deposits coeval with the most recent period of MRT fault activity 100-39 ka (Figs. 3.7 and 3.10).

Terrace ages

Bidda Terrace (T7) surface age

We used a depth profile to determine the abandonment age of the Bidda Terrace (T7) using cosmogenic nuclides. Samples from a soil pit constrain the variation in 10Be concentration from the surface to a depth of 2 m (Fig. 3.9). Surface-subsurface sampling of T7 and underlying

Cgm deposits from the Bidda Terrace (Fig. 3.6), combined with soil descriptions and modeling of the concentration as a function of depth allow for addressing the effect of inheritance on the age determination (Anderson et al., 1996; Hancock et al., 1999; Repka et al., 1997). Concentrations from samples of the upper 50 cm of the profile show evidence of mixing and/or surface erosion after deposition (points falls away from modeled curve). Below 50 cm the depth profile reveals an exponential decay of concentration with depth. Fit of a curve through these data models a surface age of 28±4 ka (Fig. 3.9). The concentration at 2m represents the inherited concentration (e.g.

Hancock et al., 1999), which is equivalent to ~2500 yrs.

Bidda Terrace age constraints – 10Be, OSL, soil analysis

Although our geochronology results would yield a reasonable age-sequence interpretation of when the strath was cut (>96-87 ka), followed by terrace aggradation of the Cgm deposits (96-87 to 28 ka) and terrace abandonment (28 ka), several other age constraints and geomorphological evidence indicate the surface age of Bidda Terrace is >>28±4 ka.

Assigning a 28±4 ka 10Be surface age for the Bidda terrace conflicts with dated stratigraphically younger deposits of Qgm (~55-48 ka) and T5 (~39 ka) that have older OSL ages

(Table 3.1). Specifically, OSL age of 39.2±8.0 ka for a well-constrained basal sequence of the T5

(Table 3.1), indicate that T5 terrace aggradation must postdate the abandonment and incision of the Bidda Terrace (see age summary diagram in Figure 3.10), sampled ~222 m lower in elevation in the Jyotipuram Canyon, overlying Cgm deposits in the footwall of the MRT and capping the fault trace above an undeformed strath contact (Figs. 3.5b, 3.6, 3.7).

An apparent younger abandonment age for the surface of the Bidda Terrace can result from processes of post-abandonment deflation or erosion that strips the upper portion of the terrace deposit. In the plot of cosmogenic nuclide concentration as a function of depth for the

Bidda Terrace (Fig. 3.9), the low Be10 concentration of the uppermost sample implies post- depositional surface modification of the upper <50 cm of the depth profile. Fitting an exponential curve to the lowest 4 samples would produce ages of ~20, 28, or 38 ka depending on how much inflation or deflation has occurred within the recent (<2000 yrs) past. A young age results from an inflation of ~20 cm in the depth profile due to human tilling or aeolian input. The ~28 ka age suggests negligible inflation or deflation since abandonment. The older age of ~38 ka, in turns, implies ~20 cm of deflation on the soil profile due to surface erosion. Analysis of the soil profile at the Bidda Terrace pit provides likely indications of surface erosion and anthropogenic soil deflation as 1) the uppermost soil horizon (0-20 cm) is a plowed Ap horizon directly above gravel and cobble rich Bt horizons (20-75+ cm); 2) silt or sand finer upper sequences are absent, which contrasts with younger terraces in our study area or others terraces of similar ages at other

Himalayan sites (Wesnousky et al., 1999; Lavé and Avouac, 2000), suggesting the finer sequences expected in upper terrace stratigraphy have been eroded (Fig. 3.11).

OSL age constraints for stratigraphically younger units (Qgm and T5), and evidence of post-abandonment modification and erosion on the surface of the Bidda Terrace, indicate an abandonment age older than ~39 ka (T5 deposits), and likely older than ~55-48 ka (Qgm) for the surface of the Bidda Terrace. We therefore bracket the Bidda Terrace age between ~96-39 ka, defined as no older than the Cgm deposits (~96-87 ka) above the strath contact (maximum) and

87 no younger than the T5 terrace deposits (~39 ka) capping Cgm in the footwall of the MRT

(minimum). Hence 10Be depth profile age result of 28±4 ka is interpreted not representative of the

Bidda Terrace abandonment age.

Terrace T5

A ~5 m thick sand and gravel rich interval sampled at the base of the T5 terrace in the

Jyotipuram Canyon, sits above a sub-horizontal strath capping the fault trace of the MRT, the

Sirban Limestone in the hanging wall, and older conglomerate units (Cgm, Qgm) in the footwall

(Fig. 3.6). The stratigraphic position of the sample is ~23 m below T5 surface and ~2m above the strath contact. The sample indicates an OSL age of 39.2±8.0 ka (Table 3.1). Using this age constraint, latest displacement on the MRT at the Jyotipuram Canyon locality predates terrace formation of T5 no older or younger than ~39 ka. Age results of the T5 deposits (~39 ka) capping the fault trace and offset Cgm deposits (~96-87 ka) together define a constrained period of fault activity of the deformation of the Cgm marker unit between ~96-87 and 39 ka for MRT near the

Chenab River.

Dugalla Terrace (T3) – FRT Geomorphic surface age constraints

We sampled the Dugalla Terrace (T3) in both the hanging wall and footwall of the FRT.

Only optically stimulated luminescence (OSL) samples yielded successful results. OSL samples came from sandy intervals within the terrace deposits rich in locally-derived Sirban-Limestone clasts, 0.75 m to 1.90 m below the T3 surface (Fig. 3.12), avoiding any potential effects of surface partial bleaching. The hanging wall (T3a) and footwall (T3b) terraces are assumed correlative because 1) of their identical surface gradients (Fig. 3.11), 2) similar degree of soil development and terrace capping gravels (Fig. 3.12), and 3) nearly coeval OSL ages data for the terrace deposits presented below.

At the Dugalla Terrace two of four soil pits (pit 1 and 4; Fig. 3.6) display consistent stratigraphy of correlating terrace across the FRT. OSL samples from 0.75 m depth in the footwall and 1.9 m depth hanging wall pits (Figs. 3.6 and 3.12) yield ages of ~7 to 14 ka, respectively (see age summary diagram in Figure 3.10; Table 3.1). Footwall pit was sampled twice, with ages of 7.6±1.1 ka and 6.7±1.1 ka (Table .1), to check reproducibility of age data for the same depositional layer, yielding a mean age of 7.1±1.1 ka. OSL samples were collected from near-surface deposits, which provide a conservative maximum surface age of T3 terrace. Thus, the abandonment age is expected equivalent or younger than the youngest OSL age (7.1±1.1 ka) sampled from T3 terrace deposits.

Temporal variation between the footwall pit (7.1±1.1 ka) and the hanging wall pit

(14.2±2.3 ka) may have resulted from several possible geological factors as such. 1) The dated samples were collected from different depositional intervals within the terrace stratigraphy of T3 at 75 cm and 190 cm depths in pit 1 and pit 4 respectively (Fig. 3.12), and thus yielding different ages. Depth correlates with age, in which the shallower and younger deposit (pit 1) is more representative of the T3 surface age. 2) Even though interpreted as the same terrace T3, the depositional history of the terrace stratigraphy could have also been slightly variable, where incremental sediment inputs partly bypasses certain areas of the fluvial system while others remain active at certain times, but then eventually becomes amalgamated into a single terrace and abandonment surface. These minor spatial and temporal variations within the terrace stratigraphy could be furthermore nearly undistinguishable to identify from the present geomorphologic of surface T3. In such scenario, the mean age range of ~7-14 ka combined from both samples is a conservative temporal representation of the terrace unit T3 given our data. 3) OSL age variation could in theory reflect effects of inheritance (overestimate) or partial bleaching (underestimate)

(Prescott and Hutton, 1994; Aitken, 1998), depending on the conditions that the dated deposits were either partially unreset during the depositional process or partially reset following burial.

However, little studies exist demonstrating OSL dating technique to either overestimate or underestimate surface ages, when correlated with other dating techniques. A recent study has in fact shown, that OSL age data were i) invariant through depth, not affected by partial bleaching induced by potential soil penetrating sunlight, and ii) consistently younger than TCN dating techniques using CL36, indicating significant effects of inheritance of the latter (Le Dortz et al.,

2011). 4) Finally, the hanging wall and footwall T3 terraces represent two temporally separate fluvial deposits. Although, we cannot refute that possibility, the combination of similar age data within uncertainties (see Table 3.1), similar characteristic upper terrace and soil horizons across the fault (Fig. 3.12), and geomorphic surfaces separated vertically by a fault scarp (Figs. 3.6, 3.5c,

3.11), provide strong supporting evidence for our interpretation of an offset and coeval geomorphic surface T3.

Including the full range of OSL ages from both footwall and hanging wall T3 terraces delineates an age range between 7.1±1.1 ka as the minimum and 14.2±2.3 ka as the maximum

(see age summary diagram in Figure 3.10). However, our preferred interpretation discussed above in point 1) provides the strongest argument that T3 surface age is younger or coeval than youngest OSL age of 7.1±1.1 ka. Given our age results for the offset terrace T3, define a constrained period of fault activity between ~14 ka and 0 for the FRT, or ~7 ka and 0 for our preferred age model.

3.8 RIASI FAULT ZONE STRUCTURAL RELATIONSHIPS

The Riasi thrust fault zone, as defined near the Chenab River area, is at minimum ~60 km long and extends ~20 km northwest and ~40 km southeast of the Chenab reentrant (Fig. 3.2).

Folded terraces and Quaternary outcrop exposures reveal that at least two discrete faults with quantifiable offset markers comprise the Riasi thrust fault zone in the Chenab River region (Fig.

3.4). In details, the thrust zone includes the northern main strand of the RT or the Main Riasi thrust (MRT), the Frontal Riasi thrust (FRT), and one or more inferred blind fault splays in

90 between (Figs. 3.4 and 3.6). The largest stratigraphic separation is across the MRT. Branch points mapped east of the Chenab River demonstrate that the ~8 km-long FRT and the other local faults are splays of the MRT (Fig. 3.7).

Main Riasi thrust (MRT)

The MRT juxtaposes Precambrian Sirban Limestone Formation against either folded

Siwalik Formation or Quaternary conglomerates (Figs. 3.5b and 3.7). Average strike of the

Sirban Formation in the hanging wall is NW-SE to E-W, with primary dips of 45-50° N to NE close to the MRT, decreasing to ~20-30° further north and up-section (Fig. 3.13). Widespread steep secondary dips (>50°) reflect hanging wall folding or secondary intra-formation faulting. In the footwall, limited bedrock exposures show steep (60-85° SW) to overturned Siwalik and

Murree Formations capped by generally less steeply dipping (<20° SW) conglomerate units (Cgm and Qgm) (Figs. 3.4, 3.5b, 3.6, 3.7). Fault dip, as measured in the field is 45-50° northeast to north-dipping reverse fault and strikes NW-SE to E-W (Figs. 3.4 and 3.6). Information of fault cut-off angles along the MRT are only available in the footwall, with dips of 85° SW to 80°NW in the steeply dipping to overturned folded Siwalik Formation bedrock, and sub-horizontal for

Quaternary gravel units (Cgm, Qgm) (Fig. 3.4). At the Jyotipuram Canyon, in the footwall of the

MRT, Cgm indicates ~25° NE dips as an apparent fault cut-off angle (Fig. 3.5b); however, the same beds are observed to be sub-horizontal only a few tens of meters south of the MRT (Fig.

3.7). Local exposure of the MRT is recognized in outcrop scale of a few meters wide, with foliated cataclasite layers and fault gouge with the Sirban Limestone in the hanging wall, and in the footwall truncated and tilted Cgm unit (Fig. 3.5b). Given uncertainty in the pre-thrusting stratigraphic thickness, minimum stratigraphic separation across the MRT, by thrusting

Precambrian Sirban Limestone onto Upper Siwalik Formation, is as much as ~14 km (Figs. 3.3).

Mapped fluvial terraces, perched as high as 350m above the Chenab River, are developed in bedrock and Quaternary gravels (Cgm, Qgm) with sub-horizontal inclinations in the hanging

91 wall and footwall of the MRT (Figs. 3.5a, 3.6). Vertical separation of 272±5 m is indicated by the difference in elevation of the offset basal Cgm deposits above a strath contact below the Bidda

Terrace (elevation 735m) in the hanging wall of the MRT, and correlative footwall deposits in the

Jyotipuram Canyon (elevation 463m) (Figs. 3.6 and 3.7). We assign a 5 m uncertainty to this estimate of vertical separation of Cgm based on the irregularity of the basal contact.

Topographically below the Bidda Terrace, terrace T5 and its underlying ~39 ka terrace deposits unconformably overly the MRT fault trace. A possible fault scarp and fault related uplift is suggested by a ~30 m step between T5 and T4 on the north and south sides of the Jyotipuram

Canyon (Fig. 3.6). This inferred footwall fault splay of the MRT indicates that faulting on the

Riasi thrust zone persisted after the formation and abandonment of terrace T5 (maximum age ~39 ka). Such an inferred footwall splay of the MRT may correlate with local exposures of other closely spaced faults south of the MRT, along strike to the southeast, near the Mari trench site

(Fig. 3.4).

Frontal Riasi thrust (FRT)

Unlike the MRT, the FRT is not exposed at the surface in most localities, although a topographic step, offset terraces, and isolated exposures of tilted Qgm mark its location. The FRT is only recognized within the reentrant of the Chenab River, and if parallels the NW-SE to E-W fault strike map pattern of the MRT (Fig. 3.4). Along strike to the southeast, the FRT merges with the main strand of the RT (Fig. 3.4). The FRT places tilted Qgm on subhorizontal Qgm, which are capped locally by terraces (Figs. 3.5c, 3.6). An anticlinal fold is generally expressed in the hanging wall of the FRT (Figs. 3.5c and 3.12). Structural relationships of the FRT are best seen in

Jyotipuram Canyon (Figs. 3.6 and 3.12). Dip of folded Qgm strata increases from 6° to 61° to the southwest towards the fault trace in the hanging wall (Fig. 3.12). Sub-horizontal dip characterizes

Qgm in the footwall. A northeast-dipping thrust fault (24-30˚) offsets Qgm and overlying

92 terraces. Differential uplift of terrace T3 (21.6±1 m) and terrace T4 (~30 m) demonstrates fault activity south of the MRT after the formation of terrace T5.

Topographic profiles constrain the vertical separation (Vs) of T3 across the FRT fault scarp (Fig. 3.11). Relative high-resolution elevation data were collected via differential GPS and total station surveying. T3 is characterized by a southwesterly gradient of 2-3° both in the hanging wall (T3a) and footwall (T3b). The topographic step mapped as the FRT scarp coincides with localized folding and faulting of Qgm and overlying T3 (Figs. 3.5c and 3.11). A topographic high in the profile ~125 m upslope from the fault trace is associated with ~25° NW- tilted Qgm suggesting that a secondary blind back-thrust to the FRT formed a popup block in the hanging wall.

Vertical separation (Vs) is determined by assuming that a 2.2° topographic gradient represents the depositional gradient of T3. The difference in elevation of T3 across the FRT measured orthogonally to the regional surface slope thus represents the Vs. Vertical separation is

12.0±1 m if the back-thrust popup block is disregarded and 21.6±1 m from the crest of the popup block to the FRT footwall (Fig. 3.11). We interpret the latter as the representative estimate of Vs to account for all deformation related to FRT.

A vertical separation of 21.6±1 m represents a minimum estimate for the total offset of

T3; given that the elevation reference utilizes modern measured surface elevation and does not take into account surface erosion. Evidence for erosion includes tilted gravel beds exposed in the

FRT fold (Figs. 3.5c and 3.11), and the lack of preservation of well-developed correlative soil strata (pit 2) at the topographic crest of the offset T3 surface (Fig. 3.12).

