On the Manifestation of Deformation and Evolution of Two Cenozoic Orogens

AN ABSTRACT OF THE DISSERTATION OF

Ellen A. Lamont for the degree of Doctor of Philosophy in Geology presented on May 25, 2021

Title: On the Manifestation of Deformation and Evolution of Two Cenozoic Orogens

Abstract approved: ______Andrew J. Meigs

The word orogenesis is derived from the Greek words oros meaning mountains and genesis meaning creation and refers to the study of the complex processes involved in the growth and evolution of mountain ranges (orogens). Orogens develop where crustal deformation builds topography and forms landscapes. This dissertation explores the role of structural deformation in two Cenozoic orogens, the NW Himalaya and the Oregon Cascades - Cenozoic mountain belts that offer well preserved records of orogenic processes and deformation in the landscape.

Chapter 1 introduces orogenesis, discusses the role of faulting in deformation, presents key unknowns, and lays out what to expect in subsequent chapters. Chapters 2 and 3 investigate the spatial and temporal evolution of the NW sub-Himalayan mountain belt from the Pliocene (~5

Ma) to the present in the context of a widely accepted model for orogenesis. These chapters characterize the spatial and temporal growth of the NW-Himalaya by integrating new geologic mapping, balanced cross-sections, detrital apatite (U-Th)/He analyses and thermal history modeling with published structural and stratigraphic data. Chapter 4 considers the role of faulting in the development of subsurface permeability and the localization of magmatism and geothermal resources in the Oregon Cascade arc-backarc region. This chapter combines new, high-resolution fault trace mapping, published estimates of regional stress, and new modeling to assess the likelihood, or tendency, for a fault to slip or dilate in the modern stress field.

Chapter 5 highlights the main findings of each chapter and discusses the broader implications for the deformation and evolution of two Cenozoic mountain belts.

On the Manifestation of Deformation and Evolution of Two Cenozoic Orogens

Ellen A. Lamont

A DISSERTATION

submitted to

Oregon State University

in partial fulfillment of the requirements for the degree of

Doctor of Philosophy

Presented May 25, 2021 Commencement June 2021

Doctor of Philosophy dissertation of Ellen A. Lamont presented on May 25, 2021

APPROVED:

Major Professor, representing Geology

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

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.

Ellen A. Lamont, Author

ACKNOWLEDGEMENTS

To my advisor, Andrew Meigs: it takes a special person to patiently and diligently nurture the growth of another person; to take their hopes and dreams and help them make it a reality. But it takes an exceptional personality to take on that responsibility with gusto, to selflessly mentor another for that person’s betterment alone, and to do it with an unwavering sense of loyalty. A simple thank you could never be enough to express the tremendous impact you have had on my life, not to mention the countless others before me and those yet to come. While my journey has been full of ups and downs, you have been my greatest cheerleader, mentor, friend, and colleague along the way. You have taught me how to be an excellent leader, role model and scientist, and

I could only hope to one day be as equally ‘bullet proof’ as you. You have my most humble gratitude and upmost respect. Thank you for believing in me.

I sincerely wish to thank the incredible support community I’ve developed over my tenure at OSU. To my committee members Jessica Creveling, Frank Sousa, Rob Harris,

Eric Kirby, and my GCR Mike Olson: thank you for your patience, guidance, and support. I’m thankful for the opportunity to have worked with each of you. You have always been willing to set aside time to help me grow as a scientist. I’ve learned so much. To the STAG community: what an incredible journey we have all been on together these last 6-years. Given this crazy rollercoaster ride called graduate school, I never would have made it through without your ready shoulders to cry on, many laughs to share, brains for the picking, and friendships to be made. I especially want to thank the STAG ladies, Danielle Woodring, Israporn (Grace) Sethanant, Marina Marcelli,

MacKenzie Mark-Mozer, Sami Cargill, and Katie Worms, plus JC and Nicole Rocco.

You have all become the closest and most supportive group of friends I’ve ever had; I look forward to taking over the world with you. To the student services staff – Lori

Hartline and Robert Allan: the two of you are the heart of graduate student success.

Thank you for always keeping our best interest at the forefront of all you do.

Most important of all, a major shout out goes to my natural and elected family for always having my back. Thanks for insisting that I go to college, encouraging me to explore the world, and to strive to greater things. Thank you for providing me with every opportunity within your power and for the sacrifices made on my behalf. I hope

I’ve made you proud. Dan, you are the reason I started this journey; I am eternally grateful. Siddarth ji, thanks for being the reason I finished. Cheers to the future. I love all of you dearly.

The work for Chapters 2 and 3 is made possible by the generous support of the USIEF

Fulbright-Nehru Research Fellowship in collaboration with the Wadia Institute of Himalayan

Geology, Dehradun, India; the GSA - Awards for Geochronology Student Research (AGeS2)

Program in collaboration with the CU-TRaIL group at the University of Colorado-Boulder; and the German Academic Exchange Service (DAAD) – Short-term Research Fellowship Program in collaboration with the Low-Temperature Thermochronology Group at the University of

Potsdam, Germany. These projects are further funded by National Science Foundation (EAR-

1759200 and EAR-1759353), the Ryoichi Sasakawa Young Leaders (SLYFF) Fellowship fund, the GSA Graduate Student Research Grant Program, and the College of Earth, Ocean, and Atmospheric Sciences at Oregon State University. Thank you Aravind A, Perumal, and the

Wadia directors for facilitating my research activities. Support for Chapter 4 comes from the

U.S. Department of Energy (DE-EEO006727) as part of a larger collaboration between Oregon

State University, the University of Utah, and the Lawrence Berkeley National Laboratory.

CONTRIBUTION OF AUTHORS

1 Aravind Anilkumar, doctoral candidate in Geology at the Wadia Institute of Himalayan

Geology, Department of Structure & Tectonics assisted with field logistics, digital and field

mapping, data and sample collection for work presented in Chapters 2 and 3.

2 Rebecca Flowers, professor of Geology at the University of Colorado-Boulder, Department

of Geological Sciences co-trained me in the apatite (U-Th)/He dating technique, through

the NSF-GSA AGeS grant program. Dr. Flowers also assisted with data analysis and

interpretation and related methods written content central to Chapters 2 and 3.

3 Jayangondaperumal, Scientist-E at the Wadia Institute of Himalayan Geology, Department

of Structure and Tectonics generously hosted me at the institution for a period of 9-months

while conducting field research for Chapters 2 and 3 through a Fulbright-Nehru Research

Fellowship. Dr. Perumal provided data resources and access to lab facilities, participated

in regular research discussions, and reviewed relevant written content.

4 Andrew Meigs, academic advisor and professor at Oregon State University, College of

Earth, Ocean, and Atmospheric Sciences was pivotal to all aspects of presented research

from experimental design, project funding, field research and sample/data collection, data

analysis and interpretation, and writing of all chapters.

5 James Metcalf, research associate and CU-TRaIL lab manager at the University of

Colorado-Boulder, Department of Geological Sciences co-trained me in apatite (U-Th)/He

dating, through the NSF-GSA AGeS grant program, central to Chapters 2 and 3.

6 Edward Sobel, applied professor of geology at the University of Potsdam, Institut für

Geowissenschaften trained me in the multi-step process of heavy mineral separation as

well as the application of low temperature thermochronology methods through the DAAD

Short-Term Research Fellowship program. Dr. Sobel further assisted in data analysis and

interpretation central to Chapters 2 and 3.

7 Francis Sousa, assistant professor (senior research) at Oregon State University, College of

Earth, Ocean, and Atmospheric Sciences was pivotal to the success of Chapters 2 and 3.

Dr. Sousa assisted in field mapping and sample collection, instructed me in the use of the

thermochronology modeling software, QTQt, assisted with data and model interpretation,

participated in regular research discussions, and was involved in the writing of methods

and data analyses components of Chapters 2 and 3.

8 Danielle Wooding, former master’s student at Oregon State University, College of Earth,

Ocean, and Atmospheric Sciences assisted with field mapping and sample collection for

samples presented in Chapter 2 and participated in regular science discussions.

TABLE OF CONTENTS

Page

1. Introduction...……………………………………………………………………… 1

1.1 Orogenesis – The Making of Mountains ...……………………………….. 2

1.2 Synopsis of Research ……………………………………………………. 2

2. Foreland accretion and sub-critical wedge state based on thermochronologic constraints in the NW Himalaya by 4 Ma...... …………………...….…………… 5

2.1 Authors & Affiliations .………………………………………………..… 6

2.2 Abstract .…………………………………………………...…………..… 6

2.3 Introduction …………………………………………………………….... 6

2.4 Himalayan Orogenic Wedge ….…………………...……………..……… 7

2.5 Sub-Himalayan Kangra Transect ………………………...………………. 8

2.6 Detrital Apatite U-Th/He Data ……………….……………………………8

2.7 Foreland Exhumation: Thermal History Modeling ..………………….….. 9

2.8 Results & Discussion ………...………………………………….….…... 10

2.8.1 Foreland Deformation ………..……………….………….…….10

2.8.2 Deformation & Climate ………………………………….….…12

2.9 Conclusions ……………………………………………………………...13

2.10 Acknowledgements …………………………………………………….13

2.11 References ..…………………………………………………………….14

2.12 Appendix A ..……………………………………………………………22

TABLE OF CONTENTS (Continued)

Page

3. Synchronous activation of northwest sub-Himalayan deformation with spatially and temporally variable distributed internal deformation since at least 4 Ma constrained by thermochronology………….….……………..………………42

3.1 Authors & Affiliations .………………………………………………..…43

3.2 Abstract .…………………………………………………...…………….43

3.3 Introduction ……………………………………………………………...44

3.4 Geologic Setting of the NW Himalayan Orogen .....……………..………46

3.4.1 Regional Tectonics …………..……………….……….….……46

3.4.2 The NW Sub-Himalaya (Sutlej River to Jhelum River) ….….…47

3.5 Methods & Results ……………..………………………...………………48

3.5.1 Balanced Cross-Section Reconstruction – Chakki Region.…….48

3.5.2 Stratigraphic and Structural Constraints……………….….……49

3.5.3 Results of Balanced Section Reconstruction…………….….… 50

3.6 Detrital Apatite (U-Th)/He Data …………………………………………51

3.6.1 Sampling Schema…………………………………………...... 52

3.6.2 Apatite (U-Th)/He Laboratory Procedures …………………… 53

3.6.3 Results of (U-Th)/He Analysis………………………………… 54

3.7 Thermal History Modeling ………………………………………………54

3.7.1 QTQt Inverse Thermal Modeling …………………………...... 55

3.7.2 QTQt Model Constraints & Parameters ……………………… 55

3.7.3 Results of Modeled Apatite Thermal Histories……………….. 56

TABLE OF CONTENTS (Continued)

Page

3.8 Discussions ……………………………………………………………... 57

3.8.1 Spatial Distribution of Shortening in NW Sub-Himalaya …...... 57

3.8.2 Temporal Distribution of Shortening in NW Sub-Himalaya..… 60

3.8.3 Diachronous foreland accretion along the Himalayan arc……... 62

3.8.4 Implications for Orogen Mechanics & Hazard Assessment ….. 63

3.9 Conclusions ……………………………………………………..…….... 65

3.10 Acknowledgements ………………..…………………………..……… 66

3.11 References ..……………………………………………………..…….. 66

3.12 Appendix B ..……………………………………………………………86

4. Fault-related permeability in Oregon’s Cascadia backarc and implications for magmatic and geothermal fluid flow..……………………...…………………95

4.1 Authors & Affiliations .………………………………………………..…96

4.2 Abstract .…………………………………………………...…………….96

4.3 Introduction ……………………………………………………………...97

4.4 Geologic Setting of the Oregon Cascades……...... ……………..………99

4.4.1 Subduction Arc Volcanism…..……………….……….….…….99

4.4.2 Regional Tectonics …………………………….....……….….. 99

4.4.3 Permeability, Fluid Transport and Heat Flow …...……….…. 101

4.5 Data & Methods ………………..………………………...……………. 103

4.5.1 Fault Mapping …………………………………..………..….. 103

4.5.2 Regional State of Stress ………....……………………….…... 104

TABLE OF CONTENTS (Continued)