Fault activity for FRT prior to T3

Although constrained displacement along the FRT postdates the terrace deformation of the Dugalla Terrace (<7-14 ka), map relations indicate that the FRT accommodated shortening

93 prior to 14 ka, possibly coeval with fault activity of the MRT (see kinematic model in Figure

3.15). An angular unconformity between folded Qgm and T3 surface in the hanging wall of the

FRT (Fig. 3.5c and 3.11), suggests fault-related folding prior to T3 terrace formation. No visible synorogenic growth that can be attributed directly to dated Qgm deposits in the same area (~55-

48 ka). Deeper buried sections of Qgm may show instead potential synorogenic growth or clearer angular unconformities associated with older generations of fault movements of the FRT, including other minor fault splays of the MRT (Figs. 3.6 and 3.7).

Structural summary

The MRT steep ramp (45-50º) mapped at the fault trace (Figs. 3.4 and 3.6), is interpreted to project at depth into a shallow dipping (~25º) thrust (Figs. 2.7 and 2.13), given that regional dips in the Sirban Limestone, Subathu, and Murree Formations shallow from 50º to 20º to the north in the MRT hanging wall (Fig. 3.13). Steep fault dip of the MRT main strand is interpreted to have resulted from tilting induced by fault displacement by the structurally lower FRT and possibly footwall folding in the Siwalik Formation (Fig. 3.15a). Activity on the MRT and FRT, including footwall folds, likely overlapped in time. Given that the Bidda Terrace is sub- horizontal and untitled, suggests that fault rotation induced by motion on the structurally lower

FRT occurred prior to terrace formation (Fig. 3.15b), and that MRT displacement persisted after rotation to 45-50° (Fig. 3.15c). Figure 3.14 delineates the following kinematic sequence: 1) slip on a ~25º-dipping MRT transfers to the structurally lower fault of the FRT stepping to the south, in which slip incrementally rotates the overlying MRT to a high fault angle (~50º); 2) erosional strath development and aggradation of Cgm; 3) abandonment of T7 and the FRT remains an active fault with renewed or continued slip on the MRT as out-of-sequence thrusting, offsetting the Cgm unit and overlying surface.

3.9 QUATERNARY SHORTENING AND PALEOSEISMOLOGY

Unconformities at the contacts of the terraces and Quaternary deposits demonstrate that folding, thrusting, and shortening across the MRT and FRT was ongoing prior to, during, and after Quaternary conglomerate deposition and terrace formation. Flat-lying hinterland-sourced fluvial conglomerate (Cgm) capped by the Bidda terrace (T7) sits above a depositional contact with 45-50° NE-tilted Sirban Limestone in the MRT hanging wall (Fig. 3.6). Similarly, folded

Siwalik Formation bedrock unconformably overlain by less deformed Cgm capped by Qgm and terraces marks the footwall of the MRT. At the FRT, folded Qgm strata capped by young terraces are faulted and offset along a fault scarp. Combining the uplift, offset, and shortening constraints from field and age data allows for measurement of the rate of deformation of offset markers units or terraces.

MRT shortening rate

Differential topographic relief between dated Cgm located 5m above a strath beneath the

Bidda Terrace in the hanging wall, and dated Cgm located 6m above a strath in the Jyotipuram

Canyon in the footwall constrains the vertical component of slip on the MRT (Fig. 3.6). Vertical separation is 272±5 m for the Cgm offset marker unit between the two sites (Fig. 3.7).

Presumably subsidence and deposition characterized the footwall as slip accrued and the hanging wall incised during MRT motion. Subsequent footwall tilting on the MRT accompanied fault- related folding on the FRT and other fault splays.

Lithological characteristics and OSL ages of Cgm deposits at the Bidda Terrace and

Jyotipuram Canyon support our interpretation that age-equivalent deposits are exposed at both sites. Age data from the two sites are 95.8±14.1 kyr and 87.0±14.1 kyr from hanging wall and footwall samples, respectively. Combining the ages, the vertical separation of 272±5 m and the

45-50˚-fault dip thus allows for the shortening and fault slip rate to be quantified. An uplift rate of 3.8-2.4 mm/yr, a slip rate of 5.4-3.1 mm/yr, and a shortening rate of 3.8-2.0 mm/yr are thus determined for the MRT (Table 3.2). These estimates assume the MRT at this locality has been

95 continuously active since ~96 ka. In the Jyotipuram Canyon, however, undeformed T5 terrace deposits capping the MRT fault trace, implies MRT activity ceased by ~39 ka. For motion on the

MRT after formation and abandonment of the Bidda Terrace (~96-39 ka), and prior to formation of terrace T5, an uplift rate of 10.8-3.3 mm/yr, a slip rate of 15.3-4.3 mm/yr, and a shortening rate of 10.8-2.8 mm/yr (Table 3.2) are indicated.

FRT shortening rate

The FRT deforms and folds gravels and geomorphic surfaces that are younger than ~7-14 ka (Table 3.1). The topographic profile of terrace T3 demonstrates structural uplift of 21.6±1 m, which includes fault-related deformation of the FRT and the back-thrust popup (Fig. 3.11). Given field measured FRT 24-30° fault dip, structural rates are calculated using the minimum and maximum age range for terrace T3, which translates to an uplift rate of 3.8-1.2 mm/yr, slip rate of

9.3-2.5 mm/yr, and shortening rate of 8.5-2.2 mm/yr (Table 3.2). Given that the dated subsurface deposits overestimate the terrace abandonment age, the abandonment age for T3 is likely close to or younger than the shallowest and youngest 7.1±1.1 ka OSL age. Thus, the rates are minimum estimates.

FRT paleoseismology

Along strike of the study sites at the Dugalla Terrace and Jyotipuram Canyon, the FRT is well delineated by a topographic break separating tilted and uplifted Qgm strata in its hanging wall with sub-horizontal geomorphic surfaces and lower relief in its footwall. One location, the

Mari paleoseismic trench sites (Fig. 3.4), was selected to quantify recent fault activity and last surface rupture of the southern and youngest splay of the RT fault zone (FRT), including information of the long-term earthquake cycle.

One of two paleoseismic trenches (T1) across the FRT exposed a distinct angular unconformity separating ~25° south-dipping, polylithic cobble and boulder alluvium (Unit 2)

96 from unconsolidated limestone-clast conglomerate and breccia (Unit 1) with ~5° south depositional dips (Fig. 3.13a). Calibrated calendar C-14 ages from detrital charcoal constrain the age of this unconformity to ~4,500 yrs old (Fig. 3.13b). Relations at the bottom of T1 suggest that the tip of the FRT occurs a few meters below the maximum depth of our excavator. Steeply dipping strata of Unit 3 cut by the low-angle thrust of the FRT and unconformably overlain by relatively undeformed Unit 2 strata were revealed in Trench T2.

Contact trench relations and C14 ages reveal that the last surface rupture earthquake on the southern splay of RT fault zone occurred no later than ~4.5 ka. No evidence for younger surface ruptures or slip events were observed for either trenches, that could be associated with a postulated 1555 earthquake in the Kashmir Himalaya (Ambraseys and Jackson, 2003; Ambraseys and Douglas, 2004; Bilham et al., 2010).

Riasi thrust fault zone (MRT + FRT)

In map view, the FRT joins the MRT along strike to the southeast at a branch point indicating that two faults merge at depth as a single fault (Figs. 3.4, 3.7, 3.13). Any documented geological displacements on the Riasi thrust (RT) fault zone characterized by multiple strands

(RFT+MRT) approximate fault activity of the fault zone along strike characterized by a single strand (MRT). Therefore, recent deformation on the FRT indicates that the fault activity persisted on the RT fault zone along strike into Holocene time.

We infer that ~25° N-NE thrusting characterizes the thrust at depth consistent with regional dip data of the Sirban, Subathu, and Murree Formations, if the MRT is a hanging wall flat (see regional cross section in Figure 3.13). Summing the amount of shortening on the two faults gives the post 100 ka shortening rate. Shortening rate for each strand is 10.8-2.8 mm/yr for the MRT and 8.5-2.2 mm/yr for the FRT. Shortening rate average over the last ~100 ka ranges of

10.8-2.2 mm/yr (6.5±4.3 mm/yr) for the RT fault zone.

This range in average shortening rate depends critically on the age uncertainties. Hence, using only the younger age constraints for each of the depositional marker units would yield higher average shortening rates. For the MRT, the Cgm unit dated as ~87-96 ka provide the offset marker unit and not the Bidda Terrace surface age (28±4), given other age constraints indicate terrace abandonment is older than ~39 ka (T5 deposits) and 48-55 ka (Qgm). Although, the younger OSL age of 87.0±14 ka can characterize the basal Cgm deposit age, yielding a higher average rate of 10.8-3.2 mm/yr, no strong geological evidence exist to exclude any of sample ages for Cgm. For the FRT, the Dugalla Terrace (T3) dated as ~7-14 ka provides the offset marker unit. In this case, geological context indicates that OSL sampling targeted subsurface deposits of various stratigraphic levels, representing maximum ages for terrace abandonment

(T3). Consequently, the stratigraphically younger subsurface OSL sample dated as 7.1±1.1 ka can be interpreted as more representative age constraint. In that respect, the FRT yields a higher shortening rate of 8.5-4.4 mm/yr, translating into a higher combined average of 10.8-4.4 mm/yr or 7.6±3.2 mm/yr for the RT fault zone in the last 100 ka.

3.10 REGIONAL TECTONICS AND DISCUSION

Our study provides constraints on the Quaternary shortening rates across faults on the

MRT and FRT. Where the Riasi thrust fault zone is characterized by two or more faults

(MRT+FRT), the shortening of 10.8-2.8 mm/yr is partitioned on the MRT between ~96-39 ka and approximately lowered to 8.5-2.2 mm/yr after ~39 ka when slip was transferred to the FRT to the south. Summing the average shortening rates for the two fault strands yield 10.8-2.2 mm/yr

(6.5±4.3 mm/yr). Using younger age constraints for the offset marker units would yield a higher average rate of 10.8-4.4 mm/yr (7.6±3.2 mm/yr).

Although rates overlap with uncertainties between the MRT and FRT, one explanation for the apparent decreasing average shortening rates in the Chenab River area, could result from increasing southward slip partitioning on the frontal folds, including the Suruin-Mastgarh

98 anticline (SMA) at the deformation front (Figs. 3.1 and 3.2). It is unknown whether shortening rates varied over the last 100 ka for the frontal folds, however, these structures could potentially have absorbed increasing India-Eurasia convergence correlating with the HFT, one of the youngest thrusts of the Himalayan fold-thrust belt (e.g. Nakata, 1989; Yeats et al., 1992).

Along strike thrusts, such as the Kotli thrust to the northwest and the Mandili-Kishanpur thrust to the southeast (Ranga Rao and Datta, 1976; Figs. 3.1 and 3.2), are likely characterized by

Quaternary fault activity, however, shortening rates for these faults remain unknown. To the south, no other surface faults were observed with clear evidence of recent deformation between the FRT and the SMA, although numerous examples of elevated strath surfaces exist and Siwalik bedrock is extensively deformed south of the FRT (Fig. 3.4). Abandoned straths and exposed bedrock between the FRT and the SMA are interpreted to reflect regional exhumation due to combination of uplift of the limbs of the frontal fold, and displacement of the MHT (see regional cross section in Figure 3.13). Whether partitioning of MHT slip between the SMA and faulting on the RT fault zone varied in space and time is not known.

An independent study found higher MRT uplift rates of 10-18 mm/yr (Vignon, 2011), translating into 10-18 mm/yr of shortening rate, and an average of 14 mm/yr. These higher

Pleistocene rates arise from assuming that their cosmogenic surface age for the Bidda Terrace

(22.5-35 ka) represents the true abandonment age. Moreover, a total vertical differential relief

(~350 m) separation across the MRT was assumed to be between the Bidda Terrace and a projected buried depth of correlative deposits south of the MRT. This contrasts with our 2725 m vertical separation based on correlation of dated Cgm deposits in the hanging wall and footwall of the MRT, avoiding any highly interpretative vertical component from other fault splays. Although the age assignments and vertical separation measurements differ between the two studies, our upper rate for the MRT (10.8 mm/yr) overlaps only with presumed lower estimate of shortening rate of 10 mm/yr, given Vignon’s [2011] lower uplift rate estimate of 10 mm/yr.

Long-term versus Pleistocene-Holocene shortening rates – How high must they be?

To compare Pleistocene-Holocene and long term shortening rates, we use a regional cross section and known estimates of fault activity across the Kashmir Himalayas (Fig. 3.14). Given information of basement cover interface, stratigraphic thickness and dip data, geometry of the RT thrust sheet consists of moderate to steeply dipping strata in a hanging wall flat against truncated and folded footwall strata with a minimum vertical throw of ~6 km (Fig. 3.14). However, exhumed Precambrian Sirban Limestone strata in the upper plate of the RT geometrically restores below the stratigraphic levels of the Murree and Subathu Formations, and at minimum, to a deeper ramp along the MHT, projected to lie north beneath the Pir Panjal Range (see supplemental info on Appendix 3.B). Restoring the Sirban Limestone to the MHT ramp yields a minimum estimate of ~50 km southward horizontal displacement on the RT. Additional shortening associated with an interpreted duplex in order to fill a structural space beneath elevated Murree and Sirban Limestone strata, indicate a total of ~70 km of shortening. Age of onset of thrusting for the RT is not known. However, we can assume that the RT has been active since ~5 Ma, when fault activity on the MBT to the north is thought to have slowed down (Kumar et al., 2003), and the onset of southward prograding conglomerate facies is observed in the Upper

Siwalik Formation (Burbank et al., 1986; Burbank et al., 1988). Direct constraints such as the appearance of Sirban Limestone clasts in the Siwalik Formation do not exist. Given our estimates of horizontal displacement (~50-70 km) and onset of the RT, translates into an extrapolated long term a shortening rate of ~10-14 mm/yr since 5 Ma.

South of the RT fault zone, in the Chenab River area, lie three fold structures, including the SMA, deforming Siwalik and Murree strata (Figs. 3.1, 3.2, 3.13). The folds are not bounded by any surface faults, underlain by interpreted blind thrust splays due to MHT slip (Figs. 3.13).

Given stratigraphic thickness and map relations of the deepest exhumed units (Lower Siwalik) at the fold axis of the SMA, the Murree strata at depth constitute cores of folds, interpreted as

100 duplexes to fill space problems beneath structures (Fig. 3.13). Depth constraints and ~2.5 N regional dip for the basement cover interface are taken analogous to known nearby regions of

Kangra reentrant, constrained by well data and reflection profiles (Powers et al., 1998). The onset of uplift and folding of the Siwalik and Murree strata at the deformation front is commonly defined by the formation of the frontal structure, and in Kashmir, the SMA (Fig. 3.1). A decline in sedimentation rates in the Upper Siwalik Formation on the southern limb of the SMA suggest the onset of folding occurred after ~1.7 Ma (Ranga Rao et al. 1988). Line length measurements of folded Siwalik and Murree strata and the SMA provide estimates of long-term shortening rate.

Estimates of total horizontal shortening are ~21 km for all frontal folds deformation, and ~12 km for the SMA, based on structural interpretations of duplexes and wedge thrusts in the cores of detachment-like folds south of the RT (Fig. 3.14; see Chapter 2). Lower Siwalik or Murree

Formations represent stratigraphic reference horizons in our measurements of shortening. If we assume a 1.7 Ma onset of tectonic uplift for the folds at the deformation front (Ranga Rao et al.,

1988), line length estimates yield shortening rates for the frontal folds and the SMA of ~12 mm/yr and ~7 mm/yr. However, using an older age of initiation of thrusting at ~3 Ma for the deformation front documented by the thermochronologic data (Gavillot et al., 2013; see Chapter

4), indicates lower shortening rates of ~7 mm/yr and ~4 mm/yr for the frontal folds and SMA, respectively.

Using our regional cross section, long term shortening rates estimated from the RT (~10-

14 mm/yr) match fairly well to our upper estimates of short-term shortening rates from both the

MRT (10.8 mm/yr) and FRT (8.5 mm/yr) using offset Quaternary units and fluvial terraces. This comparison suggests that our higher estimates of shortening rates for the MRT and FRT are more representative for the average rate of the RT fault zone (10.8-8.5 mm/yr). Similarly, our estimate of long-term shortening rate for the frontal folds (~5mm/yr), including the SMA (~3 mm/yr), closely replicates results from geomorphic studies using uplifted Pleistocene-Holocene fluvial

101 terraces along the SMA with estimates of <6 mm/yr (Vignon 2011; see Chapter 3). Given similarities between long- to short- term shortening rates for the SMA, we assume our estimates for all frontal folds are representative for deformation over Pleistocene-Holocene timescales.

Comparisons of plate, geodetic and geologic convergence rates.

Convergence rate of southern Tibet with respect to India as measured by geodetic data constrain the amount of India-Asia convergence absorbed by faults and folds within the Himalaya

(Jade et al., 2004; Banerjee and Bürgmann, 2002; Bilham et al., 1997; Schiffman et al., 2013).

This geodetic convergence is assumed to represent interseismic strain accumulation on the basal décollement, estimating the percent of India-Asia convergence absorbed within the Himalaya.

Such estimate allows for comparison with the geological rates and identification of slip deficits on faults, slip partitioning between active faults, and short- versus long-term patterns of shortening within the thrust belt (Schiffman et al., 2013; Lavé and Avouac, 2000; Wesnousky et al., 1999; Kumar et al., 2006).

In central Nepal, for example, geodetic measurements for the central Himalayas range between 192.5 to 20.52 mm/yr (Argus and Gordon, 1991; Bettinelli at al., 2006; Bilham et al.,

1997). Geological shortening rates on the HFT since Miocene-Pliocene range from 14 to 24 mm/yr (Mugnier et al., 2004), and 19 to 21 mm/yr during the Holocene (Lavé and Avouac, 2000).

Comparable geologic and geodetic rates in Nepal suggest that nearly all of the India-Eurasia convergence absorbed in the Himalaya is accommodated by the HFT (Lavé and Avouac, 2000).

Along strike to the northwest in India, geodetic convergence rate measurements range from 141 to 18.83 mm/yr in the western Himalaya (Banerjee and Bürgmann, 2002; Jade et al., 2004).

Geologically determined shortening on the HFT over Miocene to recent timescales range from 4 to 16 mm/yr (Powers et al., 1998; Kumar et al., 2001; Kumar et al., 2006; Malik and Nakata,

2003; Malik et al., 2010), and suggest on average at least ~60% of the Himalayan shortening in western Himalaya is accommodated across the HFT at the deformation front.

102

Our constraints on shortening rate allows for similar geologic to geodetic comparisons.

India-Asia plate convergence rates at the longitude of the Kashmir Himalaya range from ~29 to

43 mm/yr (Bettinelli et al, 2006; Avouac et al., 2006; Mohadjer et al., 2010). Of the ~29 to 43 mm/yr convergence, estimates of geodetic convergence across the Kashmiri Himalaya range from

11.1 to 18.8 mm/yr (Jade et al., 2004; Schiffman et al., 2013).

Known active faults and folds in the Kashmir Himalaya that accommodate convergence from south to north, include frontal folds in the footwall of the RT fault zone (i.e. the SMA), the

RT and equivalent faults along strike, and active faults on the southwest side of the Kashmir basin (Fig. 3.1) (Gavillot et al., 2010b; Hebeler, 2010; Madden et al., 2010; Thakur et al., 2010;

Vignon, 2011; Meigs et al., 2012; this study). The sum of the Pleistocene-Holocene shortening rates for the frontal folds (5.9-4.7 mm/yr) plus our preferred higher rates for the RT (10.8-4.4 mm/yr) plus Kashmir basin faults (<<1 mm/yr) equals 17 to 9.5 mm/yr, which falls within the range of the geodetically-constrained convergence of 18.8 to 11.1 mm/yr across the Kashmir and northwest Himalaya. Thus frontal folding plus internal deformation within the Kashmir

Himalaya can account for nearly all of India-Eurasia convergence absorbed in the Himalaya.

Our results indicate that the RT is the largest structure in the Kashmir Himalaya accommodating on average (~60%) of the Himalayan shortening via surface faulting and not the frontal folds (~40%), which includes the SMA (~20%) at the deformation front. Although subsurface structural geology beneath the SMA remains poorly constrained, a major emergent thrust south of the SMA is not needed (see Fig. 3.15), given that the RT can account for most of the shortening absorbed in the Kashmir Himalaya.

Compared to the western and central Himalaya, although the magnitude of Tibet-India convergence differs, the sum of the percentage of Himalayan shortening absorbed by active faults and folds in the Kashmir Himalaya remains essentially the same. An important difference between the central and Kashmir Himalaya is active distributed shortening on upper plates faults

103 that coincides with local basement structures below the MHT to the north of the deformation front (Raiverman et al. 1983). A marked structural reentrant forms at the Chenab River (Figs. 3.1 and 3.2). Basement topography may contribute to the focusing of displacement on the RT instead of the deformation front in the Kashmir Himalaya.

Earthquake potential on the RT

Discovery of internal surface-rupturing reverse faults such as the RT and earthquakes like the 2005 Mw 7.6 earthquake on the Balakot-Bagh fault (BBF) in Pakistan Azad Kashmir

(Kumahara and Nakata, 2006) indicates that seismic sources within the NW Himalayas include the MHT (the basal décollement) and upper plate faults. Located ~200 km to the northwest of the

RT, the BBF with shortening rate estimates of 1.4-4.1 mm/yr (Kaneda et al., 2008), is a ~100 km fault system in nearly identical structural setting as the RT with unique Precambrian limestone in its hanging wall. The RT mapped at minimum ~60 km long, could generate a similar Mw earthquake event or greater than the BBF, by incorporating along-strike fault rupture of the adjoining fault strands of the Mandili-Kishanpur thrust (MKT) to the southeast and/or the Kotli thrust (KT) to the northwest, delineating a total potential fault rupture as long as 150 km and 250 km respectively (Fig. 3.1).

A recent paleoseismic trench across the BBF scarp indicates two constrained events at

500 and 2200 yr BP in an exposed stratigraphic section of ~4 ka, translating into a ~2 kyr recurrence interval for Mw similar to the 2005 Kashmir earthquake (Kondo et al., 2008).

Comparatively, our paleoseismic trench site on the RT also indicates a millennial recurrence interval with its last event at ~4.5 ka. Lack of historical earthquakes that can be attributed to any active faults in the Kashmir Himalaya west of the 2005 Mw 7.6 event in Pakistan Azad Kashmir, suggest a large seismic deficit slip exist for the Indian region of Kashmir (Schiffman et al., 2013).

That seismic slip deficit could be accommodated in part by the RT, but paleoseismic data remain

104 still too limited to indicate whether more than one event in the Kashmir Himalaya is sourced from the RT or adjoining fault segments.

Consequently, the southern state of Jammu&Kashmir, including especially the area near the town of Riasi, can be characterized to have potentially high seismic hazards. For instance, an ongoing tunnel construction for the Jammu-Srinagar railway passes close to or crosscuts the RT fault zone (Fig. 3.4), stressing the need to implement earthquake resistant infrastructure.

Constraints of the seismogenic zone in Kashmir Himalaya

No known evidence of active surface faults is found to north of the Balapora fault (BF)

(<< 1 mm/yr; Madden et al., 2010; Meigs et al., 2012) on the northern side of the Pir Panjal

Range (Fig. 3.1), except much further north of the Himalayan range such as the Karakoram fault in the Ladakh region. A width of ~30-50 km for the seismogenic zone, from the deformation front

(SMA) to the surface projection of the basal RT ramp, delineates a minimum extent of the potentially fully locked zone. The RT represents one of the main structures that appear to periodically release seismic strain on millennial recurrence intervals, and with shortening rates as high as 10.8 mm/yr for the last 100 ka. Given evidence of faulting on the BF, the Pir Panjal

Range, an area of high topography topping ~4000m in elevation (Fig. 3.1), may essentially represents the locking line, delineating a ~100 km wide zone, in which to the north, strain is accumulated in the decoupled creeping section. However, given our regional cross section, earthquakes could be also be generated along the MHT décollement predicted at a depth >12.5 km north of the Pir Panjal Range (Fig. 3.14).

3.11 CONCLUSION

New mapping and geochronologic data demonstrate that internal deformation of the

Kashmir Himalaya in the Chenab River area occurs at least on two fault strands of the Riasi fault

105 system with Quaternary displacements, including one with evidence of Holocene surface rupture.

The main strand of the RT fault zone, the Main Riasi thrust (MRT), places Precambrian Sirban

Limestone on folded and truncated Pleistocene unconsolidated fluvial conglomerates (Cgm or

Qgm). The MRT is a 45-50° northeast-dipping reverse fault, which underlies the Vaishno-Devi

Mountains (Figs. 3.4-3.7). A southern and frontal strand, the Riasi frontal thrust (FRT), cuts and folds Quaternary alluvial gravel deposits (Qgm). The FRT is a northeast dipping low angle thrust fault (24-30˚), and represents a fault splay kinematically linked to the MRT at depth. The FRT and MRT root into the main Himalayan décollement (Figs. 3.7 and 3.13).

A Pleistocene fluvial terrace of the Chenab River, the Bidda Terrace (T7), sits 350m above the modern river in the hanging wall of the MRT. Underlying the surface of the Bidda

Terrace, consists of a distinctive Chenab River conglomerate (Cgm) overlying a strath contact with tilted Sirban Limestone. Cgm in the footwall constrains the vertical separation to 272±5 m.

OSL age results from the hanging wall and footwall units yield ages of 95.8±14.1 kyr and

87.0±14.2 ka, respectively (Table 3.1). Undeformed younger basal terrace deposits of T5 with an

OSL age of 39.2±8.0 kyr unconformably overlie the MRT. Together, these contact and age relations constrain fault activity for the MRT between ~96-39 ka (Fig. 3.10), which yields a shortening rate of 10.8-2.8 mm/yr (Table 2).

South of the MRT, the 25° north-dipping FRT deforms and folds alluvial gravel deposits

(Qgm) and fluvial terraces. The Dugalla Terrace (T3) has been uplifted, offset and warped along the FRT by 21.6±1 m (Fig. 3.11). A suite of OSL age data from T3 terrace subsurface deposits sampled in the hanging wall and footwall of the FRT indicate an abandonment age of ~7-14 ka

(Table 3.1), yielding a shortening rate of 8.5-2.2 mm/yr (Table 3.2). A preferred interpretation for the T3 surface age uses the youngest and shallowest dated sample of 7.1±1.1 ka. Fault activity for the FRT recorded in the offset T3 terrace is constrained to be between ~7ka - 0, yielding a

Holocene shortening rate of 8.5-4.4 mm/yr (Table 3.2).

106

Along strike, the MRT and FRT merge at a branch line (Fig. 3.4), which implies that the two strands join to form a single 25° N-NE dipping fault at depth, consistent with regional dip data in the RT thrust sheet (Figs. 3.7 and 3.13). Fault activity on the RT fault zone persisted from

100 ka to the present, although where the fault zone consists of multiple strands, the slip was partitioned onto the MRT from ~100 to 39 ka, then onto footwall splays and FRT after 39 ka.

Fault activity on the FRT was likely coeval to MRT prior to 100 ka, given angular unconformities of deformed bedrock, and kinematic constraints that necessitate MRT footwall tilting and out-of- sequence displacement (Fig. 3.14). Fault displacements on the MRT (10.8-2.8 mm/yr at ~96-39 ka timescale), and FRT (8.5-4.4 mm/yr at ~7-0 ka timescale), indicate continuous fault activity on the RT fault zone since ~100 ka.

Averaging Pleistocene-Holocene shortening rates of the MRT and FRT (Table 3.2) yield a range of 10.8-4.4 mm/yr (7.6±3.6 mm/yr). Using regional cross constraints (Fig. 3.13), our upper estimates of short-term shortening rates are consistent with a long-term rate of ~10 mm/yr for the RT fault zone. With the same exercise, long-term rates for structures south of the RT are

~5mm/yr for the frontal folds, including ~3 mm/yr for the Suruin-Mastgarh Anticline (SMA) at the deformation front. The sum of shortening rates of the RT and the frontal folds can account for nearly all of geodetically-constrained India-Tibet convergence of 11.1-18.8 mm/yr absorbed in

Northwest and Kashmir Himalaya (Jade et al., 2004; Schiffman et al., 2013). Compared to western and central Himalaya, the largest active structure in the Kashmir Himalaya is the RT accommodating on average (~60%) of the Himalayan shortening via surface faulting and not the frontal folds or the deformation front.

107 the interseismic locked zone, which delineates a zone of potentially high seismic hazards for the

Kashmir Himalaya, can be constrained to be at least ~30-50 km from the deformation front

(SMA) to the RT ramp, but likely extends ~100 km to the surface projection of an interpreted mid-crustal ramp along the MHT underlying the Pir Panjal Range (Fig. 3.13)

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Table 3.1 OSL age results

Optically Stimulated Luminescence Age Information1 Sample Depth Num. of Dose Rate De3 ± 2 USU num. OD4 (%) OSL age ± 2 (ka) num. (m) aliquots2 (Gy/ka) (Gy)

IB- 10 USU -582 12.5 20 ( 35) 2.76 ± 0.15 264.2 ± 28.27 20.1 ± 4.5 95.79 ± 14 .14

IDP -1 USU -583 30 21 ( 42) 2.99 ± 0.16 260.0 ± 33.08 23.6 ± 5.3 86.97 ± 14 .17

IDP -6.1 USU -818 25 24 ( 55) 2.83 ± 0.16 110.8 ± 17.50 33.4 ± 6.3 39.18 ± 7 .37

IDP -3 USU -584 0.7 24 ( 36) 1.87 ± 0.10 14.22 ± 1.405 24.6 ± 4.2 7.59 ± 1 .06

IDP -2 USU -586 0.7 29 ( 39) 1.88 ± 0.13 12.59 ± 1.645 31.5 ± 4.6 6.71 ± 1 .13

IDP -5 USU -817 1.9 23 ( 50) 0.98 ± 0.06 13.98 ± 1.715 39.8 ± 6.7 14.25 ± 2 .27

IDP -7 USU -829 5.5 22 ( 35) 0.44 ± 0.04 24.02 ± 3.675 27.4 ± 5.5 54.69 ± 10 .63

IDP -8 USU -816 30 23 ( 43) 2.42 ± 0.14 115.6 ± 13.59 24.8 ± 4.7 47.74 ± 7 .46

IAC -6.2 USU -825 15 20 ( 29) 1.21 ± 0.07 94.42 ± 9.93 21.1 ± 4.1 77.97 ± 11 .46

ITC -9 USU -826 20 23 ( 36) 1.74 ± 0.09 148.2 ± 12.40 17.2 ± 3.4 84.95 ± 11 .21 1 Age analysis using the single-aliquot regenerative-dose procedure of Murray and Wintle (2000) on 1-mm small-aliquots of quartz sand 2 Number of aliquots used in age calculation and number of aliquots analyzed in parentheses 3 Equivalent dose (De) calculated using the Central Age Model of Galbraith et al. 1999 except where noted otherwise 4 Overdispersion (OD) represents variance in De data beyond measurement uncertainties, OD >20% may indicate significant scatter due to depositional or post-depositional processes 5 De calculated using the Minimum Age Model of Galbraith et al. 1999