Page

4.5.3 Slip & Dilation Tendency .....…………………………….…... 105

4.6 Results ……………………………………………………….….…….. 107

4.6.1 Tendency Analysis …………………………………………... 107

4.6.2 Analysis of Variation in Stress Tensor Magnitudes…………. 110

4.7 Discussion ...…………………………………………………………… 111

4.7.1 Faulting, Stress & Slip Tendency .…………………………… 111

4.7.2 Tendency, Permeability & the Localization of Magmatism on Faults ……………………………………………….……. 112

4.7.3 Implications for Geothermal Energy Exploration in the Oregon Cascade Backarc.……………………..……………... 114

4.8 Conclusions ……………………………………………………..…….. 115

4.9 Acknowledgements ……………………………………………..…..… 116

4.10 References ……………………………………………………..…….. 117

4.11 Appendix C ………………………………………………………….. 137

5. Conclusions……………………………………………………………………... 150

LIST OF FIGURES

Figure Page

2.1 Spatial and Geologic Context of the Study Area ………………………………...17

2.1A Geologic setting of the Himalayan orogen ………………...………..… 17

2.1B Kangra Reentrant map and AHe sample sites …...………….…………. 17

2.1C Modeled AHe exhumation ages along Kangra Transect ……………… 17

2.2 U-Th/He apatite sample location ...……………………………………………… 19

2.2A Map …………………………………………………………………… 19

2.2B Structural and stratigraphic position ………...………………………… 19

2.2C Time ……………………………………………………………………19

2.3 Time-temperature pathways (tT) from QTQt thermal model given depositional and AHe age inputs for Sample 1 ……………………………………………...... 20

2.4 Space-time structural evolution …………………………………….……….…...21

2.4A-C Pattern of new (bold), on-going (solid) and uncertain (dashed) faulting ………………………...……………………………………21

2.4D Cumulative slip rate for each time interval ……………….………….....21

3.1 Location map of the NW Himalaya (India) and Potwar Plateau (Pakistan) regions with labeled geographic features and major cities …………..………...... 75

3.2 Stratigraphy of the Cenozoic Foreland Basin Sequence involved in deformation within the Chakki study area, Himachal Pradesh ……....………...... 77

3.3 Structural framework of the Chakki study area of the NW sub-Himalaya.……… 78

3.3A Geologic map across the deformed foreland sequence of the Chakki region ……………………………….……………...………… 78

3.3B Deformed balanced cross-section reconstruction across the Chakki sub-Himalaya …...…………………………………..……….………...78

3.3C Restored, line-length balanced cross-section across the Chakki sub-Himalaya …………………………………………………..……...78

LIST OF FIGURES (Continued)

Figure Page

3.4 Location and analysis of apatite (U-Th)/He samples from the Chakki sub-Himalaya.…………………………………………………………………... 80

3.4A Location map of AHe samples across the NW sub-Himalaya from new (blue points -this study) and previous (black points - Lamont et al., Chapter 2; Gavillot et al., 2018) studies …………….….………. 80

3.4B Assessment of thermal resetting of individual apatite grains …...…..… 80

3.4C Example thermal history model for Sample 27 on the SMA deformation front …………………………..…………………..……... 80

3.5 Compilation of new and published balanced cross-section reconstructions from across the NW Himalaya with modeled apatite (U-Th)/He (AHe) fault and fold-related exhumation initiation ages ……………………………..…….... 82

3.6 Summary map schematic of lateral structural variations as observed by AHe dates across the NW sub-Himalaya ……………………………………..……..... 84

4.1 Geologic context of the Oregon Cascade backarc study area ………..……….... 123

4.2 Comparison of 10m vs. 1m resolution digital elevation models for fault mapping ……………………………………………………………………….. 125

4.3 Visualization of stress state and slip tendency ..……………………………….. 128

4.4 Slip (Ts), B. dilation (Td), and C. combined (Tsd) tendency for a minimum principal stress (σ3) direction of 80° in Andersonian normal fault-like stress regime.…………………………………………………………………... 129

4.5 Slip (Ts), B. dilation (Td), and C. combined (Tsd) tendency for a minimum principal stress (σ3) direction of 80° in Andersonian strike-slip fault-like stress regime.…………………………………………………………………... 131

4.6 Plot of slip (left) and dilation (right) tendency with respect to variations in stress magnitudes for different orientation of σ3 (≈ θHmin)..…………….………….. 134

LIST OF FIGURES (Continued)

Figure Page

4.7 Interpretation of permeability in the Oregon Cascade backarc…….…………… 135

4.7A Fault Termination at Mt. Jefferson.………….……………...………... 135

4.7B Fault Intersection – Volcanic Vents..………………..……….………. 135

4.7C Newberry – Walker Rim Step Over...…………………………..….… 135

4.7D Fault Termination at Crater Lake...……………………………..….… 135

LIST OF TABLES

Table Page

4.1 Compilation of Stress State Data for the Study Area and Neighboring Regions ………………………………………………………...……..……….. 126

4.2 Stress tensor geometries used in individual tendency analyses ……………….. 127

4.3 Summary of tendency analysis results for all tested orientations of σ3 under both normal and strike slip stress regime endmembers ………………………... 133

LIST OF APPENDICES

Appendix Page

A. Chapter 2 Supplemental Materials ……...……………………………… 22

A-1 Stratigraphy of the sub-Himalayan Kangra Transect .………… 23

A-2 Detrital Apatite U-Th/He…...………………………...………... 23

A-3 Foreland Exhumation - Modeling Cooling Histories ……..…... 26

A-4 Shortening Rate Calculations ………...……………………….. 28

A-5 References ……………………………………………………... 30

B. Chapter 3 Supplemental Materials ……...……………………………… 86

B-1 (U-Th)/He Samples & Analysis Data Tables ……… .………… 87

B-2 QTQt Thermal History Models..……………………...………... 93

C. Chapter 4 Supplemental Materials ……...…………………………….. 137

C-1 Model Results for Tendency Analysis. ……………...……….. 138

LIST OF APPENDIX FIGURES

Figure Page

2.1S Plots of sample stratigraphic ages versus AHe ages …………….……………. 32

2.1S-A Distribution of individual AHe grain ages versus stratigraphic age color-coded by sample .…………...……….………………………..……… 32

2.1S-B Distribution of average sample AHe date versus stratigraphic age …32

2.2S All modeled time-temperature pathways for samples along the Kangra Transect ...……………………………………………….…………………….. 34

2.3S Interpretative diagrams with estimates of initiation of fault-related exhumation cooling along major fault structures across the Kangra Transect and the associated cooling ages of samples in exhumed hanging walls and fold cores …………….36

3.1S All modeled time-temperature pathways for samples along the Chakki Transect ……………………………………………………………...…………93

4.1S Slip (Ts), B. dilation (Td), and C. combined (Tsd) tendency for a minimum principal stress (σ3) direction of 60° in an Andersonian normal fault-like stress regime.………………………………………………………...……….. 138

4.2S Slip (Ts), B. dilation (Td), and C. combined (Tsd) tendency for a minimum principal stress (σ3) direction of 60° in an Andersonian strike-slip fault-like stress regime.………………………………………………………...……...…140

4.3S Slip (Ts), B. dilation (Td), and C. combined (Tsd) tendency for a minimum principal stress (σ3) direction of 100° in an Andersonian normal fault-like stress regime.………………………………………………………...……...…142

4.4S Slip (Ts), B. dilation (Td), and C. combined (Tsd) tendency for a minimum principal stress (σ3) direction of 100° in an Andersonian strike-slip fault-like stress regime.………………………………………………………...………...144

4.5S Slip (Ts), B. dilation (Td), and C. combined (Tsd) tendency for a minimum principal stress (σ3) direction of 120° in an Andersonian normal fault-like stress regime.………………………………………………………...………...146

4.6S Slip (Ts), B. dilation (Td), and C. combined (Tsd) tendency for a minimum principal stress (σ3) direction of 120° in an Andersonian normal fault-like stress regime.………………………………………………………...………...148

LIST OF APPENDIX TABLES

Table Page

2.1S Sample Location and Stratigraphic Age Constraints …………………………..37

2.2S Summary of (U-Th)/He Sample Analysis with corrected age...……...………...38

2.3S (U-Th)/He Thermal History Model Input Parameters …………………………40

3.1S Sample Location and Stratigraphic Age Constraints …………………………..87

3.2S Summary of (U-Th)/He Sample Analysis with corrected age...……...………...88

3.3S (U-Th)/He Thermal History Model Input Parameters …………………………90

LIST OF ABBREVIATIONS

Chapter 2 & 3

Abbreviation Definition

Ma ……………………………………………………………...... mega annum m.y. ……………………………………………………………... million years km ………………………………………………………………..… kilometers mm/yr ………………………………………………...….. millimeters per year

AHe ………………..………………………. apatite helium thermochronology

PRZ …………………………………………………….. partial retention zone t - T ………………………………………………………... time - temperature

MATA …………………………………….. minimum age of thrust activation

MFT …………………………………………………….. Main Central Thrust

ST ………………………………………………………………... Soan Thrust

JT ………………………………………………………... Jawalamukhi Thrust

PR ……………………………………………………………….. Paror Thrust

PAL …………………………………………………………. Palampur Thrust

MBT ……………………………………………….…. Main Boundary Thrust

MCT …………………………………………………….. Main Central Thrust

JA ………………………………………………………..…. Janauri Anticline

SA ……………………………………………….……………. Soan Anticline

SMA ……………………………………………….. Surin Mastgarh Anticline

SRT ……………………………………….……………….. Salt Range Thrust

PA ………………………………………………………….…. Paror Anticline

LIST OF ABBREVIATIONS (Continued)

Chapter 2 & 3

Abbreviation Definition

MHT …………………..…………………………….. Main Himalayan Thrust

US ………………………………………………….. Upper Siwalik Formation

MS ……………………………………………...… Middle Siwalik Formation

LS …………………………………………………. Lower Siwalik Formation

UD …………………………………… Upper Dharmsala (Murree) Formation

LD …………………………………… Lower Dharmsala (Murree) Formation

DEDICATION

This dissertation is dedicated to my fellow blue-collar scholars who dared to challenge the status quo, journey beyond the norms and knowns, and wound up in a brave new world…May your dreams carry you forward.

CHAPTER 1:

INTRODUCTION

1.1 Orogenesis – The Making of Mountains

The word orogenesis is derived from the Greek words oros meaning mountains and genesis meaning creation and refers to the study of the complex processes involved in the growth and evolution of mountain ranges (orogens). Orogens develop in a range of environments, including at plate boundaries or intracontinental settings, and result from extensional, contractional, and translational processes. Some of the most recognizable orogens develop at convergent plate margins where shortening and lithospheric thickening create landscapes and build topography.

A suite of geodynamic and degradational variables such as gravity, flexure, isostacy and climate that sustain or denude the landscape and modulate constructional processes.

Understanding the patterns and coupling between constructional and destructional processes acting on an orogen, as well as the timing and/or partitioning of strain on orogenic structures is critical for addressing:

(1) how deformation is manifested in mountain ranges;

(2) how the spatial and temporal evolution of an orogen is encoded within a landscape and

subsequently deciphered;

(3) how earth resources are localized in deformation zones;

(4) how associated hazards will impact vulnerable communities.

1.2 Research Synopsis

This dissertation explores the role of structural deformation in two different Cenozoic orogens, the NW Himalaya and the Oregon Cascades. Cenozoic orogens are young and thus offer a better chance of preserving records of orogenic processes and their drivers. Excellent records are required to evaluate processes occurring at the land-surface interface, such as the coupling between climate and tectonics.

In the Himalaya, orogenic growth is the product of ongoing Indo-Eurasian continental collision.

Active deformation is largely concentrated near the front of the orogenic wedge where a large portion of the overall plate convergence is absorbed. Because the distribution of shortening and changes in global climate (e.g. the Asian monsoons) co-evoluted, the Himalaya have long served as the locus of studies of climate-tectonic coupling and their respective roles in driving mountain range evolution. The work presented from the NW Himalaya seeks to constrain the spatial-temporal distribution of shortening accommodated in the deformed foreland, as well as the timing of foreland accretion at the orogenic deformation front. Interpreting the timing and pattern of deformation of the deformed foreland in the context of critical wedge mechanics provides a framework to understand the interrelations between internal deformation, erosional efficiency, and other variables that govern mountain building.

The work presented in Chapters 2 and 3 aims to understand when, where, and how deformation is partitioned across the active structures within the NW sub-Himalaya. We integrate new geologic mapping, balanced sections and detrital (U-Th)/He analyses with published structural and stratigraphic datasets to develop models of orogenic evolution for the Kangra Reentrant and the larger NW sub-Himalayan fold-and-thrust belt. These studies provide a new understanding of orogenic deformation mechanics in the NW sub-Himalaya, demonstrating the important role of distributed deformation following frontal accretion in mountain range growth and evolution. Further, the results suggest climatically driven erosion, although a major driver of material efflux in landscapes, contributed to and likely prolonged a pattern of internal deformation triggered by accretion at the orogenic deformation front.

In the Oregon Cascades arc-backarc, structural evolution is driven by synchronous Basin and

Range extension, oblique translation and rotation of the forearc, and subduction-related arc volcanism. This unique transtensional rift zone results in the development of a dense and irregular network of faults that dissect the arc-backarc landscape. Such young, hot, and

structurally complex environments are well poised for geothermal exploration because faulting contributes to the development of subsurface permeability and the localization of geothermal resources and volcanism, as shown in similar settings. Chapter 4 explores the potential coupling between faulting and fluid flow across the Oregon Cascade arc-backarc. This study uses slip and dilation tendency analysis, a 1st order assessment of potential for failure and/or dilation on faults under a given stress state. Fault networks play a central role in the development of subsurface permeability, localization of magmatism, and geothermal resources. Results from the analysis indicate an 80° azimuth for the direction of minimum principal stress in the arc- backarc best explains the array of mapped fault orientations across the landscape and observations of the broadly distributed occurrence of volcanism concentrated on structures of the same vast orientations. This analysis demonstrates the potential for enhanced permeability in several Oregon Cascade locations warranting more detailed investigation.

CHAPTER 2:

FORELAND ACCRETION AND SUB-CRITICAL WEDGE STATE BASED ON THERMOCHRONOLOGIC CONSTRAINTS IN THE NW HIMALAYA BY 4 MA

2.1 Authors & Affiliations

Ellen A. Lamont 1, Francis Sousa 1, Andrew J. Meigs 1, Jayangondaperumal 2, Rebecca M.

Flowers 3, James Metcalf 3, Aravind Anilkumar 2, Danielle Woodring 1, Edward R. Sobel 4

1 Oregon State University, College of Earth, Ocean, and Atmospheric Sciences

2 Wadia Institute of Himalayan Geology, Department of Structure and Tectonics

3 University of Colorado-Boulder, Department of Geology

4 University of Potsdam, Institut für Geowissenschaften

2.2 Abstract

Models demonstrate that climate can control the structural evolution of mountain belts and that Plio-Quaternary global climate change modulated Himalayan wedge behavior. The key test of this hypothesis is the timing of foreland accretion. If foreland accretion predates climate change then climate is not the primary driver of mountain building. New detrital apatite

(U-Th)/He data from NW-Himalaya foreland thrust sheets demonstrate that accretion was underway by 4 Ma, prior to the Plio-Quaternary global climate change. Climate cannot, therefore, be invoked as the principal driver of Himalayan tectonics in the late Cenozoic.

2.3 Introduction

Cenozoic orogenic growth, linkage to the Asian Monsoon, and Plio-Quaternary changes in global climate (Molnar, 2004) make the Himalaya an ideal mountain belt to investigate the relationship between crustal deformation and surface processes. Critical wedge theory provides a widely accepted framework to predict deformation behavior in response to changing material influx and efflux (Davis et al., 1983). Whereas, distributed internal deformation is expected from both frontal accretion (Meigs and Burbank, 1997) and climate-related increases in erosional efficiency (Whipple, 2009), the timing of foreland accretion relative to Plio-

Quaternary climate change at 3-4 Ma (Molnar, 2004) provides a key constraint for the role of climate as a driver of deformation within the Himalayan orogenic wedge.

Published estimates of the timing of foreland accretion and wedge widening is uncertain and varies significantly along the length of the Himalaya (Fig. 2.1A). Activation of the thrust front in Nepal is suggested to have initiated by ~2 Ma (Mugnier et al., 2004; van der Beek et al.,

2006). In India, sedimentological (e.g. Ranga Rao et al., 1988) and paleomagnetic (e.g. Sinha et al., 2007) data imply thrust formation after 1.7 Ma in the Kashmir Himalaya, whereas low temperature thermochronology suggests the onset of folding at the deformation front by ~4 Ma

(Gavillot et al., 2018). New thermochronometric data from Pakistan (Ghani et al., 2020) corroborate sedimentological evidence (Burbank and Beck, 1989) of thrust front initiation by

4.5-5.2 Ma.

New apatite U-Th/He (AHe) data from five major sub-Himalayan thrust sheets within the

Kangra reentrant of NW India, coupled with the critical wedge model, allow us to interpret wedge deformation patterns in the context of crustal and surface processes acting through the

Plio-Quaternary (Fig. 2.1B). Integration of these AHe ages with published geologic, stratigraphic, and geochronologic data (e.g. Powers et al., 1998) reveal the spatio-temporal record of orogenic growth. These new data suggest that foreland accretion by 4 Ma shifted the wedge to a subcritical state along more than 600 km of the Himalayan front from Pakistan to

NW India. Climate enhanced erosional removal after the Plio-Quaternary transition prolonged the period of distributed deformation and wedge taper recovery prompted by accretion.

2.4 The Himalayan Orogenic Wedge

Three morphotectonic belts comprise the Himalayan orogenic hinterland: the High, Lesser and

Sub-Himalaya (Fig. 2.1A)(Gansser, 1964). Major thrust faults, the Main Central Thrust (MCT),

Main Boundary Thrust (MBT), and Main Frontal Thrust (MFT), bound these belts and sole

into the Main Himalayan Thrust, the basal detachment, at depth (Nábělek et al., 2009). The

MCT and MBT formed by ~20 and ~10 Ma, respectively (Hodges, 2000; Meigs et al., 1995).

Between the MBT and MFT (Ranga Rao and Datta, 1976), several orogen-parallel thrusts, back thrusts, fault-cored folds and piggyback basins reflect shortening of sub-Himalayan foreland strata. Foreland basin deposits are primarily sourced from hinterland thrust sheets (Brozovic and Burbank, 2000). The deformation front is represented by a series of discontinuous folds and emergent or blind thrusts along the MFT (Yeats and Thakur, 2008).

2.5 The Sub-Himalayan Kangra Transect

The Kangra reentrant (Fig. 2.1B), a structural recess defined by the sinuous MBT trace, represents the widest exposure of deformed foreland (~100 km) along the length of the

Himalaya. The active MFT and associated Janauri Anticline mark the local deformation front

(Delcaillau et al., 2006). A balanced cross-section (Kangra transect; Fig. 2.1C) reveals the geometry and shortening accommodated within the Kangra sub-Himalaya (Powers et al.,

1998). Constraints on the structural geometry and depth to décollement along the Kangra transect include surface mapping (Ranga Rao and Datta, 1976), well data (Oil and Natural Gas

Commission data), and seismic reflection profiles (Powers et al., 1998). Five major south- vergent thrusts deform the foreland from south to north, respectively: the MFT; Soan (ST);

Jawalamukhi (JT); Paror (PT); and Palampur (PAL) thrusts, and several back thrusts. Hanging wall anticlines are associated with the MFT (JA), Soan (SA), and Paror (PA) thrusts (Fig. 2.1B).

2.6 Detrital Apatite U-Th/He Data

AHe samples were collected from the lowest stratigraphic position representing the most deeply exhumed exposed strata in the hanging wall of each of the five thrust sheets (Figs. 2.1 and 2.2). AHe analyses of 14 samples from the formations of the Siwalik Group yielded a total

of 66 new single-grain dates that range from 2.1- 463.2 Ma. Most dates are <14 Ma. Analytical methods and individual grain analytical data are reported in Appendix A.

Stratigraphic constraints provide basin depositional age brackets used to determine the timing and magnitude of post-depositional heating and cooling in thermal models. Depositional age control for Siwalik units is based on magnetostratigraphy (Meigs et al., 1995). Lower, Middle and Upper Siwalik strata depositional ages are 12.3-10.9 Ma, 10.9–6.8 Ma and 6.8–4.6 Ma, respectively (Fig. 2.2C). Structural and seismic data along the Transect indicate that strata at the lowest structural level are exhumed from depths of ~2-3 km (MFT and ST) to ~3.5-4 km

(JT; Fig. 2.2B) (Powers et al., 1998). AHe dates are mostly younger than depositional ages and thus directly constrain post-depositional thermal history (Fig. 2.1C; Appendix A – Fig. 2.2S).

2.7 Foreland Exhumation: Modeling Cooling Histories

Any individual (U-Th)/He date is consistent with a range of thermal histories. Closure temperature can vary significantly amongst grains based on cooling rate, grain size, actinide concentration, and apatite chemistry (e.g. Sousa and Farley, 2020). AHe dates across the JT thrust sheet include reset samples proximal to the thrust at the deepest stratigraphic levels (Fig.

2.2). Samples from upper Middle Siwalik and higher stratigraphic levels (samples 10-14, Fig.

2.2B) are not fully reset or are not reset. We utilize the thermal history modeling software package, QTQt (Gallagher, 2012) to extract thermal history information from single grain data.

Helium diffusion parameters of Flowers et al. (2009) account for potential closure temperature variation. All individual samples were modeled and include all single grain data as inputs.

Appendix A describes model details and presents all model results.

Integration of stratigraphic and AHe data in QTQt distinguishes samples characterized by full thermal resetting of all measured grains (e.g. Sample 1, Fig. 2.3) from samples that are not fully reset (NFR; Fig. 2.1C). To interpret the chronology of thrusting, we focus on fully reset samples

collected from the oldest exposed hanging wall strata. QTQt models using AHe grain dates and sample depositional ages return a suite of potential thermal pathways that transition from burial

(heating) to exhumation (cooling). We interpret this transition as the onset of thrust-related exhumation. We conservatively define the youngest modeled inflection as the point by which cooling must initiate (e.g. Fig. 2.3). Initiation ages younger than this point are not allowed by the AHe data and depositional ages.

Thermal modeling indicates that cooling, and therefore exhumation and faulting, was underway by 4 Ma on the MFT (Fig. 2.3), by 2.5 Ma on the ST, and by ~2 Ma on the JT (Fig. 2.1C).

Isothermal lateral translation above the MHT is precluded by the fault-bedding geometry.

Timing of exhumation on the PR and PAL faults is unconstrained, as samples did not yield dateable apatite grains. Depositional ages of deformed units provide the only brackets for motion on the PAL and PR faults (<11 Ma and <7.5 Ma, respectively).

2.8 Results & Discussion

2.8.1 Foreland Deformation

We divide deformation of the NW foreland into two periods – (I) 4-2 Ma and (II) 2-0 Ma - based on the onset timing of thrust-related exhumation interpreted from thermal models (Fig.

2.4A-C). Prior to 4 Ma, deformation was likely concentrated in the orogenic hinterland. For each sampled structure, maximum shortening rates are calculated using the Powers et al. (1998) balanced cross-section and fault-related exhumation age (details in Appendix A-4).

(I) The MFT and ST are active between 4-2 Ma. Thermal models for the MFT hanging wall indicate exhumation onset by 4 Ma, marking accretion of the undeformed foreland to the orogenic wedge (Fig. 2.3). Ongoing MFT deformation and onset of the ST by 2.5 Ma focus deformation near the front during this period. This timing compares favorably with the pre-4

Ma initiation age of the Surin-Mastgarh Anticline (SMA), the deformation front in Jammu &

Kashmir (Gavillot et al., 2018) and the ~5 Ma onset of the Salt Range thrust in Pakistan

(Burbank and Beck, 1989; Ghani et al., 2020). To the SE in Nepal, initiation of the MFT and foreland accretion occurred after ~2 Ma (Mugnier et al., 2004; van der Beek et al., 2006), which reveals a diachronous pattern of foreland accretion along strike from NW to SE. Average shortening rates for the MFT and ST are 0.6 and 1.5 mm/yr, respectively.

(II) Distributed thrusting characterizes wedge deformation following foreland accretion from

2-0 Ma. Exhumation ages indicate motion on the JT and footwall structures by 2 Ma. The minimum shortening rate for the JT and related structures is 6.9 mm/yr. Shortening is assumed to continue on the MFT, ST and other faults through this period (Thakur et al., 2014). This pattern of distributed deformation is also observed to the NW of the Kangra reentrant with exhumation ages ≤ 4 Ma on structures north of the SMA (Gavillot et al., 2018). Accretion followed by distributed deformation characterizes several global thrust belts including the

Andes, Taiwan, and Pyrenees (Jordan et al., 1993; Le Béon et al., 2014; Meigs and Burbank,

1997). Critical taper models indicate that frontal accretion causes orogenic widening, reduces wedge taper and forces internal deformation to restore taper (Davis et al., 1983).

Assuming that the ~14 mm/yr geodetic S. Tibet-India convergence rate across the NW

Himalaya is a proxy for the long-term rate (Banerjee and Bürgmann, 2002; Powers et al., 1998), individual-structure shortening rates should sum to the geodetic rate (Fig. 2.4D). Before 4Ma, hinterland structures accommodate the shortening. Between 4-2 Ma, roughly ½ the geodetic rate (~7 mm/yr; including the PR and MBT) is accounted for by active foreland structures.

Ages of the PAL and PR faults are unconstrained and thus likely lower the slip deficit. Whereas the age of the PAL is unknown, the stratigraphic separation, unit thicknesses and fault geometry suggest the thrust accommodates between 11-15 km of slip. After 2 Ma, the sum of individual shortening rates roughly equals the geodetic rate (~13.5 mm/yr). Note that the sum assumes that the MBT continues to move after 2 Ma, as argued by some (Mukherjee, 2015; Thiede et

al., 2017). Thakur et al. (2010) report that PAL is active. Finally, the Piedmont Fault, a blind thrust and associated fold in the footwall of the MFT (Thakur et al., 2014) absorbs at most a few meters of shortening.