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1 2 1 3.8 2.9 2.8 6.8 8.5 2.2 5.3 8.5 4.4 6.4 ±0.9 ±4.0 ±3.2 ±2.1 Shtr (mm/yr) Shtr Shtr (mm/yr) Shtr (mm/yr) Shtr Shtr (mm/yr) Shtr 1 1 9.3 2.5 5.9 3.4 Slpr Slpr Slpr Slpr 3. 5.38 4.25 1.13 15.3 4.34 9.81 5.46 9.26 5.02 7.14 2.12 1 1 Ur Ur Ur Ur 3.8 2.4 3.1 0.7 3.3 7.1 3.7 3.8 1.2 2.5 1.3 3.8 2.5 3.1 0.6 Min Min Min Min Max Max Max Max verage verage verage verage Range Range Range Range A A A A uncertainty uncertainty uncertainty uncertainty Ages in italics correspond to youngest or or youngest to correspond italics in Ages

(ka) T R fset marker units. Using these parameters, are calculated calculated are parameters, these Using units. marker fset f f M f 39.18±7.97 Shut o Shut 7.1±1.1 7.1±1.1 sed. age (Ky) age sed. sed. age (Ky) age sed. surf. age (Ky) age surf. surf. age (Ky) age surf. 86.96±14.17 86.96±14.17 W W W W F F F F - sed. age (Ky) age sed. sed. age (Ky) age sed. surf. age (Ky) age surf. (Ky) age surf. 14.25±2.27 95.79±14.14 95.79±14.14 W W W W H H H H and RFT and T R fset, fault dip, and range of ages for hanging wall and footwall o footwall and wall hanging for ages of range and dip, fault fset, f 27±3 27±3 47.5±2.5 47.5±2.5 Fault Dip (degrees) (degrees) Dip Fault (degrees) Dip Fault Fault Dip (degrees) (degrees) Dip Fault (degrees) Dip Fault fset (m) fset (m) fset fset (m) fset (m) fset f f f f 272±5 272±5 21.6±1 21.6±1 (96 ka - 0) - ka (96 ka) 39 - (96 (14 ka - 0) - ka (14 0) - ka (7 T T T T R R able 3.2 able Min vertical o vertical Min o vertical Min Min vertical o vertical Min o vertical Min T Shortening rates calculation for the M the for calculation rates Shortening M o vertical fault of values Measured M RF uncertainties. corresponding with (Shtr) rates shortening and (Slpr), rates slip (Ur), rates uplift average max, min, unit. marker a for constraints age possible oldest RF

132

Appendix A OSL does rate information age model interpretatio ns

Dose Rate Information1 2 Sample Grain size H2O Cosmic num. USU num. ( m) (%) K (%) Rb (ppm) Th (ppm) U (ppm) (Gy/ka) IB-10 USU-5823 90-150 2.8 1.60 ± 0.04 88.2 ± 3.5 12.0 ± 1.1 1.3 ± 0.1 0.05 ± 0.01 IDP-1 USU-583 90-150 1.9 1.77 ± 0.04 106.0 ± 4.2 12.1 ± 1.1 1.7 ± 0.1 0.02 ± 0.00 IDP-3 USU-584 90-180 1.7 0.90 ± 0.02 52.6 ± 2.1 6.9 ± 0.6 1.5 ± 0.1 0.17 ± 0.02 IDP-2 USU-5864 75-180 1.6 0.90 ± 0.09 52.6 ± 5.3 6.9 ± 0.7 1.5 ± 0.2 0.17 ± 0.02 IDP-8 USU-8165 90-150 10.8 1.50 ± 0.04 97.6 ± 3.9 11.4 ± 1.0 1.6 ± 0.1 0.02 ± 0.00 IDP-5-T3 USU-8175 150-250 1.9 0.51 ± 0.01 22.0 ± 0.9 2.7 ± 0.2 0.7 ± 0.1 0.15 ± 0.02 IDP-6.1 USU-818 90-150 1.1 1.47 ± 0.04 109.5 ± 4.4 13.9 ± 1.3 1.7 ± 0.1 0.02 ± 0.00 IAC-6.2 USU-825 90-150 0.4 0.67 ± 0.02 42.4 ± 1.7 4.0 ± 0.4 1.0 ± 0.1 0.04 ± 0.00 ITC-9 USU-826 75-150 0.0 1.19 ± 0.03 52.1 ± 2.1 3.8 ± 0.3 1.2 ± 0.1 0.03 ± 0.00 IDP-7 USU-829 75-250 0.2 0.21 ± 0.01 8.5 ± 0.3 0.9 ± 0.2 0.4 ± 0.1 0.10 ± 0.01 1 Radioelemental concentrations determined by ALS Chemex using ICP-MS and ICP-AES techniques, dose rate is derived from concentrations by conversion factors from Guerin et al. 2011 2 In-situ gravimetric water content, assumed 3±3% for moisture content to represent burial history for values <3% 3 Th:U ratio is >3SD of USU dose rate samples and may have possible dose rate disequilibrium 4 Dose rate derived from chemistry data from USU-584, errors increased to 10% to reflect dose rate derived from different sample 5 Dose rate calculated from an average of two grain sizes

Equivalent dose (De) Distributions: Probability density functions and radial plots

1. USU-582, IB-10

USU-582 ! "#$%&$'()$* +,- . /+012 +$'()$* +,- * +,-$13)$+00%0-

0 100 200 300 400 500 De (Gy)

133

2. USU-583, IDP-1 USU-583 CAM ! "#$%&$'()$* +,- . /+012 +$'()$* +,- * +,-$13)$+00%0-

-200 0 200 400 600 De (Gy)

3. USU-584, IDP-3 USU-584 ! "#$%&$'()$* +,- CAM . /+012 +$'()$* +,- * +,-$13)$+00%0- MAM

0 10 20 30 40 De (Gy)

4. USU-586, IDP-2 USU-586 CAM

MAM

! "#$%&$'()$* +,- . /+012 +$'()$* +,- * +,-$13)$+00%0-

0 20 40 60 De (Gy)

5. USU-817, IDP-5-T3

134

USU-817 ! "#$%&$'()$* +,- CAM . /+012 +$'()$* +,- * +,-$13)$+00%0- MAM

-20 0 20 40 60 80 De (Gy)

6. USU-829, IDP-7 USU-829 ! "#$%&$'()$* +,- . /+012 +$'()$* +,- CAM * +,-$13)$+00%0- MAM

0 20 40 60 80 De (Gy)

7. USU-816, IDP-8 USU-816

! "#$% &$'()$* +,- . /+012 +$'()$* +,- * +,-$13)$+00%0- CAM

0 100 200 300 De (Gy)

8. USU-818, IDP-6.1

135

USU-818 ! "#$%&$'()$* +,- . /+012 +$'()$* +,- CAM * +,-$13)$+00%0-

-100 0 100 200 300 De (Gy)

9. USU-825, IAC-6.2 USU-825 ! "#$%&$'()$* +,- . /+012 +$'()$* +,- * +,-$13)$+00%0- CAM

0 50 100 150 200 De (Gy)

10. USU-826, ITC-9 USU-826 ! "#$%&$'()$* +,- CAM . /+012 +$'()$* +,- * +,-$13)$+00%0-

0 100 200 300 De (Gy)

136

137

CHAPTER 4:

Late Cenozoic – Quaternary exhumation history of the Kashmir Himalaya from the

Siwalik-Murree belt to the High Himalaya using (U-Th/He) thermochronometry.

Gavillot, Y., A. J. Meigs, D. Stockli, D. Yule, M. M. Malik

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4.1 ABSTRACT

Apatite and zircon (U-Th)/He cooling ages are used to quantify the late Cenozoic to Quaternary exhumation pattern associated with fault activity across the Kashmir Himalayas. Here we present thermochronological data from thirty samples, totaling of 74 individual single-grain apatite and zircon (U-Th)/He dated aliquots. Cooling age data were collected from (1) molasse sediments of the Murree and Siwalik formations exhumed by active faults and folds in the Sub-Himalayan belt

(the deformed foreland) and from (2) metasediments and plutonic rocks exhumed in the hinterland thrust sheets. Apatite (U-Th)/He (AHe) cooling ages for the molasse sediments are consistently younger than the source sediment indicating that Sub-Himalayan belt samples are reset. Mean cooling age data based on single grain populations from each sample range from ~1–

10 Ma. Single grain and mean age probability density plots reveal a period of rapid cooling and exhumation between 1.9–3.2 Ma throughout the Sub-Himalaya. Estimates of exhumation rates for the sum of Sub-Himalayan structures are 2.8-2.2 mm/yr. In the hinterland, in the Pir Panjal

Range, samples from the hinterland MBT and MCT thrust sheets yield AHe cooling ages >~5

Ma, in which three samples haves ages between 4.7–7.2 Ma, likely coeval with fault-related cooling on the MBT. Further to the north in the hinterland, published AFT coeval young cooling ages (< 3 Ma) and high exhumation (2.8-1.3 mm/yr) across the Kishtwar Window, >100 km north of the deformation front, suggests tectonic-driven rapid exhumation across the orogenic wedge, coincides with localities of predicted changing ramp geometry and/or active orogenic growth.

Zircon (U‐Th)/He (ZHe) samples from the hinterland are younger than the ages of the metasedimentary or plutonic source rocks. Probability density plots of hinterland ZHe data show a pronounced spike in cooling between ~16–20 Ma, a period where MCT motion is well documented throughout the Himalaya. ZHe mean sample and single-grain age population from

Sub-Himalayan samples contains a nearly identical cluster of 17–22 Ma. Cooling patterns across the Kashmir Himalayas do not correlate with precipitation, suggesting climate forcing is

139 decoupled from erosion and exhumation. Distributed and coeval thrusting rather than localized break-forward deformation characterizes fault-related exhumation for the orogenic wedge development in the Sub-Himalayan belt since at least ~2-3 Ma.

4.2 INTRODUCTION

In fold-thrust belts, break-forward thrust sequences, defined by the pattern of sequential thrust emplacement from the hinterland to the foreland (e.g. Dahlstrom, 1969; Price, 1981; Boyer and Elliott, 1982; Suppe, 1983), commonly characterize the long-term kinematic evolution of orogenic growth in many well-studied mountain belts (i.e. Southern Appalachians, Canadian

Rockies, Swiss Alps, Sevier fold-thrust belt). However, in detail, some fold-thrust belts (i.e.

Taiwan, Eastern Precordillera, Kashmir Himalaya) show evidence of deformation and major surface-rupture earthquakes within the orogen, north of the active deformation front (Ji et al.,

2001; Lee et al., 2001; Kaneda et al., 2008; Rockwell et al., 2013). Thus an emerging image for fold-thrust belts is that the pattern of active thrusting is temporally and spatially variable, and can be distributed on two or more active structures at any given time across an orogenic wedge.

The Himalaya, an iconic mountain belt with extensive crustal deformation, is characterized by a series of south-verging thrust splays emplaced in sequence at 20-22 Ma for

Main Central thrust (MCT; DeCelles et al., 1998b), ~10-11 Ma for Main Boundary thrust (MBT;

Meigs et al., 1995, Burbank et al., 1996), and <3 Ma for the Main Frontal thrust or Himalayan

Frontal thrust (HFT) at the active deformation front (Burbank et al., 1996, Valdiya, 1998;

Mugnier et al., 1999; Sangode and Kumar, 2003 Kumar et al., 2006; Van der Beek et al., 2006).

In Nepal, the active thrust front accommodates most of the plate convergence absorbed in the

Himalaya fold thrust belt, releasing stored interseismic strain via fault surface-rupture during moderate to large earthquake events (e.g. Bilham et al., 1997; Lavé and Avouac, 2000; Lavé et al., 2005; Mugnier et al., 2004; Sopkota et al., 2013). Active thrusting within the orogen on the

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MCT remains a matter of debate.

Two competing interpretations characterize the modern pattern of orogen-scale deformation across the width of the fold-thrust belt. Correlation of young cooling ages with the topographic pattern in the crystalline rich hinterland is interpreted as evidence of recent fault- related exhumation, including out-of-sequence thrusting near the MCT (e.g. Wobus et al., 2003;

2005; Hodges et al., 2004; Blythe et al., 2007). Alternatively, similarity between the geologic shortening rates of the HFT and geodetic rates inferred for the Main Himalayan thrust (MHT), the plate-boundary basal décollement (Schelling and Arita, 1991; Schelling, 1992, Pandey et al.,

1995; DeCelles et al., 1998a; Powers et al., 1998; Lavé and Avouac, 2000; Hodges et al., 2000), suggest that nearly all convergence is accommodated at the deformation front (Bilham, 1997;

Lave and Avouac, 2000, Mugnier et al., 2004). Hinterland deformation and erosion observed in thermochronological and morphological data is thought to primarily reflect geometry of a mid- crustal ramp in the MHT (e.g. Brewer and Burbank, 2006; Bollinger et al., 2006; Robert et al.,

2009).

Structural architecture of the Kashmir Himalaya, defined as the region between the Ravi

River in India and the Jhelum River in Pakistan, differs from the central and western Himalaya. A frontal fold characterizes the deformation front with no evidence of an emergent thrust. Similar to

Taiwan, surface-rupture earthquakes occur north of the deformation front. Within the Sub-

Himalaya of Kashmir, active distributed shortening occurs on structures ~40 km to ~150 km north of the deformation front, on the Riasi thrust and Balakot-Bagh fault respectively (e.g.

Kaneda et al., 2008; Kondo et al. 2008; Gavillot et al., 2010b). In the High Himalaya hinterland, evidence of active thrusting is unknown. The northern splay of the Main Central thrust (MCTII;

Searle and Goodin, 2003) has no correlative in the Kashmir Himalaya, except locally across the

Kishtwar Window (Fig.4.1), a locality characterized by young cooling ages (Kumar et al., 1995).

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Given evidence of distributed coeval thrusting across the Sub-Himalaya in Kashmir, competing interpretations of the mechanisms driving Himalayan exhumation can be tested. This study explores whether exhumational variability can be attributed to climate forcing (Thiede et al., 2004, 2005, 2009; Deeken et al., 2011, Adlakha et al., 2013) or tectonic forcing (Burbank et al., 2003; Blythe et al., 2007). Here we present low-temperature thermochronological (U-Th)/He data from the Kashmir Himalaya that extend from the deformation front in the foreland to the Pir

Panjal Range south of the Kashmir basin in hinterland (Fig. 4.1). Our data set comprises 15 apatite (AHe) and 16 zircon (ZHe) samples, including 74 individual single-grain dated aliquots.

Importantly, our study provides the first data set of reset detrital cooling ages of foreland molasse sediments in the Kashmir or western Himalaya. Combining these new data with previously published low-temperature data from neighboring areas, we constrain the cooling history and thrust belt kinematics in ~18 000 km2 area across the Kashmir Himalaya. Exhumation pattern of the Kashmir Himalaya orogen primarily reflects the structural kinematics along the MHT.

Precipitation gradients do not correlate with pattern of cooling ages in the Kashmir Himalaya.

4.3 GEOLOGIC SETTING

The Himalaya mountain belt developed through a series of south verging thrusts, formed during continental collision between the India and Eurasia plates (e.g. Le Fort, 1975; Molnar and

Tapponnier, 1975; 1977; Besse et al., 1984). All major Himalayan thrusts root at depth into the

Main Himalayan thrust (MHT), the plate boundary basal décollement along which Indian basement underthrusts Tibet (Schelling and Arita, 1991; Schelling, 1992, Pandey et al., 1995;

DeCelles et al., 1998a; Powers et al., 1998; Lavé and Avouac, 2000; Hodges et al., 2000).