2.8.2 Deformation & Climate

Critical wedge theory is widely utilized to understand the relative impact of mass added to the orogen, via tectonic fluxes, and removed from the orogen, via erosional fluxes in thrust belt evolution. When these fluxes are balanced, a ‘critical’ taper characterizes the orogen such that stresses everywhere within the wedge are near failure (Davis et al., 1983). If material influx exceeds efflux due to frontal accretion and instantaneous widening, the wedge becomes

‘subcritical’. Subsequent narrowing from internal deformation behind the thrust front acts to rebuild wedge taper. Similar patterns of narrowing and internal deformation are expected for scenarios where the erosive efflux of mass outpaces tectonic influx and for frontal accretion.

A thrust front jump by ~100 km south of the MBT by 4 Ma accreted the undeformed foreland to the wedge. Prior to accretion, shortening is thought to have been accommodated primarily on the MBT and internal structures (Mukherjee, 2015; Thiede et al., 2017). Following MFT initiation, distributed internal deformation characterized the spatial and temporal pattern of shortening. Thus, at the onset of the Plio-Quaternary climate transition, the NW Himalayan wedge was subcritical. The climate transition marked the onset of a period of colder and more variable climatic conditions, which is associated with increased sediment fluxes globally

(Molnar, 2004). Although some argue that climate-driven enhanced erosion was associated with MFT initiation (Hirschmiller et al., 2014), exhumation ages of structures along more than

600 km of the orogenic front indicate that the MFT initiated prior to the climate shift. Enhanced erosion following the climate transition likely acted to sustain distributed deformation and delay a return to critical taper.

2.9 Conclusions

Detrital AHe data from deformed foreland basin sediments provide new constraints on the timing of structure-related exhumation that, combined with critical wedge models, address the relationship between crustal deformation and surface processes in the NW Himalaya. In the

Kangra reentrant, the sub-Himalayan belt accreted to the wedge by 4 Ma. Frontal accretion shifted the Himalayan wedge to a subcritical state prior to the Plio-Quaternary climate transition. Subsequent deformation was distributed on imbricate thrusts between the MBT and

MFT from 4 Ma to the present with the cumulative shortening rate across the Kangra sub-

Himalaya roughly equaling the modern geodetic convergence rate by 2 Ma. Enhanced erosion following the climate transition likely sustained the subcritical wedge established following accretion of the foreland.

2.10 Acknowledgements

This material is based upon work supported by the National Science Foundation under Grant

Nos. EAR-1759200 and EAR-1759353. Opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Thanks to the AGeS program for its support.

The Fulbright-Nehru Research Program of the US-India Education Foundation, the German

Academic Exchange Service, the Ryoichi Sasakawa Young Leaders Fellowship Fund, and

OSU provided support. Mineral separations supported by Dr. R. Patel (Kurukshetra University,

India), GeoSeps, Moscow, ID, and J. Worster. Shri P. Kumar and the directors at the Wadia

Institute of Himalayan Geology are thanked for their generous assistance.

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Burbank, D. W., and Beck, R. A., 1989, Early Pliocene uplift of the Salt Range: Temporal and constraints on thrust wedge development, in Malinconico, L. L., and Lillie, R. J., eds., Tectonics of the Western Himalaya, Volume Special Paper 232: Boulder, Geological Society of America, p. 113-128.

Davis, D. M., Suppe, J., and Dahlen, F. A., 1983, Mechanics of fold-and-thrust belts and accretionary wedges: Journal of Geophysical Research, v. 88, no. B2, p. 1153-1172.

Delcaillau, B., Carozza, J.-M., and Laville, E., 2006, Recent fold growth and drainage development: The Janauri and Chandigarh anticlines in the Siwalik foothills, northwest India: Geomorphology, v. 76, p. 241-256.

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Gallagher, K., 2012, Transdimensional inverse thermal history modeling for quantitative thermochronology: Journal of Geophysical Research: Solid Earth, v. 117, no. B2.

Gansser, A., 1964, Geology of the Himalayas, London, Interscience Publishers, 289 p.:

Gavillot, Y., Meigs, A. J., Sousa, F. J., Stockli, D., Yule, D., and Malik, M., 2018, Late Cenozoic foreland‐to‐hinterland low‐temperature exhumation history of the Kashmir Himalaya: Tectonics, v. 37, no. 9, p. 3041-3068.

Ghani, H., Sobel, E. R., Zeilinger, G., Glodny, J., Zapata, S., and Irum, I., 2020, Palaeozoic and Pliocene tectonic evolution of the Salt Range constrained by low‐temperature thermochronology: Terra Nova.

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Hodges, K. V., 2000, Tectonics of the Himalaya and Tibet from two perspectives: Geological Society of America Bulletin, v. 112, no. 3, p. 324-350.

Jordan, T., Allmendinger, R., Damanti, J., and Drake, R., 1993, Chronology of motion in a complete thrust belt: the Precordillera, 30-31 S, Andes Mountains: The Journal of Geology, v. 101, no. 2, p. 135-156.

Le Béon, M., Suppe, J., Jaiswal, M. K., Chen, Y. G., and Ustaszewski, M. E., 2014, Deciphering cumulative fault slip vectors from fold scarps: Relationships between long‐term

and coseismic deformations in central Western Taiwan: Journal of Geophysical Research: Solid Earth, v. 119, no. 7, p. 5943-5978.

Meigs, A. J., and Burbank, D. W., 1997, Growth of the south Pyrenean orogenic wedge: Tectonics, v. 16, p. 239-258.

Meigs, A. J., Burbank, D. W., and Beck, R. A., 1995, Middle-late Miocene (>10 Ma) formation of the Main Boundary thrust in the western Himalaya: Geology, v. 23, no. 5, p. 423- 426.

Molnar, P., 2004, Late Cenozoic increase in accumulation rates of terrestrial sediment: How might climate change have affected erosion rates?: Annu. Rev. Earth Planet. Sci., v. 32, p. 67- 89.

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

Mukherjee, S., 2015, A review on out-of-sequence deformation in the Himalaya: Geological Society, London, Special Publications, v. 412, no. 1, p. 67-109.

Nábělek, J., Hetényi, G., Vergne, J., Sapkota, S., Kafle, B., Jiang, M., Su, H., Chen, J., and Huang, B.-S., 2009, Underplating in the Himalaya-Tibet collision zone revealed by the Hi- CLIMB experiment: Science, v. 325, no. 5946, p. 1371-1374.

Powers, P. M., Lillie, R. J., and Yeats, R. S., 1998, Structure and shortening of the Kangra and Dehra Dun reentrants, Sub-Himalaya, India: Geological Society of America Bulletin, v. 110, p. 1010-1027.

Ranga Rao, A., Agarwal, R., Sharma, U., Bhalla, M., and Nanda, A., 1988, Magnetic polarity stratigraphy and vertebrate palaeontology of the Upper Siwalik Subgroup of Jammu Hills, India: Journal of the Geological Society of India, v. 31, no. 4, p. 361-385.

Ranga Rao, A., and Datta, N., Geological map of the Himalayan foot-hills, in Proceedings Himalayan Geology Seminar, Section III: Oil and Natural Gas Resources, Oil and Natural Gas Commission: New Delhi, India, Sheet A, scale1976, Volume 1.

Sakai, H., Sakai, H., Yahagi, W., Fujii, R., Hayashi, T., and Upreti, B. N., 2006, Pleistocene rapid uplift of the Himalayan frontal ranges recorded in the Kathmandu and Siwalik basins: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 241, p. 16-27.

Sinha, R., Kumar, R., Sinha, S., Tandon, S. K., and Gibling, M. R., 2007, Late Ceonzoic fluvial successions in northern and western India: an overview and syntheis: Quaternary Science Reviews, v. 26, p. 2801-2822.

Sousa, F. J., and Farley, K. A., 2020, A Framework for Evaluating Variation in (U-Th)/He Datasets: Minerals, v. 10, no. 12, p. 1111.

Thakur, V., Joshi, M., Sahoo, D., Suresh, N., Jayangondapermal, R., and Singh, A., 2014, Partitioning of convergence in Northwest Sub-Himalaya: estimation of late Quaternary uplift

and convergence rates across the Kangra reentrant, North India: International Journal of Earth Sciences, v. 103, no. 4, p. 1037-1056.

Thakur, V. C., Jayangondaperumal, R., and Malik, M. A., 2010, Redefining Medlicott-Wadia's main boundary fault from Jhelum to Yamuna: An active fault strand of the main boundary thrust in northwest Himalaya: Tectonophysics, v. 489, p. 29-42.

Valdiya, K. S., 2003, Reactivation of the Himalayan frontal fault: Implications Current Science, v. 85, no. 7, p. 1031-1040. van der Beek, P., Robert, X., Mugnier, J. L., Bernet, M., Huyghe, P., and Labrin, E., 2006, Late Miocene–recent exhumation of the central Himalaya and recycling in the foreland basin assessed by apatite fission‐track thermochronology of Siwalik sediments, Nepal: Basin Research, v. 18, no. 4, p. 413-434.

Whipple, K. X., 2009, The influece of climate on the tectonic evolution of mountain belts: Nature Geoscience, v. 2, no. 25 January 2009, p. 97-104.

Yeats, R. S., and Thakur, V. C., 2008, Active faulting south of the Himalayan Front: Establishing a new plate boundary: Tectonophysics, v. 453, no. 1, p. 63-73.

; ;

–

B, Fig. 1 B, Fig.

Jawalamukhi Jawalamukhi

–

Main Boundary Thrust; MFT MFT Thrust; Boundary Main

–

narrow narrow black lines with axial JT Anticline.

–

Main Central Thrust; MBT MBT Thrust; Central Main

–

. Kangra Reentrant map and AHe sample sites (white numbered dots; dots; numbered (white sample sites AHe map and Reentrant Kangra .

. Modeled AHe exhumation ages along Kangra Transect (A Transect along Kangra ages exhumation AHe Modeled .

Geologic Geologic setting of the orogen Himalayan from (modified Hodges, 2000). Major

. .

black black barbed Folds lines.

–

Palampur Thrust. Palampur Thrust.

–

South Tibetan Detachment; MCT MCT Detachment; Tibetan South

MFT initiation age constraints. constraints. age initiation MFT

Paror Anticline. PAL PAL Anticline. Paror

–

Tibetian Suture Zone; STD Zone; Suture Tibetian

Indo PA Fault. aror

–

Spatial Spatial and Context Geologic of the Study Area

–

: :

Figure faults: ITSZ existing ovals: White Frontal Thrust. Main Table S1). foreland Undeformed is south of Faults MFT. PR Thrust. 1998). et Powers al.,

Figure 2.2: U-Th/He apatite sample location in (A) map, (B) structural and stratigraphic position, and (C) time across Jawalamukhi Thrust sheet (JT; sample 6 in footwall). Kangra transect (A-B) - solid black line (Fig. 1B). LS, MS, and US: Lower, Middle, and Upper Siwalik, respectively. Qal: Quaternary Alluvium. Magnetostratigraphic section and correlation from

Meigs et al. (1995). MPS – Local magnetic polarity stratigraphy. MPTS – Global polarity time scale (Cande and Kent, 1992)

Figure 2.3: Time-temperature pathways (tT) from QTQt thermal model given depositional and

AHe age inputs for Sample 1. Dashed box – depositional age range. Solid line – youngest burial to exhumation tT path (vertical dashed line). Model parameters and QTQt models for all samples found in Appendix A.

Figure 2.4: Space-time structural evolution. A-C. Pattern of new (bold), on-going (solid) and uncertain (dashed) faulting for (A) >4 Ma, (B) 4-2 Ma, and (C) <2 Ma intervals. D. Cumulative slip rate for each time interval calculated from modeled AHe exhumation ages and retrodeformed slip from Powers et al. (1998) balanced section (Appendix A). Fault names as in Fig. 1. Dash line - NW Himalaya geodetic convergence rate (Banerjee and Bürgmann, 2002).

Dark grey box – MFT. MBT* rate (arrows give range) from Theide et al. (2017).

APPENDIX A.

SUPPLEMENTAL MATERIALS

A-1. Stratigraphy of the sub-Himalayan Kangra Transect

Deformed Siwalik molasse and Dharmsala shallow marine deposits comprise the NW sub-

Himalayan strata. Down-section, the Dharmsala Formation contains marine to continental fluvio-deltaic sediments deposited concurrent to the growth of the early Tibetan Plateau (Lyon‐

Caen and Molnar, 1985). It is subdivided into a Miocene Upper (UD) green-grey sandstone and an Eocene-Oligocene Lower Dharmsala (LD) purple claystone (Powers et al., 1998). Up- section, the Siwalik Group represents a time-transgressive, upward coarsening sequence of onlap deposits related to progressive orogenic growth and Indian craton flexure (Lyon‐Caen and Molnar, 1985). It is divided into the Plio-Pleistocene Upper Siwalik Formation (US) – a conglomerate to coarse sandstone; the Miocene Middle Siwalik Formation (MS) - a coarse to medium sandstone; and the Miocene Lower Siwalik Formation (LS) - a siltstone. Piggyback basins formed between the major thrusts, localizing deposition of thick, late Pleistocene and younger alluvial fills (< 200m thick in the Jawalamukhi thrust hanging wall) (Dey et al., 2016), which unconformably overlie thick successions of deformed Siwalik and Dharmsala strata.

A-2. Detrital Apatite U-Th/He

(U-Th)/He thermochronology is a low-temperature method based on the radiogenic decay of

U, Th, and Sm, which is sensitive to relatively shallow crustal exhumation and cooling (Ehlers and Farley, 2003; Farley, 2002; Flowers et al., 2009; Gavillot et al., 2016; Sousa et al., 2016).

These data allow for comparison of timing and patterns of fold growth and faulting on >105 yr timescales, applicable to deformed foreland settings where traditional thermochronometers cannot capture exhumation histories from such shallowly buried and re-exhumed structures.

Helium retention during exhumation occurs at temperatures below 40-80°C for apatite and 120-

220°C for zircon depending on grain size, cooling rate, and the effective Uranium concentration

(Farley, 2002; Flowers et al., 2009; Reiners et al., 2005). Given the roughly 5 km depth to

décollement is a proxy for burial depth (Figure 2B), thermal resetting of apatite is likely in sub-

Himalayan strata. Resetting of zircon is also possible for samples in the footwall, as the section thickens northward in the reentrant, and along thrust sheets exhuming deeper stratigraphic intervals (Dharmsala/Murree). Because our interest is in understanding the youngest deformation history of structures, zircons were not analyzed in this study.

The low closure temperature of the apatite (U-Th)/He (AHe) thermochronometer allows for direct constraints on the timing of cooling through the uppermost few kilometers of the crust

(Ehlers and Farley, 2003; Farley, 2002; Flowers et al., 2009; Gavillot et al., 2016; Sousa et al.,

2016). Post-depositional thermal resetting of detrital apatite depends on the AHe date relative to the depositional age of the unit sampled. If AHe dates, for example, are older than a sample depositional age, those AHe dates are not reset. After determining that a sample is reset, detrital

AHe data from a given sample can be used to directly constrain the timing of that sample’s cooling through the AHe partial retention zone (PRZ; Figure S1).

Sixteen samples from the five principal thrust sheets were collected for AHe analysis (Figure

1B; Supplement Table 1). A typical sample consisted of 12-15 kg of coarse-grained sandstone material extracted from the lowest exposed stratigraphic level in the immediate hanging wall above faults and in the core of folds. We opted for large sample sizes to ensure sufficient quantity of high-quality grains. Similarly, in the hanging wall of the Jawalamukhi Thrust, additional samples were extracted in 500 m stratigraphic intervals starting from immediately above the fault exposure to the core of the Lambargaon syncline (Figure 2A). Samples across this thrust sheet were taken from sample locations where the Siwalik unit depositional age is constrained at the chron level based on paleomagnetic data (Figure 2C) (Meigs et al., 1995).

A total of 66 single grain AHe dates were determined at the CU-TRaIL Lab in Boulder,

Colorado for the 14 samples (Table S2). Two samples, JT9 and PR1, yielded no dateable grains due to insufficient grain quantity and/or quality and thus were not analyzed.

Standard heavy liquid mineral separation procedures were used to isolate apatite. All analyses were carried out at the University of Colorado-Boulder TRaIL (Thermochronology research and Instrumentation Lab). Individual apatite grains were hand-selected under a stereographic microscope in alcohol, assessed for grain quality (large size, euhedral and inclusion-free), measured to determine the alpha-ejection correction, and packed into Nd tubes for analysis. Nb tubes (packets) were then loaded into an ASI Alphachron He extraction and measurement line, placed in the UHV extraction line (~3 X 10-8 torr) and heated with a 25W diode laser to ~800-

1100°C for 5 to 10 minutes to extract the radiogenic 4He. The degassed 4He was then spiked with approximately 13 ncc of pure 3He, cleaned via interaction with two SAES getters, and analyzed on a Balzers PrismaPlus QME 220 quadrupole mass spectrometer. This procedure was repeated at least once to ensure complete mineral degassing. Degassed grains were then removed from the line and taken to a Class 10 clean lab for dissolution. Apatite grains, still enclosed in the Nb tubes, were placed in 1.5 mL Cetac vials, spiked with a 235U - 230Th –

145Nd tracer in HNO3, capped, and baked in a lab oven at 80°C for 2 hours. After dissolution, samples were diluted with 1 to 3 mL of doubly-deionized water. Sample solutions, along with normal solutions and blanks, were analyzed for U, Th, and Sm content using an Agilent 7900 quadrupole ICP-MS. After U, Th, and Sm measurement, He dates and all associated data were calculated on a custom spreadsheet using the methods described in Ketcham et al. (2011). The natural occurring 238U/235U ratio used in data reduction is 137.818 after Hiess et al. (2012).

Every batch of samples includes standards run sporadically throughout the process to monitor procedures and maintain consistency from run to run. Long term averages of Fish Canyon Tuff zircons and Durango fluorapatites run in the CU TRaIL are 28.7 ± 1.8 Ma (n=150) and 31.1 ±

2.1 (n=85), respectively.

Overall, strata sampled from the immediate hanging wall above thrust sheets yielded cooling ages younger than the sediment depositional age (Figure S1). Similarly, successive samples

from across the Jawalamukhi hanging wall switch from younger to older than the sample stratigraphic age progressively up-section away from the fault, respectively (Figure 2). Thus, the observation that the exhumation is younger than the depositional age indicates that the cooling resulted from fault-related uplift with respect to the surface. Alpha-ejection corrected

AHe ages vary from 2.1 to 463.2 Ma. Most dates are younger than 13.9 Ma with uncertainties on the order of a few hundred thousand years. Four individual grain results were discarded from further analysis based upon their extremely low U-Th-Sm abundances, which likely explains their uncharacteristically old results (Table S2).

A-3. Foreland Exhumation - Modeling Cooling Histories

(U-Th)/He dates obtained for individual apatite grains during lab analysis represent the integrated thermal history of that grain since the start of helium retention as it passed through its grain-specific closure temperature. The complex relationship between the physical processes acting on a grain and the time-temperature (t-T) dependence of helium diffusion within a grain requires inverse modeling to constrain possible thermal histories consistent with the AHe data and other geologic and geochronologic information (Flowers et al., 2015;

Gallagher, 2012).

We used the QTQt thermochronological modeling software of Gallagher (2012) to determine t-T pathways for each multi-grain sample. QTQt utilizes a transdimensional Bayesian Monte

Carlo Markov Chain statistical approach to find the best-fit t-T pathways to explain the AHe data. The RDAAM model of Flowers et al. (2009) was incorporated to account for variations in apatite kinetic behavior. By modeling each sample’s ensemble of grains together rather than separately, we can extract t-T pathways that best explain all input grain parameters, since each grain within a thermally reset sample experienced the same exhumation history post-reburial.

Model inputs include only the depositional age of the unit from which the sample was taken, estimates of surface temperature and the AHe ages derived from each grain. The model outputs

are a swath of equally acceptable t-T pathways representing all allowable paths consistent with the input parameters.

For our analyses, we implemented a minimal set of thermal history constraints for our models to allow for a wide search space. For all models, t-T boundary conditions are set to 80 ± 80 Ma for time and 85 ± 70°C for temperature. Depositional age ranges of the Siwalik Group are well constrained in the Kangra region from published magnetostratigraphic data obtained in the hanging wall of the Jawalamukhi thrust sheet (Meigs et al., 1995). For samples extracted directly from the Jawalamukhi hanging wall, depositional ages are more precisely known down to the chron-level. These ages provide further time and temperature constraints through which the t-T pathways must pass before being re-exhumed to the surface today. The modern mean and paleo-depositional sub-Himalayan surface temperatures were assumed as 17.5 ± 2.5°C

(See Table S3 for model parameters). We used 500,000 pre- and post-burn in model iterations to define the range of best-fit t-T pathways and to evaluate the sensitivity of the modeled pathways within that range, respectively.

For this study, the tightly constrained geologic context of each sample allows for extraction of detailed thermal history information, especially during the critical time period where the sample transitions from reheating to cooling. Detailed local geologic constraints along the

Kangra transect allow us to interpret the post-depositional reheating phase as driven by burial by sedimentary overburden and the subsequent cooling phase as related to structurally controlled exhumation. In this framework, the transition between these phases represents the local onset of structural deformation, which we use to infer patterns of fold growth and thrust activity.

A-4. Shortening Rate Calculations

Shortening distances for each of the sampled foreland deformation structures were derived from pin line restorations of each thrust sheet segment of the balanced cross-section of Powers et al. (1998). By retrodeforming the section back to its undeformed state using two reference points fixed to both the stable section and the thrust sheet to be restored, we can calculate the amount of slip on each fault (Woodward et al., 1989). In our calculations, we assume that all slip on the given fault was accommodated over the period of time between the onset of fault- related exhumation derived from our AHe thermal models to the present. The extension of time from onset to present reflects the observation of Quaternary activity on all foreland structures and that there are no constraints on motion beyond the onset of cooling. For structures without reset AHe dates (e.g. the Paror Fault), we estimate a range of possible minimum shortening rates bracketed by the 4 Ma and 2 Ma divisions shown in Fig. 4B and 4C.

To calculate the shortening rate, we use the equation:

Shortening Rate = Slip Distance / Onset of Fault-Related Exhumation

The balanced section interpretation and restoration on individual structures constrains the amount of shortening on each structure (Power et al., 1998). Since we selected the onset of exhumation from the model with the youngest transition from heating to cooling, all other models for the switch only yield an older onset age for a given structure. Thus, all older inflection points produce a lower and slower rate than the pathway with the youngest transition from heating to cooling. Thus, the youngest date results in the highest rate (or a maximum) for a structure. In short, the rates are inversely proportional to the age (for a fixed distance per structure).

Our values for the foreland slip rate calculations are as follows:

Fault Name Distance (km) Age(s) (Ma) Slip Rate (mm/yr) Main Frontal Thrust 2.5 4.0 0.6 Soan Thrust 3.7 2.5 1.5 Jawalamukhi Thrust 13.8 2.0 6.9 + Footwall Paror Thrust 3.5 2.0 – 4.0 0.9 – 1.8

Shortening rates were not calculated for the Palampur Fault because both timing and slip are unconstrained. However, a conservative estimate of slip on the Palampur Fault is 15 km, indicating that the fault is a significant source of shortening in the foreland. Rates vary from

3.8 – 7.5 mm/yr for a 4 and 2 Ma initiation age, respectively. Shortening rates on the Main

Boundary Thrust were taken from Thiede et al. (2017) for the nearby Dhauladhar Range and are estimated as 2.6-3.5 mm/yr since 8 Ma.

Assuming that the geodetic rate is a proxy for the long-term shortening rate (Gordon and Stein,

1992), the shortening budget for the NW Himalaya is ~11-14 mm/yr (Banerjee and Bürgmann,

2002; Schiffman et al., 2013). As such, we estimated shortening rates for each of the active faults for the time periods >4 Ma, 4-2 Ma, and 2-0 Ma and summed their shortening rates (Fig.

4D). Any shortening not accounted for by one of our known and dated structures in the shortening budget is considered a ‘deficit’ that requires accommodation by other structures.

Although not a foreland structure, slip on the Main Boundary Thrust is included as its shortening rate has been previously estimated and the structure has been inferred as active by previous studies. Note that if we assume that shortening on the PAL was occurring during 2-0

Ma, shortening on the Main Boundary Thrust would necessarily be significantly reduced to maintain the shortening budget.

Other possibilities to explain the apparent shortening deficit in the sub-Himalaya as suggested by our new data include: (1) modern geodetic rates are not applicable at longer, geologic timescales; (2) unknowns in the modeling, such as geothermal gradient, the magnitude of heat advection from faulting, effects of groundwater circulation, residence times at depth, and cooling rates, may impact exhumation cooling ages.

A-5. References:

Banerjee, P., and Bürgmann, R., 2002, Convergence across the northwest Himalaya from GPS measurements: Geophysical Research Letters, v. 29, no. 13, p. 1652.

Cooperdock, E. H., Ketcham, R. A., and Stockli, D. F., 2019, Resolving the effects of 2-D versus 3-D grain measurements on apatite (U–Th)∕ He age data and reproducibility: Geochronology, v. 1, no. 1, p. 17-41.

Dey, S., Thiede, R. C., Schildgen, T. F., Wittmann, H., Bookhagen, B., Scherler, D., and Strecker, M. R., 2016, Holocene internal shortening within the northwest Sub‐Himalaya: Out‐ of‐sequence faulting of the Jwalamukhi Thrust, India: Tectonics.