The Himalayan fold-thrust belt is commonly divided into four tectonostratigrphic units separated by major regionally extensive thrusts (e.g. Gansser, 1964; Hodges, 2000; DiPietro and

Pogue, 2004). Along the outer deformation belt, the Sub-Himalaya (SH) consists of Cenozoic strata, including the Siwalik and Murree molasse units, bounded to the south by the Himalayan

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Frontal thrust (HFT) (Fig. 4.1). The HFT is the most external and youngest surface expression of the MHT propagating in the Indo-Gangetic foreland basin and accommodates active shortening at the deformation front (Hagen, 1956; Hodges, 2000; Yeats et al., 1992; Mugnier et al., 1999; Lavé and Avouac, 2000, DiPietro and Pogue, 2004). Towards the north in the hinterland, the Lesser

Himalaya (LH) is composed largely of pre-Tertiary low-grade metasediments in the hanging wall of the Main Boundary thrust (MBT). To the north, high-grade High Himalaya crystalline (HHC) rocks are exposed between the Main Central thrust (MCT) and the South Tibetan Detachment system (STD), or the Zanskar Shear Zone (ZSZ) (Fig. 4.1). Both the STD and ZSZ carry

Paleozoic Tethys Himalaya (TH) strata, and represent the northernmost boundary between the

Himalayan thrusts and the Tibetan zone (Le Fort, 1975; Hodges, 2000; DiPietro and Pogue,

2004).

Timing of major thrust emplacement across the Himalaya is characterized by a break- forward thrust sequence. Motion on the MCT initiated by 22-20 Ma (DeCelles et al., 1998b), and the MBT developing by ~10-11 Ma (Meigs et al., 1995, Burbank et al., 1996; Sangode and

Kumar, 2003). Both thrusts have evidence of activity or reactivation into the Pliocene (Nakata,

1989; Macfarlane, 1993; Hodges et al., 1996; Harrison et al., 1997; Kumar et al., 2003). For the

HFT, an onset of thrust-related folding is proposed at ~1.77-1.5 Ma constrained by paleomagnetic and sedimentological studies (Burbank et al., 1996, Valdiya, 1998; Sangode and Kumar, 2003;

Kumar et al., 2006), or ~2-3 Ma from thermochronological data (Van der Beek et al, 2006; this study).

Thermochronology data have now been widely used across the Himalaya to document space-time crustal exhumation related to both tectonic and surface processes. In the central

Himalaya and parts of western Himalaya, past work focused in the hinterland units of crystalline units in the Lesser and High Himalayan units due to their more favorable lithologies for thermochronometers. North of the MBT, data generally indicate cooling history (>5 Ma), older

143 than that of the Sub-Himalaya units of Nepal (~2 Ma) (Copeland et al., 1991; McFarlane, 1993;

Arita et al., 1997; Wobus et al., 2003; Van der Beek et al., 2006). However, dense data sets across or near the MCT indicate a pattern of young cooling (2-4 Ma; as young as <1 Ma), high exhumation rates, and sharp age gradients (Copeland et al., 1991;Wobus et al., 2003; 2005;

Hodges et al., 2004; Huntington and Hodges, 2006; Blythe et al., 2007; Patel and Carter, 2009).

Young cooling ages near the MCT correlate with topographic and precipitation gradients, which have been interpreted to be the result of active faulting (Wobus et al., 2005; Thiede et al, 2004;

2005; 2009; Deeken et al., 2011). Others argue that orographic precipitation and surface processes are decoupled from tectonic-driven erosion and exhumation (Burbank et al., 2003;

Blythe et al. 2007, Patel and Carter, 2009). Deformation associated with overthrusting and crustal underplating along the basal décollement has been proposed as a mechanism to explain the cooling age pattern within the interior of the Himalayan orogen (Bollinger et al., 2006; Brewer and Burbank, 2006; Whipp et al al., 2007).

4.4 ARCHITECTURE OF THE KASHMIR HIMALAYA

Along strike, the Kashmir Himalaya, display differences in the structural and stratigraphic architecture when compared to central and western Himalaya. The orogenic front is characterized by a wide Sub-Himalayan belt (Hodges, 2000; DiPietro and Pogue, 2004), which consists of a thick succession of Oligocene-Pliocene Murree formations and Siwalik formations foreland strata (Fig. 4.2). Map relations and regional cross sections (lines A-A’ and B-B’; Fig.

4.3), indicate stratigraphic thicknesses of 2-3.6 km for the Murree Formations and ~6 km for

Siwalik units, consistent with known maximum unit thicknesses in Kashmir (Karunakaran and

Ranga Rao, 1976; Thakur and Rawat, 1992; Powers et al., 1998; Shah, 2009). Active distributed shortening occurs on major faults north of the HFT within the Sub-Himalaya, with evidence of

Holocene to modern surface-rupture earthquakes on the Riasi thrust and Balakot-Bagh fault

(Kaneda et al., 2008; Kondo et al. 2008; Gavillot et al., 2010b; see Chapter 3).

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From south to north, a broad fold, the Suruin-Mastgarh anticline (SMA), defines the deformation front (Fig. 4.2). A foreland-directed emergent thrust fault similar to the HFT in central and western Himalaya does not bound the SMA, and neither limb is cut by a fault at the surface. Active emergent thrust faulting within the Sub-Himalaya, however, occurs roughly 40 km north of the deformation front, on the Riasi thrust (RT) and its along-strike correlative the

Mandili-Kishanpur thrust (MKT). Further north, additional fault splays of the RT includes the

Tanhal thrust (TT) and Mundun thrust (MT). Deepest stratigraphic levels are generally exposed in the hanging wall or fold axis of the various structures. The RT shares structural and stratigraphic similarities with an en-echelon series of faults to the northwest, which includes the Kotli thrust

(KT) and the Balakot-Bagh fault (BBF) (Fig. 4.1; Ranga Rao and Datta, 1976; Iqbal et al., 2004;

S.H. Hussain et al., 2004). Key similarities include their position within the orogenic belt behind the deformation front and unique hanging wall stratigraphic successions marked by thick (~5 km)

Precambrian limestone and Paleocene limestone (Ranga Rao and Datta, 1976; Thakur and Rawat,

1992; S. Hussain et al., 2004; Iqbal et al., 2004; A. Hussain et al., 2009). The RT fault zone represents one strand of a seismically-active emergent thrust fault system that extends stepwise more than 200 km northwestward to the Balakot-Bagh fault (source of the Mw 7.6 2005 Kashmir earthquake in Pakistan; A. Hussain et al., 2009; MonaLisa et al., 2009). Published estimates of

Pleistocene-Holocene shortening rates include <6 mm/yr for the SMA, 8.8-4.4 mm/yr for the RT, and ~3 mm/yr for BBF (Kondo et al. 2008; Vignon, 2011; Gavillot et al., 2010b; Hebeler et al.,

2010).

In the hinterland, the MBT and MCT, or locally known as the Murree thrust and Panjal thrust respectively (Wadia, 1934; Thakur and Rawat, 1992), form a narrow <10 km wide thrust zone that bound the Pir Panjal Range, a series of peaks that top over ~4000m south of the

Kashmir Basin (Fig. 4.1). The Lesser Himalaya units are bounded by MBT and MCT and consist of the Ramban Formation, low-grade slate and phylitte, likely Proterozoic in age (Wadia, 1934;

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Bhatia and Bhatia, 1973; Thakur and Rawat, 1992), which is thought to be younger or coeval to the Precambrian Sirban Limestone (Gavillot et al., 2010b and references therein; see Chapter 3).

North of the MCT, series of highly deformed and folded medium-grade (garnet to biotite) crystalline and metasediments correlate to the basal Haimanta Formation recognized along strike in Chamba of late Precambrian to early Paleozoic age (Bhatia and Bhatia, 1973; Thakur and

Rawat, 1992; Deeken et al., 2011 and references therein). Constituting most of the higher elevation of the Pir Panjal Range and overlying the Haimanta Formation, the ~8 km thick

Paleozoic-Cenozoic passive margin sediments represent the Kashmir Tethys Himalaya strata and include the Permian Panjal volcanics (Thakur and Rawat, 1992). One key difference between the

Kashmir Himalaya and the Himalaya to the east is a dominantly unobserved MCT in the hinterland root zone (i.e. MCTII; Searle and Goodin, 2003), except across a local structural window (Kishtwar Window) to the southeast of the Kashmir basin (Figs. 4.1 and 4.2). There is no known geological evidence of Quaternary displacement on either MBT or MCT. A recent study on the MBT using fluvial terraces confirms no evidence of recent faulting (Vignon, 2011).

4.5 APATITE AND ZIRCON (U-TH)/HE THERMOCHRONOMETRY

Analytical Methods

Apatite and zircon (U-Th)/He thermochronometry provide a powerful dating method to constrain the timing and magnitude of cooling and exhumation in response to deformation and erosion (e.g., Stockli et al., 2000; Ehlers and Farley, 2003). For detailed description of the dating technique, see Wolf et al. [1998], Farley and Stockli [2002], and Stockli [2005]. Helium (He) 4, a decay product of several radiogenic isotopes (235U, 238U, 232Th, 147Sm), is expelled from apatite at temperatures above ∼80°C and almost totally retained below ∼40°C (termed the He

Partial Retention Zone, HePRZ) (e.g., Wolf et al., 1998; Farley and Stockli, 2002). Zircon is characterized by a higher He closure temperature, which ranges from ∼170–190°C (e.g., Reiners

146 et al., 2004; Reiners, 2005) and a HePRZ that spans a temperature range from ∼120–180°C

(Tagami et al., 2003; Stockli, 2005). U-Th/He or He dating differs from the widely used fission track dating. Fission tracks or linear zone of damage that result from the spontaneous fission of

23 U within the crystal lattice begin to accumulate at temperatures lower than that of the “partial annealing zone” (PAZ; e.g. Gleadow and Fitzgerald, 1 7). In a setting of rapid cooling and exhumation, apatite fission track (AFT) and apatite He (AHe) data are expected to show related age results, given the slightly higher temperature of AFTPAZ of ~70-120°C (Gleadow and

Fitzgerald, 1987; Ketcham et al., 1999), which partly overlaps with apatite HePRZ (40-80°C).

Zircon fission track (ZFT) ages record cooling below the ~240°C closure temperature and the

PAZ spans 250-350°C (Cerveney et al., 1988; Bernett et al., 2004; Tagami and Dimitru, 1996;

Tagami et al., 1996, 1998l Rahn et al., 2004).

Thermal constraints for the Kashmir Himalaya are required to translate a cooling age to a measure of exhumation rate. Mean annual surface temperature is 20°C for the Sub-Himalaya valleys and 10°C for the Pir Panjal and High Himalaya. A geothermal gradient of 18-24 °C/km for the Sub-Himalaya as determined in western India and Nepal (Argrawal et al., 1994; Van der

Beek et al., 2006), and 35-45°C for the Pir Panjal and High Himalaya (Deeken et al. 2011; this study), are chosen as appropriate available thermal constraints for the Kashmir Himalaya. Apatite

(U‐Th)/He (AHe) and zircon (U‐Th)/He (ZHe) thermochronometry thus provide cooling history information for rocks as they passed through the upper ∼1.5–4 km and ~4–9 km of the crust, respectively.

(U-Th)/He dating, particularly AHe dating provide a low temperature system to document the last exhumation, and is appropriate to constrain recent activity on individual thrusts

(e.g. Ehlers and Farley, 2003). Fault‐related unroofing of sufficient magnitude exhumes samples from below the HePRZ, thus date faulting directly [e.g., Stockli, 2005]. Tectonically-driven exhumation in the upper plate of a given structure, may result from displacement of nearby

147 surface fault, or alternatively from sequential progressive uplift due to structural lower faults

(Gavillot et al., 2010a). For samples of siliciclastic units that never experienced temperatures as high as the HePRZ, detrital apatite and zircon ages preserve provenance and source-terrane cooling information.

Mean AHe and ZHe ages were calculated on the basis of 2–4 inclusion‐free apatite aliquots and 1-3 zircon aliquots replicate analyses. Uncertainties (2-sigma) of single ages reflect the reproducibility of replicate analyses of laboratory standard samples. Estimated analytical uncertainties are ~6% (2-sigma) for apatite and ~8% (2-sigma) for zircon He ages, respectively.

If multiple aliquots were dated, then mean sample ages are reported with the standard deviation of the aliquot population about the mean.

Sampling Methods

For this study, 15 apatite and 16 zircon samples, totaling 74 individual single‐grain apatite and zircon aliquot analyses were dated (Tables 2-3, Data Set S1). Field sampling targeted hanging wall and footwall units of various thrust sheets in both the foreland and hinterland to document thrust-related exhumation across the Kashmir Himalaya. Hence, in our study, foreland samples refer to the region between the SMA and the MBT. Hinterland sample refer to region between the MBT and the ZSZ (Figs. 4.1 and 4.2). Collection of samples was done through various methodologies to delineate potential effects of topography to the extent possible via local vertical transects, river valley profiles transects, mid valleys profiles, and by along strike ridges.

Foreland samples comprise of detrital apatite and zircon grains collected from Sub-

Himalaya deformation belt consisting of Cenozoic molasse. Sampling targeted sand and silt rich intervals belt that include the Oligocene to Early Miocene Murree formations (Lower and Upper

Murree), and the basal sequences of the Early to Middle Miocene sequences of the Siwalik formations (Lower and Middle Siwalik) (Fig. 4.2). Apatite- or zircon-bearing sediments from

148 deep stratigraphic levels record reset detrital cooling ages depending on the geothermal gradient and closure isotherm of a given thermochronometer. No samples were collected from the carbonate‐dominated Precambrian Sirban Limestone (Fig. 4.2), a unit not expected to yield detrital apatite or zircon (Fig. 4.1).

Hinterland samples comprise of low- to med-grade igneous and metasedimentary rocks from the Lesser Himalaya (LH) and High Himalaya (HH). Sampling interval was a ~15 km-wide transect across thrust sheets of the MBT and MCT respectively, bounding the Pir Panjal Range of

Kashmir (Figs. 4.2 and 4.4). LH sampling targeted local silt rich intervals within the flysh-type

Precambrian Ramban Formation (Wadia, 1937; Bhatia and Bhatia, 1973; Thakur and Rawat,

1992). For the HH, target samples included gneiss, schist and metavolcanic intervals correlating to the Precambrian to early Paleozoic Haimanta Formation (Bhatia and Bhatia, 1973; Thakur and

Rawat, 1992). Given that hinterland samples represent deeply exhumed crustal rocks with protolith ages predating the collision history, low-temperature cooling likely reflects exhumation associated with major thrusting events (MBT and MCT). Kashmir Tethys Himalaya strata in the

MCT upper plate were not sampled in this study.

Paleo-temperature estimates

Compilation stratigraphic thickness, stratigraphic overburden and paleotemperatures from the foreland samples are listed in Table 1. Estimates of thicknesses are constrained from map relations, dip data and cross sections along lines A-A’ and B-B’ (Figs 4.2-4.3). In locations where units are not well exposed, our cross sections and previous studies constrain stratigraphic thicknesses (Karunakaran and Ranga Rao, 1976; Tharkur and Rawat, 1992; Power et al., 1998).

Measurements of overburden includes only the lower part of the Upper Siwalik Formation, as age of deposition (5-1.7 Ma), which accumulated coevally with fault activity at the deformation front in part (Ranga Rao et al. 1988; Burbank et al., 1996, Valdiya, 1998; Mugnier et al., 1999;

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Sangode and Kumar, 2003 Kumar et al., 2006; Van der Beek et al., 2006). We assume a ~1000 m of the total 3000-3300 m represents the pre-thrust stratigraphic thickness for the deformation front in the Kashmir Himalaya.