Ehlers, T. A., and Farley, K. A., 2003, Apatite (U-Th)/He thermochronometry: methods and applications to problems in tectonic and surface processes: Earth and Planetary Science Letters, v. 206, p. 1-14.

Farley, K. A., 2002, (U-Th)/He dating: Techniques, calibrations, and applications: Reviews in Mineralogy and Geochemistry, v. 47, no. 1, p. 819-844.

Flowers, R. M., Farley, K. A., and Ketcham, R. A., 2015, A reporting protocol for thermochronologic modeling illustrated with data from the Grand Canyon: Earth and Planetary Science Letters, v. 432, p. 425-435.

Gallagher, K., 2012, Transdimensional inverse thermal history modeling for quantitative thermochronology: Journal of Geophysical Research: Solid Earth, v. 117, no. B2.

Gavillot, Y., Meigs, A., Yule, D., Heermance, R., Rittenour, T., Madugo, C., and Malik, M., 2016, Shortening rate and Holocene surface rupture on the Riasi fault system in the Kashmir Himalaya: Active thrusting within the Northwest Himalayan orogenic wedge: Geological Society of America Bulletin, v. 128, no. 7-8, p. 1070-1094.

Gordon, R. G., & Stein, S., 1992, Global tectonics and space geodesy: Science, v. 256, no. 5055, p. 333-342.

Hiess, J., Condon, D. J., McLean, N., and Noble, S. R., 2012, 238U/235U systematics in terrestrial uranium-bearing minerals: Science, v. 335, no. 6076, p. 1610-1614.

Ketcham, R. A., Gautheron, C., and Tassan-Got, L., 2011, Accounting for long alpha-particle stopping distances in (U–Th–Sm)/He geochronology: Refinement of the baseline case: Geochimica et Cosmochimica Acta, v. 75, no. 24, p. 7779-7791.

Lyon‐Caen, H., and Molnar, P., 1985, Gravity anomalies, flexure of the Indian plate, and the structure, support and evolution of the Himalaya and Ganga Basin: Tectonics, v. 4, no. 6, p. 513-538.

Meigs, A. J., Burbank, D. W., and Beck, R. A., 1995, Middle-late Miocene (>10 Ma) formation of the Main Boundary thrust in the western Himalaya: Geology, v. 23, no. 5, p. 423- 426.

Reiners, P. W., Ehlers, T. A., and Zeitler, P. K., 2005, Past, present, and future of thermochronology: Reviews in Mineralogy and Geochemistry, v. 58, no. 1, p. 1-18.

Schiffman, C., Bali, B. S., Szeliga, W., and Bilham, R., 2013, Seismic slip deficit in the Kashmir Himalaya from GPS observations: Geophysical Research Letters, v. 40, no. 21, p. 5642-5645.

Sousa, F. J., Farley, K. A., Saleeby, J., and Clark, M., 2016, Eocene activity on the Western Sierra Fault System and its role incising Kings Canyon, California: Earth and Planetary Science Letters, v. 439, p. 29-38.

Wolf, R., Farley, K., and Kass, D., 1998, Modeling of the temperature sensitivity of the apatite (U–Th)/He thermochronometer: Chemical Geology, v. 148, no. 1-2, p. 105-114.

Woodward, N. B., Boyer, S. E., and Suppe, J., 1989, Balanced geological cross-sections: American Geophysical Union, Short Courses in Geology, v. 6, p. 132.

ially ially

eported eported errors are

1 1 line could be part

s s are samples without highly

versus stratigraphic age. stratigraphic versus

age

coded coded by sample. Squares represent samples with tightly

AHe AHe

color

. Distribution of average sample of average . Distribution

AHe AHe grain age versus stratigraphic age

graphic graphic AHe ages ages. versus If AHe dates are younger than ages, stratigraphic the sample is likely thermally

level level depositional ages defined by published magnetostratigraphy (Meigs et al., 1995). Circle

: Plots of sample strati

. Distribution of individual

Figure reset; however, if AHe dates are older ages, than stratigraphic the sample is not fully reset. Samples to close the age 1 reset. constrained, chron constrained depositional ages. Instead, circle samples use the age range of the entire unit from which they were extracted. R magnetostratigraphy. the from associated derived

Figure 2.2S: All modeled time-temperature pathways for samples along the Kangra Transect

Figure 2.2S: Continued

Figure 2.3S: Interpretative diagrams with estimates of initiation of fault-related exhumation cooling along major fault structures across the Kangra Transect and the associated cooling ages of samples in exhumed hanging walls and fold cores (also see Figure 2.1C). Fully thermally reset samples with their minimum exhumation onset ages reported in red. Samples that were not fully reset are shown in black text.

Table S1. Sample Location and Stratigraphic Age Constraints

TABLE S1. SAMPLE LOCATION AND STRATIGRAPHIC AGE CONSTRAINTS Identification Location Stratigraphy Sample Sample Latitude Longitude Elevation Magneto-Chron Lower Age Range Upper Age Unit Name ID Name (°N)* (°E)* (m) † (Ma) § Range (Ma) § 1 J1 31.60802 75.94765 325 Middle Siwalik N/A 7.091 11.052 2 J2 31.62332 75.99685 482 Middle Siwalik N/A 7.091 11.052 3 S1 31.68572 76.13355 484 Middle Siwalik N/A 7.091 11.052 4 S2 31.73662 76.18182 677 Middle Siwalik N/A 7.091 11.052 5 SMA1 31.87313 76.29877 482 Middle Siwalik N/A 7.091 11.052 6 JT2 31.89543 76.30437 581 Middle Siwalik N/A 7.091 11.052 7 JT1 31.89473 76.30037 543 Lower Siwalik C5An.2n 12.184 12.401 8 JT3 31.89110 76.31922 807 Lower - Middle Siwalik C5r.1r / C5r.1n 10.949 11.099 9 JT4 31.90158 76.32538 981 Middle Siwalik C5n.2n 9.920 10.949 10 JT5 31.91320 76.37770 742 Middle Siwalik C4Ar.2r 9.308 9.580 11 JT6 31.93082 76.39905 845 Middle - Upper Siwalik C4n.2r 7.562 7.650 12 JT7 31.94393 76.40517 834 lower Upper Siwalik C3Bn 6.935 7.091 13 JT9 31.96362 76.43917 676 upper Upper Siwalik C3r / C3An.1n 5.894 6.137 14 JT8 31.97970 76.47182 781 upper Upper Siwalik C3n.3r 4.890 4.980 15 PR1 31.99377 76.52777 812 middle Upper Siwalik N/A 6.137 7.091 16 PAL1 32.08454 76.59804 1128 Lower Dharmsala N/A 16.500 20.000 * Latitude & Longitude reported in decimal degrees in the WGS84 (G1762) reference system. Collected with a Garmin 64SX GPS. † Samples extracted from same location as those of Meigs et al., 1995 magnetostratigraphy and use associted Chrons. § Age range from Cande & Kent 1992; 1995. Those with a chron use the chron age. The others use the full US, MS, LS, LD Range

Table 2.2S: Summary of (U-Th)/He Sample Analysis with corrected age

TABLE S2. SUMMARY OF (U-TH)/HE SAMPLE ANALYSIS WITH CORRECTED AGE Sample Uncorrected +/- Corrected +/- U Th 147Sm He Mass rs e (U) †† Th/U Ft*** Name Date (Ma) † (Ma) § Date (Ma) (Ma) § (ppm)* (ppm)** (ppm)* (nmol/g)** (ug) §§ (um)##

Sample 1 J1_a01 2.0 0.1 3.0 0.3 24.7 6.9 n.m. 26.36 0.28 0.3 1.5 42 0.67 J1_a02 3.0 0.1 3.8 0.2 56.6 27.2 26.0 62.95 0.48 1.0 7.3 70 0.80 J1_a03 3.8 0.1 4.9 0.3 58.8 7.7 16.0 60.60 0.13 1.3 5.6 61 0.78 J1_a04 5.6 0.1 7.9 0.4 29.3 19.8 28.4 33.92 0.68 1.0 2.0 48 0.71 J1_a05 2.6 0.1 3.6 0.2 12.8 7.0 6.6 14.40 0.55 0.2 3.6 50 0.72 J1_a06 3.9 0.1 5.5 0.4 11.9 9.4 2.7 14.08 0.79 0.3 2.9 50 0.72

Sample 2 J2_a01 9.2 0.8 11.7 2.2 5.5 3.2 6.4 6.23 0.58 0.3 5.3 65 0.78 J2_a02 7.6 0.4 10.5 1.2 2.6 2.7 1.8 3.22 1.06 0.1 3.7 52 0.72 J2_a03 4.1 0.1 5.2 0.2 28.8 8.4 47.2 30.80 0.29 0.7 5.3 66 0.79 J2_a04 4.9 0.1 5.7 0.2 26.4 6.3 11.8 27.88 0.24 0.7 22.1 102 0.86 J2_a05 5.1 0.1 6.9 0.3 83.3 6.1 11.8 84.71 0.07 2.3 3.0 52 0.74 J2_a06 3.2 0.1 4.1 0.1 36.5 114.0 52.3 63.32 3.12 1.1 5.6 64 0.77

Sample 3 S1_a01 3.2 0.2 4.3 0.4 14.9 23.7 9.3 20.44 1.59 0.4 4.3 60 0.76 S1_a02 4.7 0.1 7.3 0.4 32.1 36.7 12.9 40.76 1.14 1.0 1.2 39 0.63 S1_a03 3.0 0.1 4.3 0.3 22.6 23.3 15.8 28.10 1.03 0.5 2.3 45 0.69 S1_a04 2.5 0.1 3.6 0.4 8.9 18.0 1.2 13.10 2.03 0.2 1.9 49 0.71 S1_a05 1.6 0.1 2.1 0.3 5.8 14.2 2.5 9.10 2.45 0.1 3.7 62 0.77

Sample 4 S2_a01 3.9 0.1 5.8 0.4 17.1 38.5 42.9 26.18 2.25 0.6 1.9 44 0.67 S2_a02 4.2 0.2 6.3 0.5 16.2 16.5 16.1 20.09 1.02 0.5 1.9 41 0.66 S2_a03 3.1 0.2 5.1 0.6 19.0 3.1 31.8 19.73 0.17 0.3 1.0 34 0.60 S2_a04 8.9 0.3 13.5 0.8 32.7 56.5 51.3 45.97 1.73 2.2 1.4 42 0.66 S2_a05 4.4 0.2 7.3 0.8 43.4 4.5 37.9 44.43 0.10 1.1 0.6 33 0.60 S2_a06 8.7 0.1 13.9 0.4 73.4 51.8 49.4 85.59 0.71 4.0 1.4 37 0.62

Sample 5 SMAI_a01 2.1 0.0 2.7 0.1 32.6 85.6 31.0 52.76 2.62 0.6 6.5 69 0.79 SMAI_a02 2.5 0.1 3.4 0.2 16.4 58.3 70.9 30.16 3.55 0.4 2.8 54 0.72 SMAI_a03 8.5 0.2 10.5 0.4 3.6 19.2 11.4 8.14 5.27 0.4 8.4 79 0.81

Sample 6 * JT2_a01 356.4 199.5 463.2 527.7 0.0 0.3 0.0 0.10 8.52 0.2 5.3 65 0.76 JT2_a02 1.8 0.1 2.9 0.2 70.1 36.3 36.3 78.59 0.52 0.7 1.0 35 0.61 JT2_a03 2.0 0.0 3.1 0.1 177.5 32.5 33.2 185.13 0.18 2.0 1.0 38 0.64 JT2_a04 4.7 0.1 6.7 0.2 48.4 54.4 57.4 61.16 1.12 1.6 1.8 45 0.69 JT2_a05 1.8 0.0 2.2 0.1 43.7 82.3 31.0 63.07 1.88 0.6 8.0 70 0.80

Sample 7 JT1_a01 6.4 0.1 7.5 0.2 12.8 8.1 15.4 14.73 0.63 0.5 22.1 102 0.86 JT1_a02 6.4 0.1 8.0 0.3 31.1 8.1 42.0 33.03 0.26 1.1 6.0 67 0.79 JT1_a03 2.3 0.0 3.5 0.1 46.4 187.1 26.1 90.40 4.03 1.2 2.2 45 0.67 JT1_a04 5.2 0.1 7.5 0.3 96.2 59.9 29.4 110.30 0.62 3.1 2.0 44 0.69 * JT1_a05 57.9 93.9 82.8 277.9 0.7 0.9 0.6 0.92 1.19 0.3 2.5 47 0.70