Minimum estimates of stratigraphic overburden overlying sampled units are 4.4-7.7 km for the Lower Murree, and 4.4-5.7 km for the Upper Murree north of the MKT or RT. For samples along the SMA, estimates of overburden are 2.8-4.4 km for the Lower Siwalik, and 2.5-

3.4 km for the Middle Siwalik (see Table 1). Using an estimate of geothermal gradient of ∼18–

24°C/km (Argrawal et al., 1994; Van der Beek et al., 2006), the thickness of overburden translates to prethrust maximum burial temperatures of 78-186°C for the Lower Murree, 71-

138°C for the Upper Murree, 50-107°C for the Lower Siwalik, and 45-82°C for the Middle

Siwalik (Table 1). Nearly all sampled units nearly reach or exceed the AHe closure temperature

(∼80°C) due to burial. Upper Murree to Middle Siwalik samples may partly overlapped the

AHePRZ (~80-45°C), and thus potentially represent partially unreset detrital cooling ages. Any sample shallower than the uppermost Middle Siwalik and Upper Siwalik were likely not buried sufficiently to thermally affect detrital apatites. In contrast, only the basal Lower Murree

Formation may had sufficient overburden of 7.7 km to affect the ZHe ages given ZHe closure temperature of ∼190–170°C. All other overlying units likely preserve the detrital and pre-existing exhumation cooling in the source terrain.

Estimates of stratigraphic overburden for sampled LH and HHC are poorly constrained to be at least ~8 km with overlying Kashmir Tethys Himalaya strata (Tharkur and Rawat, 1992) and thrust sheet thicknesses. Assuming a varying geothermal gradient of 35-45°C for the hinterland samples, values comparable to those used in neighboring studies (Kumar et al., 1995; Deeken et al., 2011), minimum paleotemperatures range from 280°C to 360°C, which greatly exceed the

AHe and ZHe closure temperatures.

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4.6 RESULTS

AHe cooling ages

Foreland samples

Samples collected from the Sub-Himalaya, between the deformation front and the MBT are referred to here as the foreland samples. From south to north, foreland samples were collected on transects along the fold axis and limbs of the Suruin-Mastgarh anticline (SMA) and across the thrust sheets of the Kishanpur-Mandili thrust (MKT), Riasi thrust (RT), Tanhal thrust (TT) and

Mundun thrust (MT) (Fig. 4.4; Ranga Rao and Datta, 1976).

At the deformation front, three samples from the Lower Siwalik Formation exhumed within the core of the SMA, yield AHe ages of 1.0±0.1 Ma, 3.0±0.3 Ma, and 4.4±0.2 Ma mean sample ages (Fig. 4.4; Table 4.2). Single grain age population for the three samples range between 0.9 Ma and 9.8 Ma (Data Set 1). As a note, single grain ages larger than twice the standard deviation within a given a sample, are not included in our calculation of mean sample ages. Age probability plots for both mean sample and single grain ages along the SMA fold trace reveal a clustering of ages <5 Ma (Fig. 4.5a). Peak cooling occurs at 1 Ma, although an equally significant age population occurs at 3 Ma and 4.4 Ma.

In the MKT thrust sheet, three samples from the Lower and Upper Murree formations yield 2.4±0.1 Ma, 2.7±0.2 Ma, and 3.0±1.5 Ma in mean AHe ages (Fig. 4.4; Table 4.2). Single grain ages show a wider distribution between 2.3 Ma and 17.8 Ma, with an erroneously old single grain age of ~153 Ma (Data set 1). Age probability plots for the MKT samples indicate that ages cluster <5 Ma and a spike in cooling ages at ~2.5 Ma characterize both the mean sample and single grain ages (Figure 4.5b). At the RT, footwall samples in the Middle Siwalik and Upper

151

Murree units yield AHe mean sample ages of 5.6±0.5 Ma and 9.2±3.3 Ma respectively. In the RT hanging wall, Lower Murree yields an AHe mean age of 2.1±0.2 Ma (Fig. 4.4; Table 4.1). Single grain ages range between 5.3 Ma and 13.6 Ma (Data set 1). Age probability plots indicate a peak cooling at ~2 Ma (Fig. 4.5c). At the TT and MT, a sample from the Upper Murree (TT hanging wall) and Lower Murree formations (MT hanging wall) yield mean ages of 1.9±0.6 Ma and

10.2±3.7 Ma respectively (Fig. 4.4; Table 2). Single grain ages range between 1.2 and 14.8 Ma

(Data set 1). Both mean- and single grain-ages probability plots indicate peak cooling between

1.8 Ma and 2 Ma (Fig. 4.5d).

In summary, AHe foreland samples range from 10.2 ± 3.7 to 1.0 ± 0.1 Ma (~1-10 Ma) in mean sample ages and 13.9 to 0.9 Ma in single grain ages (Table 4.2 and Data Set 1). A large number of foreland samples indicate a regional pattern of young cooling <3 Ma across the Sub-

Himalaya (Fig. 4.4). Mean ages cluster at <5 Ma and combining all samples together emphasizes cooling between 1.9-3.2 Ma (Fig. 4.5e).

Most of the foreland samples resided below or close to the AHe closure temperature (see discussion of paleo-temperature in previous section) and thus represent reset detrital cooling ages.

One exception is the stratigraphically shallowest sample in Middle Siwalik that is only partially reset. By definition an unrest detrital cooling age is older than the stratigraphic age (Cerveny et al., 1988; Copeland & Harrison, 1990; Brewer et al., 2003). Plotting stratigraphic age with each

AHe mean sample cooling age reveals that most samples plot younger than the depositional age, which indicates the ages are reset (Fig. 4.6a). Partially reset ages may record either i) less stratigraphic burial and colder paleotemperature for the uppermost units (Middle Siwalik); ii) slow localized tectonic uplift and longer residence in the AHePRZ for samples in the footwall of thrust sheets; iii) or grains resistant to thermal perturbation.

Plots of AHe cooling ages versus distance across the Sub-Himalayan structures (SMA,

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MKT, RT, MT) indicate apparent age gradients across the fault or fold traces (Fig. 4.7). For the

SMA, young cooling is localized at the fold axis. For faults, younger ages characterize the hanging wall relative to footwall, as expected given the thrust sense of motion. Both stratigraphically deep (Upper Murree) and shallow (Middle Siwalik) footwall samples yield older

AHe ages than hanging wall samples. This pattern is interpreted to result from differential exhumation across the fault.

Hinterland samples

Hinterland samples collected north of the Sub-Himalaya represent AHe cooling associated with exhumation within the orogen (Fig. 4.4). AHe ages from four hinterland samples range between ~5 to 21 Ma (Fig. 4.4, Table 4.2). All sample mean ages are characterized by high standard deviations that reflect the diverse population of single grain ages (Data Set 1). Three of those samples have mean ages between 4.7 Ma and 7.2 Ma (4.7±1.6 to 7.2±5.6 Ma) (Table 4.2;

Fig. 4.4). Although hinterland AHe data are few, probability plots using mean ages suggest a peak in mean sample cooling ages between 4.5 Ma and 9 Ma, which is an older age cluster than in the Sub-Himalaya (Figs. 8a-b). A 21.1±6.7 Ma sample in the hanging wall of the MCT represents a significantly older AHe age than other hinterland samples, and may represent poor quality apatite crystals. Hinterland AHe peak cooling at 4.5-9 Ma for the MBT and MCT thrust sheets in the Pir Panjal Range is interpreted to reflect slow fault-related exhumation.

ZHe cooling ages

Both detrital and igneous or metamorphic zircon (U‐Th)/He (ZHe) samples were dated from the same locations as the AHe samples (Fig. 4.4), which provide direct age correlation between the two thermochronometers. A few samples did not yield apatite, but did contain zircon.

Hinterland ZHe mean sample ages range between 13 Ma and 22 Ma (Table 4.3; Fig. 4.4).

Hinterland zircon samples are no older than ~26 Ma and are younger than the corresponding

153 protoliths ages of the Precambrian igneous or metasedimentary Lesser Himalaya and High

Himalaya source unit, even when age uncertainty is considered (Table 4.2, Fig. 4.9a). Probability density plots of both mean and single grain ages show a large cluster of ages characterized by a high probability spike between 16 Ma and 21 Ma. A secondary spike occurs between 6 Ma and

8.5 Ma (Fig. 4.9a). Given published constraints of thrust activity of the MBT and MCT, two spikes reasonably reflect periods of rapid exhumation associated fault activity on the MCT (20-22

Ma; e.g. DeCelles et al., 1998b) and possibly the MBT (5-11 Ma; Meigs et al., 1995, Burbank et al., 1996; Sangode and Kumar, 2003; Kumar et al., 2003). Hinterland zircon samples show no clear correlation of cooling ages with respect to elevation, although the elevation change along the valley is relatively small.

Foreland sample ages range from 105 Ma to 11 Ma (Fig. 4.4), and single grain ages are distributed between 113 Ma to 4.2 Ma (Table 4.3). Most sample ZHe ages are either older or coeval to corresponding sedimentary depositional age. A plot of stratigraphic versus single grain

ZHe ages demonstrates that the majority of the grains is detrital and records a cooling age of the source terrain or is a recycled detrital age (Fig. 4.6b; Table 4.3). A few exceptions exist, notably for the stratigraphically deepest samples from the Lower Murree Formation, which include a number of ZHe cooling ages that are significantly younger than the depositional age (Oligocene-

Lower Miocene). These ZHe samples are thus reset.

Probability density plots using both mean and single grain ages contain the same cluster and probability spike of 16–21 Ma than the hinterland samples suggesting a causal link (Fig. 9b).

Correlation of the same cluster of ages in both the Siwalik and hinterland samples reflects a period of cooling, exhumation in the hinterland source terrain associated with the MCT, and subsequent deposition in the foreland. The lag time between exhumation and deposition cannot be determined because the stratigraphic ages are not sufficiently well known.

154

AFT cooling age comparison

Published apatite and zircon fission track age data constrain hinterland exhumation from the Kishtwar to Window (KW) northward to the Zanskar Shear Zone (ZSZ) (Fig. 4.4; Kumar et al., 1995). The KW is a 45 km-wide structure interpreted to be formed by thrust duplication of the

LH in duplex above the basal décollement. The overlying MCT acts as the roof thrust to the duplex (DiPietro and Pogue 2004; Yin 2006; Searle et al., 2007). Rocks from within the KW thus represent structurally deeper levels of the hinterland, and rocks above the MCT are relatively shallower.

Apatite fission track (AFT) data record younger cooling (<3 Ma) within the KW duplex than >3.1 Ma above the structure (Fig. 4.4; Kumar et al., 1995). South of the KW, AFT ages of

>3.1 Ma are similar to the AHe hinterland samples of 4.5-7.2 Ma from the Pir Panjal Range (Fig.

4.4). North of the KW, the AFT ages generally increase from south to north and define a pattern of progressively older cooling to Zanskar Shear Zone (ZSZ; Figs. 4.4). Two young AFT ages (<

3 Ma) occur north of the KW, however, the ages are spatially limited to a narrow locality within a granitoid dome structure equally characterized by old AFT cooling (>3.1 Ma) (Kumar et al.,

1995).

In comparison AHe age data across the Sub-Himalayan structures indicate equally young cooling ages (< 3 Ma) than the AFT ages in the KW. In contrast, across the MBT and MCT thrust sheets in the Pir Panjal Range, which separates the Sub-Himalaya from the High Himalaya, AHe cooling ages are older (> 3 Ma) than AFT ages in the KW (Fig. 4.4). Less than 3 Ma cooling ages in both the KW and the Sub-Himalaya suggest that in both areas cooling and exhumation are linked (Fig. 4.4). Given that the AHe thermochronometry is characterized by a slightly lower closure temperate (AHe ~80°C) than AFT (~120°C), predicted AHe cooling ages within the KW

155 are predicted to be younger than AFT age data. Pattern of younger AHe ages than corresponding

AFT ages is documented in a similar study in the Nepalese High Himalaya (Blythe et al., 2003).

4.7 EXHUMATION RATES

A plot of AHe and AFT cooling ages against sample elevation defines the exhumation rates (Fitzgerald and Gleadow, 1988; Mancktelow and Grasemann, 1997; Ehlers and Farley,

2003). In the Sub-Himalaya, given limited topographic relief, foreland transects show poor age- elevation trends (Fig. 4.10), which precludes using pseudo-vertical transects to constrain the exhumation rate (ER).

An alternative method to calculate ER is to use a sample age as the time since cooling through a given closure temperature and employing the ER equation:

ER= [(Tc-Ts)/(dT/dz)]/A, where Tc is the closure temperature (°C), Ts is the average surface temperature (°C), dT/dz is the geothermal gradient (°C/km), and A is the sample age (Ma) (Dodson, 1973). For the foreland samples, we assume a geothermal gradient of 18-24°C (Argrawal et al., 1994; Van der Beek et al., 2006) and a surface temperature of ~20°C. Table 4 lists calculated ERs for the youngest AHe ages of each the major structures. The SMA predicts values of 3-1.0 mm/yr. Given a study of uplifted terraces at the deformation front suggests cumulative uplift rates of 1.8 mm/yr (Gavillot et al., in preparation; see Chapter 2), ER of 1 mm/yr represents a more appropriate average minimum estimate for the SMA. Thrusts north of the deformation front predict ERs of 1.2 mm/yr for the MKT, 1.4 for RT, and 1.8 for the MT, delineating a total range of 1.8-1.2 mm/yr.

Summing the rates between the SMA (>1.0 mm/yr) and Sub-Himalayan thrusts (1.8-1.2 mm/r), yield a minimum total range of 2.8-2.2 mm/yr.

156

Rates of exhumation are also calculated for the hinterland samples north of the MBT. An average surface temperature of ~10°C and a range of values 35-45°C for the geothermal gradient in the Pir Panjal and High Himalaya are used based on previous studies in the region (Kumar et al., 1995; Deeken at al., 2011). In the Pir Panjal Range, the MBT and MCT thrust sheets have lower ER values than that of the Sub-Himalaya, with <0.2 mm/yr based on relief transect (Fig.

4.11a) and 0.4 mm/yr using the ER equation (Table 4). At the KW (Fig. 4.4), AFT data indicate that ERs are high within the structure, with a 1.3 mm/yr rate suggested by age-elevation relations

(Fig. 4.11b) and an equation-derived estimate of 2.9 mm/yr (Table 4.4). Further north in the upper plate of the KW, ERs decrease to the northeast and values are <1 mm/yr from vertical transects and ER equation (Fig. 4.11c-d; Table 4.4).

4.8 DISCUSSION

Exhumation pattern in the Sub-Himalaya: evidence of distributed deformation at 105 yrs timescales

Cooling AHe age data across the Sub-Himalayan belt in Kashmir indicate a regional pattern of AHe ages < 3 Ma (Fig. 4.4). At the deformation front (SMA), although a peak cooling occurs at 1 Ma, mean AHe ages span a protracted cooling history between 1 Ma and 4 Ma (Fig.

4.5 a). At internal thrusts north of the deformation front (MKT, RT, MT), peak cooling occurs at

~2 Ma for hanging wall samples (Fig. 4.5b-d). A combined peak cooling period of 1.9-3.2 Ma characterizes all Sub-Himalayan structures (Fig. 4.5e). Contrasting age gradients between footwall and hanging wall samples collected on thrust transects suggest cooling and exhumation are driven by thrust motion on individual faults (MKT, RT, MT). Compartmentalization in fault- related exhumation suggests the Sub-Himalayan belt deforms by thin-skinned thrusting, arguing against possible effect of regional basement uplift. Thus, we interpret regional distribution of young cooling ages in the foreland samples reflect a pattern of distributed deformation via coeval

157 thrusting both at the deformation front and internal thrusts. Estimates of exhumation rates of 3-1 mm/yr for the SMA, and 1.8-1.2 mm/yr for internal thrusts (Fig. 4.10; Table 4.4), are consistent with our interpretation of rapid exhumation associated with coeval thrusting across the Sub-

Himalaya. Active faulting is known to occur on the Riasi thrust, ~40 km north of the deformation front, documented to have Pleistocene-Holocene shortening rate of 8.8-4.4 mm/yr using offset terraces (Gavillot et al., 2010b; see Chapter 3). Along strike, the MKT and MT are interpreted to represent southeastern continuation of the RT as regional active fault system north of the deformation front (Gavillot et al., 2010b; Tharkur et al., 2010; Vignon, 2011; see Chapter 3).

Although onset of thrusting and fault history likely differs between the deformation front and faults to the north within the Sub-Himalaya, our data constrain that distributed deformation with coeval thrusting across the Sub-Himalaya has been ongoing since at least 1.9-3.2 Ma.

Constraints on age of thrusting for Sub-Himalayan structures

There are no resolved inflection points in our cooling age data and pseudo-vertical transects, which would mark the onset of rapid cooling (e.g., Ehlers and Farley, 2003). Peak cooling AHe ages at ~2-3 Ma is interpreted to reflect ongoing and active thrusting on multiple structures in the Sub-Himalaya. Given that cooling ages record time since a particle path travelled through the AHe closure isotherm, 2-3 Ma peak cooling represent a minimum age of onset of thrusting for the RT, MKT, and MT. Major fault activity on the MBT to the north is thought to have slowed down by ~5 Ma (Kumar et al., 2003), and would suggest an onset of southward propagating thrusts between ~5-3 Ma. At the deformation front, an older age population of reset

Lower Siwalik samples collected along the SMA fold axis suggests fault-related exhumation was ongoing since at least 3-4 Ma. This age constraints is older than a previous published estimate for onset of fold-related uplift on the SMA at ~1.7 Ma, using decrease in sedimentation rates in the

Upper Siwalik (Ranga Rao et al., 1988). Fault-related exhumation and fault activity on the SMA potentially increased during the Quaternary, explaining a sample mean AHe age of ~1 Ma (Fig.

158

4.4). Intermittent fault activity explains the cooling history on the SMA, since its inception as an active structure at 3-4 Ma. Such mode of operation for the deformation front has been proposed for the HFT in Nepal (Robert et al., 2005; Van der Beek et al., 2006).

Exhumation pattern in the hinterland

In the hinterland front ranges of the Kashmir Himalaya, along the Pir Panjal Range, AHe age data indicate >4.7 Ma exhumation in the hanging wall of the MBT and MCT (Fig. 4.4). In comparison to the young AHe cooling ages in Sub-Himalaya faults, hinterland samples in the Pir

Panjal Range record older ages and slower exhumation rates (<1 mm/yr), not characterized by

Quaternary rapid cooling.

ZHe age data similarly indicate an older cooling history with a peak cooling between 16-

21 Ma and possibly 4.7-8 Ma, interpreted to represent earlier fault-related exhumation associated with the MCT and MBT respectively. Hence our age data suggest little to no active thrusting occurs on the MBT and MCT in the Pir Panjal of Kashmir, consistent with our field observation and previous interpretations (e.g. Wadia, 1934; Nakata et al., 1989; DiPietro and Pogue, 2004), including a geomorphic study that constrains no recent fault activity on the MBT since at least

~14-17 kyr (Vignon, 2011).

Further north, across the Kishtwar Window (KW), however, AFT age data (Kumar et al.

1995), indicates young cooling <3 Ma and high exhumation rates (2.8-1.3 mm/yr) within vicinity of the mapped structural window of a LH duplex (Figs. 4.4, 4.11, 4.12; Table 4.4). Older AFT ages (>3.1 Ma) and slower exhumation rates (<1 mm/yr), both south and north of the KW, including the High Himalaya Range and the Zanskar Shear Zone, suggest that the KW is the primary structure in the hinterland characterized by Quaternary and rapid exhumation north of the

MBT.

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Quaternary surface displacement is not known for the KW thrusts. Curvilinear map pattern along the MCT roof thrust bounding the KW, suggest little to no surface thrusting occurs along mapped fault boundaries (e.g. Stephenson et al., 2000; DiPietro and Pogue, 2004). Given distribution of young AHe ages and high exhumation localized only in the vicinity of the KW, a structure interpreted to overlay LH duplexes at depth (DiPietro and Pogue, 2004; Yin, 2006,

Searle et al., 2007), we interpret the exhumation pattern to translation over a mid-crustal ramp along the MHT décollement (Fig. 4.3). Deformation pattern of overthrusting and/or duplex formation along a changing geometry of the basal décollement within the orogen can produce localized regions of increased uplift (Molnar and Lyon-Caen, 1988), a mechanism invoked to explain young ages and rapid exhumation in the Nepalese hinterland (Bollinger et al. 2006;

Brewer and Burbank 2006; Robert et al., 2009; Herman et al., 2010). Another hinterland structural window, the Rampur Window, in neighboring western Himalaya, is also characterized by young cooling ages (Lal et al., 1999; Schulp et al., 2003; Thiede et al., 2004; Thiede et al.,

2005; Thiede at al., 2009; Vannay et al., 2004).

Kinematics of the Kashmir Himalaya orogen

AHe and AFT age data and exhumation vary systematically from south to north across the Kashmir Himalaya orogenic wedge (Figs. 4.4 and 4.12). In the Sub-Himalaya, regional pattern of young cooling ages and high ERs of 2.8-2.2 mm/yr, are attributed to two or more active thrusts that merge at depth with the MHT basal décollement (Fig. 4.3). In the hinterland, cooling ages are older with significantly lower ERs (<1 mm/yr) for basal thrust sheets in the Pir Panjal

Range and for the rocks above MCT in the High Himalaya, except for the KW duplex with cooling ages and ERs of equal magnitude than the in Sub-Himalaya. Comparable ERs and timing of exhumation for the Sub-Himalayan structures and the KW duplex, favors the interpretation that active thrusting is linked between the two zones.

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Frontal thrust slip balances slip on the basal décollement along duplexes and/or mid- crustal ramps in thin-skinned thrust belts (e.g. Dahlstrom, 1969; Price, 1981; Boyer and Elliot,

1982). Exhumation variability within a thrust belt is a function of 1) the structural position within the wedge, and 2) the variation in the steepness of active fault segments, where rapid exhumation occurs in association with ramps and slow exhumation with flats (e.g. Molnar and Lyon-Caen,

1988). Using this structural framework of thrust belts kinematics, cooling ages and exhumation rates across the Kashmir Himalaya dictate a pattern of exhumational variability controlled by geometry of the MHT, in which any active ramps along the MHT or upper-plate faults translate horizontal motion into zones of high vertical uplift. Given cooling ages and exhumation rates are in agreement between the Sub-Himalaya and the KW, suggest duplex (KW) displacement is transferred along the basal décollement with little to no out-of-sequence thrusting in the hinterland. Young cooling and high exhumation in the Sub-Himalaya therefore represents the frontal structures accommodating up-dip displacement of basal décollement transferred from thrusting at deeper crustal levels (Fig. 4.3). Absence of young cooling ages on the MCT and MBT thrust sheets in the Pir Panjal Range (Figs. 4.2 and 4.4) is interpreted to reflect lower exhumation rates due to horizontal translation on a flat section of the MHT (Fig. 4.3).

Climate forcing?

A dynamic link exists between tectonic and surface processes in actively deforming orogens, in which exhumation is controlled by the balance between tectonic uplift and erosion

(e.g. Dahlen and Suppe, 1988; Willett, 1999). Given the low closure temperature of AHe system, thermal perturbation in the shallow crust can affect cooling ages that result from topographic and erosion gradients (Ehlers and Farley, 2003).

In many locations in the hinterland of central and the western Himalaya, the MCT generally coincides with an abrupt rise in topography, fluvial knick-points (Hodges et al., 2004),

161 and a well-documented focused monsoonal precipitation on the south flank of the High Himalaya

(Bookhagen and Burbank, 2006). Correlation of young AFT cooling age data sets and precipitation pattern has been interpreted by some studies into a model interpretation that focused precipitation and enhanced erosion drives fault-related exhumation, arguing that climate-induced erosion controls the pattern of thrusting (Jain et al., 2000; Thiede et al., 2004, Thiede et al, 2005;

Wobus et al., 2005). Alternatively, others studies report uniformly young cooling ages for ~10-30 km across the MCT hanging wall, a region characterized by a large precipitation drop over the same distance across the topographic step in the High Himalaya. These results suggest that precipitation is decoupled from erosion and exhumation, or does not have significant effect in perturbing cooling ages compared to the overall tectonic process (Burbank et al., 2003; Blythe et al. 2007, Patel and Carter, 2009).

Precipitation data from the Kashmir Himalaya (Bookhagen and Burbank, 2006), combined with relief, cooling ages, and a structural cross section from the deformation front to

ZSZ can be used to quantitatively assess the spatial relationships between cooling and precipitation (Fig. 4.13). Pattern of young cooling correlates very well with known areas of active thrusting on the Sub-Himalaya structures, or hinterland orogenic growth at the KW associated with active duplexing. From south to north, precipitation is high across the Sub-Himalaya until the Pir Panjal Range, which then steadily drops northward into the High Himalaya (Fig. 4.13).

High precipitation correlates with young cooling ages in the Sub-Himalaya, however, in the hinterland across the KW, young cooling ages are associated with a precipitation low (Fig. 4.13).

Uncorrelation between precipitation and cooling ages is also shown with a precipitation high at the base of the Pir Panjal, a locality characterized by older cooling ages than either in the Sub-

Himalaya to the south of the KW to the north. This anti-correlation between cooling ages and precipitation pattern for areas north of the Sub-Himalaya indicates that in Kashmir, climate- induced erosion does not significantly perturb exhumation process. Young ages and rapid

162 exhumation decoupled from the precipitation pattern with the orogen most closely coincide with predicted location of a mid-crustal ramp beneath the mapped duplex (Fig. 3), a structural control shown to explain young ages and rapid exhumation in the Nepalese Himalaya hinterland

(Bollinger et al., 2006; Brewer and Burbank, 2006; Whip et al., 2007; Robert et al., 2009).

Young AFT cooling ages (<3 Ma) and high exhumation rates (2.8 mm/yr) also occur along strike, in the neighboring Dhauladhar Range, southeast of the Kashmir Himalaya (Figs. 4.4 and 4.12). These data span across the MCT and are interpreted as evidence of climate-enhanced exhumation (Deeken et al., 2011; Adlakha et al., 2013). Here at the latitude near the Kangra reentrant, the Himalayan orogen is characterized by a steep orographic range front (the

Dhauladhar Range) with focused precipitation gradient (Bookhagen and Burbank, 2006), and no structural window or major thrusts north of the MCT across the High Himalaya hinterland until the Zanskar Shear Zone (Fig. 4.2 and 4.4). Although knowledge of cooling ages and exhumation is not known for the Sub-Himalaya south of the Dhauladhar Range, we would argue that orogenic growth along the MHT driving upper plate exhumation and tilting of the MBT and MCT thrust sheets, can equally explain the pattern of young cooling ages in the Dhauladhar Range.

Structural control and wedge behavior in the Kashmir Himalaya orogen

Utilizing our new age data and their relations with deformation pattern, topographic and precipitation pattern across the Kashmir Himalaya, we can use a comparative approach to constrain the primary forcing mechanisms modulating wedge behavior interpreted in terms of critical taper theory model predictions (Davis, 1983; DeCelles and Mitra, 1995; Mitra, 1997;

Horton, 1999).

Wedge model theory predicts that thrust belts strive for a critical condition, in which the sum of the gravitational body force and horizontal compressive force is balanced by the sliding friction along the basal décollement of the wedge (Davis et al., 1983; Dahlen and Suppe, 1988).

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The critical condition or “critical taper” is attained only if the sum of the surface slope and décollement slope reaches a specific value dependent on the balance between the influx of material by tectonic processes and removal of material by surface processes (Willet, 1999). Any changes in the geometrical or mechanical parameters, such as erosion or geometry of the basal décollement, force the wedge out of critical condition, and become either sub-critical or supercritical.

An unifying phenomenon from a plethora of thermochronology studies throughout the

Himalaya, is a well-documented evidence for young and rapid cooling ages in the hinterland region, used as proxy for the location of mid-crustal ramp, generally coinciding with the MCT or physiographic transition. In Nepal and parts of northwest India, out-of-sequence thrusting driven by climate-enhanced erosion is one competing model interpretation explaining this pattern of localized exhumation north of the active deformation front (e.g. Hodges et al., 2004; Wobus et al., 2003, 2005; Thiede et al., 2004, 2005). Hence increased precipitation and removal of material by surface processes modulate a sub-critical wedge by internal deformation. Alternatively, overthrusting, duplexing, or underplating of material across a mid-crustal ramp explains the same pattern (e.g. Bollinger et al., 2006; Brewer and Burbank, 2006; Robert et al., 2009; Herman et al.,

2010). In this model interpretation, mechanism of accretion of crustal material (underplating and/or duplexing) and transfer of displacement along the basal décollement, dictates a balanced critically tapered deforming wedge.

In Kashmir Himalaya, our model interpretation suggests that young ages and sum of fault-related exhumation rates at the orogenic front correlates with comparable ages and exhumation rates across a hinterland duplex. This deformation pattern suggests that in Kashmir, displacement absorbed by the hinterland duplex is largely transferred up-dip along the basal décollement into multiple active structures in the Sub-Himalaya. Although deformation pattern in he Kashmir hinterland supports a wedge deforming by crustal accretion (critical wedge),

164 distributed shortening on coeval thrusts in the Sub-Himalaya suggests internal surface faulting modulates the orogenic front (sub-critical).

Given our cooling data and associated deformation pattern across the Kashmir Himalaya suggest effects of precipitation is decoupled from fault-related exhumation and consequential to the physiography, we propose that wedge behavior is primarily controlled by changes in the structural architecture along the basal décollement. Key evidence that basement structures modulate the deformation and exhumation pattern comes from the Sub-Himalaya orogenic wedge in the Kashmir. Unique characteristics include active distributed shortening on two or more thrusts within the Sub-Himalaya belt, and an active regional fault system (BBF to KT to RT to likely the MKT), located 40-150 km north of the active deformation front (Fig. 4.1). This fault system is characterized by inliers of Precambrian limestone strata up to 4-5 km thick, kinematically requiring a large structural ramp along the MHT to juxtapose the strata to the surface (Fig. 4.3). A similar effect of basement ramp is well-documented in the geophysical data beneath Salt Range thrust in Pakistan, which is interpreted to explain a highly emergent thrust front (Pennock et al., 1989, Baker, 1987, Leathers, 1987; Jaumé and Lillie, 1988). Southeast to the Kashmir Himalaya, in Chamba, an interpretation of no structural ramp in the hinterland explains the thermochronology data with the lack of young cooling ages within the interior of the orogen (Deeken et al., 2011). Hence in Kashmir, distributed deformation with multiple active thrusts in the Sub-Himalaya, characterized by potentially infrequent but large earthquakes

(Gavillot et al., 2010b; see Chapter 3), may ultimately be controlled by pre-existing Precambrian basin architecture with highly variable geometry, similarly proposed on the effects of accreting passive margins indentations and old rift margins (i.e. Zagros, Talbot and Alavi, 1996;

McQuarrie, 2004).

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4.9 CONCLUSION

Apatite and zircon (U‐Th)/He cooling ages quantify the recent exhumation pattern associated with fault activity across the Kashmir Himalayas. Utilizing our data with other low- temperature data from neighboring areas (Kumar et al., 1995), we constrain the cooling history and thrust belt kinematics across a ~190 km-section of the Kashmir Himalaya. Cooling age data were collected from foreland strata (Murree and Siwalik), Lesser Himalaya, and High Himalaya units in Kashmir. Samples were collected from various transects across folds and faults in the

Sub-Himalaya (SMA, MKT, RT, TT, MT), and in the hinterland (MBT, MCT). We interpret regional pattern of young AHe cooling ages (<3 Ma) in the foreland (Fig. 4.4), peak cooling ages at 1.9-3.2 Ma (Fig. 4.5), reset detrital cooling ages uncorrelated to most stratigraphic units (Fig.