Sample 8 JT3_a01 1.7 0.1 2.4 0.3 27.1 16.4 23.2 30.93 0.61 0.3 2.6 49 0.72 JT3_a02 2.6 0.0 3.0 0.1 33.8 8.2 12.8 35.76 0.24 0.5 25.9 116 0.88 JT3_a03 7.6 0.1 8.7 0.2 37.7 6.0 7.4 39.15 0.16 1.6 32.9 113 0.88 JT3_a04 2.4 0.0 2.8 0.1 36.1 7.1 11.0 37.77 0.20 0.5 23.6 104 0.86 JT3_a05 2.4 0.1 2.9 0.3 22.5 6.4 19.4 24.00 0.29 0.3 8.4 75 0.82

Sample 9 JT4_a01 5.1 0.1 5.8 0.2 14.8 3.6 12.4 15.65 0.24 0.4 30.6 115 0.88 * JT4_a02 34.7 1.2 39.9 2.7 1.5 0.3 1.1 1.58 0.18 0.3 30.0 108 0.87 JT4_a03 3.4 0.1 4.3 0.1 34.6 10.5 16.4 37.07 0.30 0.7 6.8 64 0.78 JT4_a04 2.8 0.0 3.4 0.1 27.9 6.5 10.4 29.43 0.23 0.5 13.5 85 0.84 JT4_a05 3.0 0.1 3.7 0.2 35.2 12.0 28.9 38.04 0.34 0.6 7.5 75 0.81

Sample 10 JT5_a01 2.9 0.1 3.5 0.3 6.0 0.4 2.0 6.11 0.07 0.1 11.9 84 0.84 JT5_a02 7.2 0.1 9.0 0.4 43.1 8.5 29.1 45.08 0.20 1.8 6.5 69 0.80 JT5_a03 7.4 0.1 9.1 0.3 52.8 3.4 13.5 53.63 0.06 2.1 8.8 73 0.81

Table 2.2S: Continued

TABLE S2. CONTINUED

Sample Uncorrected +/- Corrected +/- U Th 147Sm He Mass rs e (U) †† Th/U Ft*** Name Date (Ma) † (Ma) § Date (Ma) # (Ma) § (ppm)** (ppm)** (ppm)** (nmol/g)** (ug) §§ (um)##

Sample 11 JT6_a01 8.1 0.2 12.5 0.6 53.0 57.5 113.3 66.52 1.08 2.9 1.2 40 0.64 JT6_a02 6.5 0.2 8.6 0.6 35.3 19.0 32.4 39.82 0.54 1.4 4.3 59 0.76 JT6_a03 5.2 0.1 7.8 0.4 35.5 13.8 27.2 38.73 0.39 1.1 1.5 41 0.66 JT6_a04 7.4 0.2 9.2 0.4 53.5 7.8 25.9 55.30 0.15 2.2 7.6 72 0.81 JT6_a05 6.8 0.1 8.4 0.3 10.4 12.6 15.3 13.37 1.21 0.5 7.9 74 0.81 JT6_a06 7.0 0.1 8.8 0.3 23.4 17.6 28.4 27.58 0.75 1.1 8.7 70 0.80 Sample 12 * JT7_a01 11.6 0.7 13.2 1.7 0.0 0.5 12.4 0.12 N/A 0.0 19.1 94 0.83 JT7_a02 5.7 0.1 7.7 0.2 29.0 2.8 15.7 29.68 0.09 0.9 3.7 52 0.74 Sample 13 * JT9 No Grains ------Sample 14 JT8_a01 6.3 6.3 7.7 0.5 13.6 17.0 35.7 17.65 1.25 0.6 10.7 80 0.82 JT8_a02 5.0 5.0 7.1 0.4 20.9 126.2 77.5 50.59 6.03 1.4 2.7 50 0.70 JT8_a03 5.4 5.4 7.5 0.5 37.8 44.0 76.6 48.18 1.16 1.4 2.1 49 0.71 JT8_a04 4.5 4.5 6.6 0.8 4.3 11.9 71.1 7.06 2.79 0.2 2.4 45 0.67 JT8_a05 3.5 3.5 5.3 0.3 34.4 81.9 77.1 53.59 2.38 1.0 2.0 43 0.66 JT8_a06 4.7 4.7 6.5 0.3 30.4 36.8 61.7 39.01 1.21 1.0 2.3 49 0.71 Sample 15 PR1_a01 2.1 2.1 4.0 0.6 3.6 61.5 61.2 18.08 16.92 0.2 0.9 32 0.52 PR1_a02 4.2 4.2 7.0 0.4 66.4 120.0 44.2 94.59 1.81 2.1 0.8 34 0.59 PR1_a03 4.7 4.7 8.3 1.2 24.5 34.7 32.2 32.63 1.42 0.8 0.5 32 0.56 Sample 16 * PAL1 No Grains ------* Grains not used in thermal history modeling † Uncorrected Date (Ma): Calculated iteratively using the 4He production equation defined as Equation 1 in Wolf et al. (1998) and assuming secular equilibrium. # Corrected date (Ma): Calculated iteratively using the absolute values of He, U, Th and Sm, the isotope specific FT corrections, and Equation 34 in Ketcham et al. (2011) assuming secular equilibrium. § +/- Ma: = 2s propagated analytical uncertainty on the U, Th, Sm, and He measurements

** Concentrations of He, U, Th and Sm: computed from their absolute amounts and the estimated dimensional mass

†† eU: effective uranium concentration calculated as U + 0.2375*Th + 0.0012*Sm §§ Mass: The mass of the crystal. Determined from the measured grain dimensions, the volume assuming the reported grain geometry, and the volume equations and mineral densities in Ketcham et al. (2011). ## rs: The radius of a sphere with an equivalent alpha ejection correction as the grain, calculated using Equation A6 in Cooperdock et al. (2019)

*** FT: The combined alpha-ejection correction for the crystal calculated from the isotope specific FT corrections, the proportion of U and Th contributing to 4He production, and assuming homogeneous parent isotope distributions.

Table 2.3S: (U-Th)/He Thermal History Model Input Parameters

TABLE S3. (U-TH)/HE THERMAL HISTORY MODEL INPUT PARAMETERS Samples Modeled Grains Thermal History Bounds Stratigraphic Constraints Model-Specific Parameters # of # of Surface # Burn-in # Post Burn-in Scale Parameter Sample Sample Sample Time Range Temperature Max ∂T/∂t Modern Surface Mean Age He Kinetic Grains Grains Temperature at Model Model Selection Type ID Name Modeled (Myr) Range (°C) (°C/Myr) Temperature (°C) Deposition (Ma) Model Used Excluded Deposition (°C) Iterations Iterations (Proposal Birth) 1 J1 Yes 6 0 80 ± 80 85 ± 70 1000 17.5 ± 2.5 9.07 ± 1.98 17.5 ± 2.5 500,000 500,000 Gaussian RDAAM 2 J2 Yes 6 0 80 ± 80 85 ± 70 1000 17.5 ± 2.5 9.07 ± 1.98 17.5 ± 2.5 500,000 500,000 Gaussian RDAAM 3 S1 Yes 5 0 80 ± 80 85 ± 70 1000 17.5 ± 2.5 9.07 ± 1.98 17.5 ± 2.5 500,000 500,000 Gaussian RDAAM 4 S2 Yes 6 0 80 ± 80 85 ± 70 1000 17.5 ± 2.5 9.07 ± 1.98 17.5 ± 2.5 500,000 500,000 Gaussian RDAAM 5 SMA1 Yes 3 0 80 ± 80 85 ± 70 1000 17.5 ± 2.5 9.07 ± 1.98 17.5 ± 2.5 100,000 100,000 Gaussian RDAAM 6 JT2 Yes 4 1 80 ± 80 85 ± 70 1000 17.5 ± 2.5 9.07 ± 1.98 17.5 ± 2.5 500,000 500,000 Gaussian RDAAM 7 JT1 Yes 4 1 80 ± 80 85 ± 70 1000 17.5 ± 2.5 12.3 ± 0.101 17.5 ± 2.5 500,000 500,000 Gaussian RDAAM 8 JT3 Yes 5 0 80 ± 80 85 ± 70 1000 17.5 ± 2.5 11.024 ± 0.075 17.5 ± 2.5 500,000 500,000 Gaussian RDAAM 9 JT4 Yes 4 1 80 ± 80 85 ± 70 1000 17.5 ± 2.5 10.435 ± 0.514 17.5 ± 2.5 500,000 500,000 Gaussian RDAAM 10 JT5 Yes 3 0 80 ± 80 85 ± 70 1000 17.5 ± 2.5 9.44 ± 0.14 17.5 ± 2.5 500,000 500,000 Gaussian RDAAM 11 JT6 Yes 6 0 80 ± 80 85 ± 70 1000 17.5 ± 2.5 7.606 ± 0.044 17.5 ± 2.5 100,000 100,000 Gaussian RDAAM 12 JT7 Yes 1 1 80 ± 80 85 ± 70 1000 17.5 ± 2.5 7.013 ± 0.078 17.5 ± 2.5 100,000 100,000 Gaussian RDAAM 13 JT9 No NAN NAN NAN NAN NAN NAN NAN NAN NAN NAN NAN NAN 14 JT8 Yes 6 0 80 ± 80 85 ± 70 1000 17.5 ± 2.5 4.935 ± 0.045 17.5 ± 2.5 100,000 100,000 Gaussian RDAAM 15 PR1 Yes 3 0 80 ± 80 85 ± 70 1000 17.5 ± 2.5 6.614 ± 0.477 17.5 ± 2.5 500,000 500,000 Gaussian RDAAM 16 PAL1 No NAN NAN NAN NAN NAN NAN NAN NAN NAN NAN NAN NAN

Thermochronologic Data: Specific (U-Th)/He analytical inputs for each analyzed grain, including the U, Th, Sm, and He concentrations as well as grain radius, can be found in Table

S2. ‘# of grains used’ refers to the number of individual grain results from a single sample used for the analysis. Excluded grains were discarded due to very small U, Th, Sm concentrations, though initial trials included all grains to test overall fit of all data.

Geologic Assumptions & Constraints: Bounds on the thermal history were set to allow for the widest possible range of reasonable modeled thermal pathways. The time range was set to far beyond the interpreted ~55 Ma age of orogen development (Najman et al., 2017). The maximum temperature was set to double the average temperature range of the PRZ. The minimum temperature is the lowest allowed model temperature at the Earth’s surface. The modern surface temperature ranges from 15-20°C. A large maximum thermal rate of change was selected to allow the model freedom to test thermal pathways from a wide range of burial and exhumation rates.

Detrital depositional ages of the sampled sediments of the Siwalik Group were constrained by the Jawalamukhi magnetostratigraphic section of Meigs et al. (1995). Samples extracted from dated stratigraphic horizons have tightly constrained, chron-level depositional ages with small errors. For less well constrained samples, formation ages based upon the magnetostratigraphy

was utilized. The sediment was deposited at the surface, but paleo-surface temperature is poorly constrained. Therefore, we assumed a 17.5° C surface temperature similar to modern surface temperatures conditions.

System & Model-Specific Parameters: Thermal history analyses were conducted using the

QTQt v.PC64R5.6.0 software of Gallagher (2012). For all model values not explicitly defined in Table 3, default software values were assumed. For a given sample, the total number of model iterations is the sum of the burn-in and post-burn-in. The burn-in models are used to narrow in on the best-fit thermal pathways and are ultimately discarded in the analysis. The post-burn-in models test the robustness of the best-fit model and are used in the inference of the thermal history. 500,000 iterations for each were conducted for samples interpreted to yield thermally reset (U-Th)/He ages to tightly constrain thermal histories. Samples indicating only partial or no resetting were analyzed using 100,000 iterations each. The selection of new time and temperature points between individual model iterations in the sampling chain is done using a Gaussian distribution. The RDAAM model of alpha radiation damage on the diffusivity of

He in apatite is employed for all models (Flowers et al., 2009).

CHAPTER 3:

SYNCHRONOUS ACTIVATION OF NORTHWEST SUB- HIMALAYAN DEFORMATION WITH SPATIALLY AND TEMPORALLY VARIABLE DISTRIBUTED INTERNAL DEFORMATION SINCE AT LEAST 4 MA CONSTRAINED BY THERMOCHRONOLOGY

3.1 Authors & Affiliations

Ellen A. Lamont 1, Francis Sousa 1, Andrew J. Meigs 1, Jayangondaperumal 2, Rebecca M.