4.6b), and age gradients across structures (Fig. 4.7), and rapid exhumation rates (2.8-2.2 mm/yr;

Fig. 4.10), to represent rapid coeval fault-related exhumation on multiple structures across the

Sub-Himalaya. Coeval distributed shortening across multiple active faults in the Kashmir Sub-

Himalaya explains the regional upper-plate exhumation since 1.9-3.2 Ma. In the hinterland front ranges of the Kashmir Himalaya, along the Pir Panjal Range, AHe age data indicate older cooling

(>4.7 Ma) and slower exhumation rates across the MBT and MCT (Figs. 4.4, 4.8, 4.11, 4.12).

Tectonic mechanisms driving upper-plate exhumation at the orogenic front thus did not significantly affect cooling and exhumation in the hinterland thrust sheets. Further north in the hinterland, active orogenic growth and duplexing on along a mid-crustal ramp of the MHT (Fig.

4.3), explains young cooling ages (<3 Ma) and rapid fault-related exhumation (2.8-1.3 mm/yr),

~100 km north of the active deformation front (Figs. 4.4, 4.11, 4.12).

Cooling patterns across the Kashmir Himalaya reflect pattern of distributed deformation and coeval thrusting orogenic wedge development in the Sub-Himalayan belt after 2-3 Ma.

Duplexing and overthrusting across a mid-crustal ramp characterize in the hinterland deformation

(Kishtwar Window), transferring slip to the orogenic front along the basal décollement. Our result

166 suggests that exhumation pattern across the width of the Kashmir Himalaya orogen primarily reflects changes in the kinematics along the MHT, and a wedge modulated by the pre-existing basin structural architecture. Local precipitation gradient does not correlate with pattern of cooling ages in the Kashmir Himalaya, suggesting climate-driven process is consequential to the physiography and exhumation pattern.

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Table 4.3 Kashmir Himalaya Zircon (U-Th)/He data Age ± U Th 147Sm He Mass Standard Sample [U]e Th/U Ft ESR (Ma) (Ma) (ppm) (ppm) (ppm) (nmol/g) (ug) deviation Suruin-Mastargh anticline (SMA) zIHS-1-1 18.4 1.47 531.1 65.3 1.4 546.1 0.12 41.4 5.61 0.76 48.00 zIHS-1-2 42.4 3.39 457.9 100.1 0.5 480.9 0.22 84.1 5.36 0.76 48.75 zIHS-1-3 32.8 2.63 366.6 64.4 0.5 381.4 0.18 52.2 5.52 0.77 50.20 zIHS 31.2 2.5 451.9 76.6 0.8 469.5 0.2 59.2 5.5 0.8 49.0 12.05 zIHS-5-1 19.6 1.57 498.5 100.9 0.8 521.7 0.20 41.7 5.21 0.75 46.83 zIHS-5-2 2.7 0.22 1936.1 274.8 2.2 1999.4 0.14 21.6 3.60 0.74 43.21 zIHS-5-3 22.8 1.83 642.6 135.8 3.3 673.9 0.21 61.1 3.58 0.74 43.14 zIHS-5 15.0 1.2 1025.7 170.5 2.1 1065.0 0.2 41.5 4.1 0.7 44.4 10.80 zIHS-6-1 19.8 1.59 555.6 76.5 0.9 573.2 0.14 47.4 8.32 0.77 50.60 zIHS-6-2 20.4 1.63 673.9 88.6 1.6 694.3 0.13 56.6 4.06 0.74 43.92 zIHS-6-3 18.6 1.49 593.5 154.0 1.0 629.0 0.26 46.7 4.13 0.74 44.07 zIHS-6 19.6 1.6 607.7 106.4 1.2 632.2 0.2 50.3 5.5 0.8 46.2 0.92

Mandili-Kishanpur thrust (MKT) zIHM-3-1 105.4 8.43 101.4 69.9 2.8 117.5 0.69 46.5 2.41 0.69 37.25 - zIHM-4-1 33.3 2.67 504.2 45.4 0.6 514.6 0.09 66.7 2.91 0.72 40.04 zIHM-4-2 56.6 4.53 102.4 51.7 1.0 114.3 0.50 26.1 5.01 0.74 45.49 zIHM-4-3 29.8 2.39 442.6 431.7 1.2 542.0 0.98 65.5 5.94 0.75 46.91 zIHM-4 39.9 3.2 349.7 176.3 1.0 390.3 0.5 52.7 4.6 0.7 44.1 14.57 zIHM-14-1 25.2 2.01 689.0 327.2 1.3 764.3 0.47 79.8 6.08 0.77 50.36 zIHM-14-2 29.1 2.33 135.5 95.6 1.2 157.5 0.71 18.4 5.16 0.74 45.55 zIHM-14-3 55.8 4.47 163.2 102.9 1.5 186.9 0.63 42.2 4.56 0.74 45.93 zIHM-14-4 18.1 1.45 1474.2 79.3 0.7 1492.4 0.05 109.6 4.85 0.75 45.32 zIHM-14 32.1 2.6 615.5 151.2 1.2 650.3 0.5 62.5 5.2 0.8 46.8 16.49

Riasi thrust (RT) zIHS-7-1 27.7 2.22 291.2 83.7 0.7 310.5 0.29 38.8 16.60 0.83 71.24 zIHS-7-2 32.6 2.61 230.2 72.7 1.7 247.0 0.32 37.0 21.03 0.85 80.46 zIHS-7-3 18.5 1.48 369.4 73.4 0.8 386.3 0.20 32.2 16.53 0.83 71.51 zIHS-7 26.3 2.1 297.0 76.6 1.1 314.6 0.3 36.0 18.1 0.8 74.4 7.15 zIHM-8-1 10.3 0.82 697.4 91.8 0.8 718.6 0.13 29.1 3.59 0.73 42.43 zIHM-8-2 16.8 1.35 117.2 36.7 0.5 125.7 0.31 9.0 15.56 0.79 55.98 zIHM-8-3 7.5 0.60 255.9 127.3 1.7 285.2 0.50 8.7 4.92 0.75 46.35 zIHM-8 11.5 0.9 356.9 85.3 1.0 376.5 0.3 15.6 8.0 0.8 48.3 4.77 zIHM-9-1 18.5 1.48 613.4 101.8 3.1 636.9 0.17 49.9 9.30 0.79 54.35 zIHM-9-2 21.9 1.75 173.5 50.7 0.8 185.2 0.29 16.4 4.16 0.75 45.32 zIHM-9-3 18.5 1.48 484.0 107.0 0.9 508.7 0.22 39.3 6.01 0.77 50.45 zIHM-9 19.6 1.6 423.6 86.5 1.6 443.6 0.2 35.2 6.5 0.8 50.0 1.98

Tanhal thrust (TT) and Mundun thrust (MT) zIHM-15-1 22.8 1.82 208.9 102.0 2.1 232.4 0.49 21.5 4.64 0.75 46.89 zIHM-15-2 32.9 2.63 174.7 125.3 0.8 203.6 0.72 27.0 4.66 0.74 45.96 zIHM-15-3 83.8 6.70 192.0 120.6 0.8 219.8 0.63 71.5 3.05 0.71 40.56 zIHM-15 46.5 3.7 191.9 116.0 1.3 218.6 0.6 40.0 4.1 0.7 44.5 32.68 zIHM-16-1 51.4 4.11 150.8 123.9 1.9 179.3 0.82 36.6 3.88 0.73 43.92 zIHM-16-2 23.9 1.91 340.8 46.6 1.2 351.5 0.14 35.4 8.04 0.78 52.61 zIHM-16-3 19.1 1.53 88.8 42.1 0.7 98.5 0.47 7.7 4.83 0.75 47.27 zIHM-16 31.4 2.5 193.5 70.9 1.3 209.8 0.5 26.6 5.6 0.8 47.9 17.41

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Table 3. (continued) MBT/MBT (Pir Panjal) zIHR-10-1 20.9 1.67 653.5 62.5 0.6 667.9 0.10 61.4 11.53 0.82 62.84 zIHR-10-2 17.1 1.37 433.6 125.0 0.5 462.4 0.29 33.9 7.34 0.79 56.86 zIHR-10-3 6.4 0.51 38.2 36.6 2.0 46.7 0.96 1.2 3.10 0.72 41.91 zIHR-10 14.8 1.2 375.1 74.7 1.0 392.3 0.4 32.1 7.3 0.8 53.9 7.51 zIHSC-11-P-1 19.1 1.53 230.3 62.7 1.2 244.7 0.27 20.2 10.39 0.80 59.08 zIHSC-11-P-2 12.9 1.03 1047.6 557.6 8.0 1176.0 0.53 60.0 3.90 0.73 43.10 zIHSC-11-P-3 7.9 0.63 925.9 132.3 1.5 956.4 0.14 30.7 5.03 0.76 46.99 zIHSC-11 (P) 13.3 1.1 734.6 250.9 3.6 792.4 0.3 37.0 6.4 0.8 49.7 5.62 zIHP-12-1 18.1 1.45 236.0 60.3 0.9 249.9 0.26 18.6 4.83 0.76 47.90 - zIHG-13-1 18.1 1.45 93.9 79.5 6.5 112.2 0.85 8.8 11.45 0.80 59.90 zIHG-13-2 24.1 1.92 95.7 64.5 1.7 110.5 0.67 11.3 9.06 0.79 56.38 zIHG-13-3 25.4 2.03 149.3 109.5 4.3 174.5 0.73 18.3 6.03 0.76 49.94 zIHC-13 22.5 1.8 112.9 84.5 4.1 132.4 0.8 12.8 8.8 0.8 55.4 3.89

Each aliquot is listed with corresponding single-grain cooling ages. When multiple aliquots are dated, then the mean sample ages are listed in bold font, reported with standard deviation of the aliquot population about the mean.

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CHAPTER 5: Conclusion

In Chapter 2, we used fluvial terraces and bedrock stratal relations, to constrain the structural geometry, deformation pattern, and shortening rates across the deformation front in

Kashmir. Map relations and multiple sections constrain the structural geometry at depth of SW- directed duplex along the basal décollement, folding an overlying roof thrust that joins a NE- directed wedge thrust rooting into an upper detachment horizon. Total bedrock long-term shortening rate predict ~6.5 mm/yr of shortening using a balanced cross section, using an onset of thrusting at ~3 Ma. River terraces are used to evaluate our structural interpretation and constrain intermediate deformation rates. Vector fold restoration of long terrace profiles indicates a deformation pattern characterized by regional uplift across the anticlinal axis and northern limb, and by fold limb rotation on the southern limb. Differential uplift across the fold trace suggests localized faulting, explaining an asymmetrical fold geometry.

Competing structural models for the SMA are tested using predicted terrace deformation patterns associated with a given structural interpretation (SW-directed duplex, NE-directed duplex, detachment fold; Fig. 2.14). Long wavelength tilting terraces on the southern limb, and vertical uplift in the northern limb is most consistent with a SW-directed duplex, folding an overlying roof thrust into a broad fold (Fig. 2.14). At depth, the duplex join a wedge thrust that correlates with surface faulting within the fold axis, explaining no emergent thrust on the southern limb of the SMA, and overall asymmetric fold geometry (Figs. 2.2-2.3).

Fault-related displacement at depth is assumed to relate to a 25-dipping ramp for the blind structure on the basis of dip data constraints. Since ~53 ka, the duplex structure absorbs 2.7-

3.0 mm/yr of shortening. Fold axis faulting absorbs an additional 1.1-2.4 mm/yr of shortening.

Summing the average shortening rates accounting for all deformation partitioned across the SMA yields a total of 3.8-5.4 mm/yr, averaging ~4.6 mm/yr. Shortening rate of ~4.6 mm/yr recorded in

196 geomorphic timescales are consistent with estimates of bedrock long-term shortening for the

SMA (6.5 mm/yr).

In Chapter 3, our main conclusion demonstrates that internal deformation of the Kashmir

Himalaya in the Chenab River area occurs on two fault strands at least of the Riasi fault system with Quaternary displacements, including one with evidence of Holocene surface rupture. At the

Chenab reentrant, we constrain fault activity on the strand of the RT fault zone, the Main Riasi thrust (MRT), to be between ~96-39 ka with a shortening rate of 10.8-2.8 mm/yr using offset

Pleistocene fluvial and alluvial deposits. South of the MRT, the Frontal Riasi thrust (FRT) is marked by a scarp and an offset Holocene terrace. Using a preferred abandonment age of 7.1±1.1 ka, yields a shortening rate of 8.5-4.4 mm/yr. Fault displacements on the MRT (10.8-2.8 mm/yr at

~96-39 ka timescale), and FRT (8.5-4.4 mm/yr at ~7 ka - 0 timescale), indicate sustained episodic fault activity on the RT fault zone since ~100 ka. Averaging Pleistocene-Holocene shortening rates of the MRT and FRT), yields a summed range of 10.8-4.4 mm/yr (7.6±3.6 mm/yr). Using regional cross constraints, our upper estimates of short-term shortening rates are consistent with a long-term rate of ~10 mm/yr for the RT fault zone.

Combining results from Chapter 2 and 3, the sum of shortening rates of the deformation front and the RT can account for nearly all of the geodetically constrained India-Tibet convergence of 11.1-18.8 mm/yr absorbed in Northwest and Kashmir Himalaya. Intermediate shortening rates using river terraces and Quaternary deposits for the main active structures in the

Sub-Himalaya of Kashmir, are 10.8-4.4 for the RT, and ~5.4-3.8 mm/yr for the deformation front.

Comparative long-term shortening of ~6.5 mm/yr using a balanced cross section for the deformation front is consistent with our shortening rates derived from terraces and Quaternary deposits, suggesting a long-term pattern of distributed deformation. The deformation front absorbs an average of 30% of the Himalayan shortening, comparatively lower than the RT (50%), implying that both the deformation front and the RT absorbs most of the Himalayan convergence.

197

This contrast to the central Himalaya, in which studies have shown that most of the Himalayan convergence is absorbed by the deformation front or thrust front (e.g. Lavé and Avouac, 2000).

Active internal surface-rupturing reverse faults and earthquake events in the last 4.5 ka on the RT indicate that seismic sources within the NW Himalayas include the Main Himalayan thrust (the basal décollement) and upper plate faults capable of generating moderate to great earthquakes, analogous to the Mw 7.6 2005 Kashmir earthquake. Although the SMA is a blind structure accommodating only part of the overall Himalayan-India shortening, it is a sink, nonetheless, for slip in large earthquakes on the MHT at depth. . The seismogenic zone that is the interseismic locked zone, which delineates a zone of potentially high seismic hazards for the

Kashmir Himalaya, can be constrained to be at least ~30-50 km from the deformation front

(SMA) to the RT ramp, but likely extends ~100 km to the surface projection of an interpreted mid-crustal ramp along the MHT underlying the Pir Panjal Range.

In Chapter 4, apatite and zircon (U-Th)/He cooling ages are used to quantify the deformation pattern at longer timescales in the Kashmir Himalaya. Utilizing our data with other low-temperature data from neighboring areas, we constrain the cooling history and thrust belt kinematics across a ~190 km section of the Kashmir Himalaya. Cooling patterns across the

Kashmir Himalaya reflect distributed deformation and coeval thrusting of an orogenic wedge in the Sub-Himalayan belt after 2-3 Ma. Duplexing and overthrusting across a mid-crustal ramp characterize the hinterland deformation (Kishtwar Window), transferring slip to the orogenic front along the basal décollement. Our result suggest that the exhumation pattern across the width of the Kashmir Himalaya orogen primarily reflects changes in the kinematics along the MHT, and a wedge modulated by the pre-existing basin structural architecture. Effects of local precipitation appears decoupled from exhumation rock record, suggesting that a climate-driven process is consequential to the physiography and exhumation pattern.

198

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