Flowers 3, James Metcalf 3, Aravind Anilkumar 2, Edward R. Sobel 4

1 Oregon State University, College of Earth, Ocean, and Atmospheric Sciences

2 Wadia Institute of Himalayan Geology, Department of Structure and Tectonics

3 University of Colorado-Boulder, Department of Geology

4 University of Potsdam, Institut für Geowissenschaften

3.2 Abstract

In an evolving orogenic system, constraints on the timing and pattern of strain accommodation within the deformed foreland are critical to decoding the processes driving the growth and localization of strain in mountain ranges. Simple break-forward models of thrust front propagation fail to account for the importance of distributed deformation in rebuilding wedge taper and its impact on the distribution of seismic hazards. Critical wedge models predict that thrust front propagation should be followed by a period of distributed internal deformation. It remains unclear, however, the extent to which this pattern manifests in active orogens. New and published maps, field data, and balanced cross-sections constrain the amount of shortening accommodated on individual structures throughout the NW sub-Himalaya. Dating of 64 new, high-quality detrital apatite grains extracted from fault hanging walls and fold cores using (U-

Th)/He thermochronology quantifies the timing and rates of shortening on individual foreland structures. Distributed internal deformation by ~2.0 Ma followed accretion of the undeformed foreland by ~4 Ma along the entire length of the NW sub-Himalaya. Balanced sections further indicate a minimum of 22-24 km of regionally uniform shortening. Spatial variations in the geometry and number of individual structures available to accommodate uniform shortening necessitates partitioning between structural domains. Distributed internal deformation

throughout the NW sub-Himalaya is characterized by rapid, on-going shortening ranging regionally from 3.0-6.5 mm/yr since ~2.0 Ma. The relative concentration of higher shortening rates on internal foreland structures rather than focused shortening at the deformation front has implications for increased seismic hazard on a wider collection of internal foreland structures.

3.3 Introduction

The progressive growth and propagation of fold-and-thrust belts (FTB) into the undeformed foreland is a widely inferred thrust sequence (Boyer, 1982; Dahlstrom, 1969; Price, 1981;

Suppe, 1983). In this model, the detachment or décollement zone at the base of the orogenic wedge, the principal source of large magnitude earthquakes in thrust belts (Avouac, 2003;

Avouac et al., 2015; Bilham et al., 2001; Lavé et al., 2005), primarily feeds slip to the surface at the deformation front. However, the observation that large magnitude, destructive earthquakes, such as the 1999 Mw 7.6 Chi-Chi (Taiwan) and the 2005 Mw 7.8 Kashmir (India) earthquakes, occurred on faults more internal to the orogenic wedge (Avouac et al., 2015; Ma et al., 1999; Yeats and Hussain, 2006), suggests that active shortening is not restricted to the deformation front. Rather, an emerging image for FTB evolution cites variable patterns of deformation inboard of the deformation front, herein referred to as distributed deformation, as an important mechanism of shortening accommodation.

Critical wedge models of FTB development predict that thrust front propagation into the undeformed foreland accretes material to the toe of the orogenic wedge. A period of distributed internal deformation ought to accompany the accretion (Boyer, 1982; Dahlen and Suppe, 1988;

Davis et al., 1983). Because distributed deformation results from both frontal accretion

(Gavillot et al., 2018; Meigs and Burbank, 1997) and hinterland focused erosion (Beaumont et al., 1992; Davis et al., 1983; Hilley and Strecker, 2004; Whipple, 2009; Willett, 1999), commonly studied records of rock uplift and erosion from orogenic hinterlands fail to uniquely

explain drivers of orogen evolution (Molnar and England, 1990). Consequently, reconstructing the timing and pattern of accretion within a deformed foreland setting provides critical context for understanding the driver of distributed deformation in the structural evolution of FTBs.

This study focuses on the spatial and temporal distribution of shortening in the sub-Himalaya, the deformed Himalayan foreland. The NW sub-Himalaya represents the leading edge of the orogen along the southern edge of the FTB (Gavillot et al., 2018; Lamont et al., (Chapter 2)).

Recent studies indicate that a number of internal structures of the NW sub-Himalaya are active

(Dey et al., 2016; Gavillot et al., 2018; Lamont et al., (Chapter 2); Powers et al., 1998; Thakur et al., 2014). Along strike changes in FTB architecture (Deeken et al., 2011; DiPietro and

Pogue, 2004; Karunakaran and Ranga Rao, 1979; Raiverman et al., 1991; Ranga Rao and Datta,

1976; Yin, 2006) and geodetic shortening rates (Kundu et al., 2014) further allude to lateral variations in deformation accommodation.

We integrate new and published geologic data from three structural domains across the NW sub-Himalaya to assess the long-term pattern of faulting and folding in the deformed foreland.

We further explore regional variability and along-strike linkages related to deformation accommodation and address orogenic wedge mechanics. Balanced cross-sections reveal a minimum of 22-24 km of total shortening have been accommodated across the deformed NW

Himalayan foreland despite lateral variations in structural architecture. New data presented include 13 detrital samples from fault hanging walls and fold cores, yielding 64 individual, high-quality apatite grains for (U-Th)/He (AHe) analysis. Estimates of the latest possible onset of fault-related exhumation interpreted from modeled AHe sample thermal histories indicate distributed deformation by 2.5 Ma followed accretion of the undeformed foreland occurring by

4 Ma. Accretion followed by distributed shortening is a pattern of foreland deformation reflected across the entire NW sub-Himalaya (Burbank and Beck, 1989; Gavillot et al., 2018;

Ghani et al., 2020).

3.4. Geologic Setting of the NW Himalayan Orogen

3.4.1 Regional Tectonics

The Himalaya forms an ~2700 km-long, arcuate mountain belt resulting from Cenozoic, Indo-

Eurasia collision (DeMets et al., 1994) after ~55 Ma (Najman et al., 2017). The orogenic wedge is defined by a series of south-vergent thrusts that root into the basal décollement, the Main

Himalayan Thrust (MHT) (DeCelles et al., 1998; DiPietro and Pogue, 2004; Hodges, 2000;

Powers et al., 1998; Raiverman et al., 1983). The Main Central Thrust (MCT) and Main

Boundary Thrust (MBT) represent the hinterland portion of the orogen and formed at ~20 Ma and ~10 Ma, respectively (DeCelles et al., 1998; Hodges, 2000; Meigs et al., 1995). These structures are thought to have accommodated at most a few kilometers of late Quaternary shortening despite their contribution to overall shortening in the Miocene and Pliocene

(Avouac, 2003; Bollinger et al., 2006; Brewer and Burbank, 2006; Hodges, 2000; Robert et al.,

2009; Wobus et al., 2006).

The MBT defines the boundary between the orogenic hinterland to the north and the deformed foreland, known as the sub-Himalaya, to the south (Figure 3.1). The sub-Himalaya is composed of a series of orogen-parallel thrusts, back thrusts, folds, and piggyback (dun) basins

(Karunakaran and Ranga Rao, 1979; Ranga Rao and Datta, 1976). Foreland deformation exposes rocks of the Cenozoic foreland basin sequence including clastic rocks of the Mio-

Pliocene Upper (US), Middle (MS), and Lower (LS) Siwalik Formations, detrital material shed from the Lesser and High Himalaya to the north, and the Oligocene-Miocene age shallow marine siltstone-mudstone deposits of the Upper and Lower Dharmsala/Murree Formations

(Figure 3.2) (Karunakaran and Ranga Rao, 1979; Powers et al., 1998; Raiverman, 2002;

Raiverman et al., 1983; Ranga Rao, 1993; Ranga Rao et al., 1988). The Main Frontal Thrust

(MFT) represents the deformation front at the orogen’s southern edge and is expressed as a series of discontinuous thrusts and fault-cored folds along strike (Raiverman et al., 1983).

Initiation of deformation on the MFT began prior to 5.2 Ma in the Salt Range (Pakistan)

(Burbank and Beck, 1989; Ghani et al., 2020), 4 Ma in the NW Himalaya (India) (Gavillot et al., 2018; Chapter 2) and by 2Ma in the central Himalaya (Nepal) (van der Beek et al., 2006).

3.4.2 The NW Sub-Himalaya (Sutlej River to Jhelum River)

This study explores contrasting expressions of shortening accommodation across the deformed foreland along the length of the NW India sub-Himalaya. The study area extends from the

Sutlej River in the Kangra Reentrant, Himachal Pradesh westward to the Chenab River near

Jammu in Jammu & Kashmir (Figure 3.1). Structural salients and reentrants are reflected by the sinuous trace of the MBT along strike (Raiverman et al., 1991; Ranga Rao and Datta, 1976).

The width of the deformed foreland between the MBT and the deformation front changes along strike. Near Jammu, the deformation front is delineated by the blind, fault-cored Surin-

Mustgarh Anticline (SMA) located ~30 km south of the MBT. In the Kangra reentrant, the width between the emergent MFT and the MBT is ~100km. The region near the outlet of the

Chakki River, henceforth referred to as the Chakki region, straddles this change in width and kinematically links the Kangra and Jammu structural domains. Within the NW Himalaya, geodetic data indicate that 11-14 mm/yr of the Indian plate convergence is accommodated within the Himalaya (Banerjee and Bürgmann, 2002; Schiffman et al., 2013).

The Kangra reentrant represents the widest exposure of deformed foreland strata across the

Himalayan orogen (Fig. 3.1). The Kangra reentrant is characterized by a series of westward plunging fault-related folds with steep forelimbs and gentle backlimbs in the south. These southern structures include the Janauri Anticline in the MFT hanging wall and the Soan

Anticline in the hanging wall of the Soan Thrust (ST)). The northern Kangra reentrant is composed of broadly spaced imbricate thrust sheets including the Jawalamukhi Thrust (JT),

Paror Thrust (PR), and Palampur Thrust (PAL) (Delcaillau et al., 2006; Malik et al., 2010;

Powers et al., 1998; Raiverman et al., 1991). Faulting and folding in the southern portion of the

Kangra region predominately exposes Siwalik strata. Toward the north, rocks of the Dharmsala

Formation emerge in thrust hanging walls. Large synclinal dun basins on the backlimbs of major thrust sheets trap Quaternary alluvium in thrust-top basins. Previous studies identify back thrusts on the southern SA and JA folds and along the Barsar back thrust toward the north

(Delcaillau et al., 2006; Jayangondaperumal et al., 2017; Kothyari et al., 2019; Kothyari et al.,

2021; Powers et al., 1998; Thakur et al., 2014). At depth, a duplex is thought to accommodate additional shortening in the JT footwall (Powers et al., 1998).

Width of the deformed foreland decreases to the northwest in the Jammu region (Figures 3.1,

3.4). Deformation of the Jammu sub-Himalaya has exposed the full Cenozoic foreland basin sequence at the surface, including rocks of the Siwalik and Dharmshala Groups and the

Paleocene-Eocene Subathu Formation (Karunakaran and Ranga Rao, 1979; Raiverman et al.,

1991; Ranga Rao and Datta, 1976). The Surin-Mastagarh anticline (SMA) is expressed as a symmetric fold exposing Siwalik strata at the surface, and is interpreted to reflect development of an antiformal stack duplex in the subsurface. Unlike other fold structures at the deformation front, the SMA is not bound on its southern limb by a foreland-directed emergent thrust fault

(Gavillot et al., 2016; Gavillot et al., 2018). The first emergent thrusts appear ~10 km to the north of the SMA and include the Riasi, Mandili-Kishampur, Tanhal, and Mundun thrusts

(Figure 3.1) (Gavillot et al., 2018; Raiverman et al., 1983; Ranga Rao and Datta, 1976). Dun basins are largely absent, with the exception of locally developed synclines.

3.5 Methods & Results

3.5.1 Balanced Cross-Section Reconstruction - Chakki Region

A deformed state and separate retrodeformed state cross-section comprise a balanced cross- section. When restored to their undeformed state, such reconstructions provide minimum

estimates of shortening accommodated by fold-and-thrust belt structures (Boyer and Geiser,

1987; Dahlstrom, 1969; Elliott, 1983; Woodward et al., 1989). We use the conservation of line- length balancing method and kink-band geometries (Woodward et al., 1989) to calculate orogenic shortening. The ~60 km-long, N55E trending section is perpendicular to the average structural grain of the sub-Himalayan FTB in the Chakki Region. Plane strain deformation is assumed across the NW Himalaya because oblique Indo-Eurasian convergence is partitioned into an arc-normal component acting on the southern FTB and an arc-parallel component driving large-scale strike-slip motion along the Karakoram Fault system in Tibet (McCaffrey and Nabelek, 1998; Nakata, 1989; Schiffman et al., 2013).

3.5.2 Stratigraphic & Structural Constraints

Stratigraphic thicknesses and ages within the Chakki Section are constrained with new field and map data, published geologic data (Karunakaran and Ranga Rao, 1979; Powers et al., 1998;

Raiverman, 2002; Raiverman et al., 1983; Ranga Rao and Datta, 1976), balanced sections

(Gavillot et al., 2018; Mukhopadhyay and Mishra, 1999; Powers et al., 1998) and magnetostratigraphy from the neighboring Jammu and Kangra regions (Johnson et al., 1983;

Meigs et al., 1995; Ranga Rao et al., 1988). Foreland deposits thicken westward from Kangra to Jammu implying that the depth to basement increases towards the west. Because measured thicknesses from wells and geophysical data are not available in the Chakki section, averages of the Kangra and Jammu unit thicknesses and basement depth were assumed. Thicknesses were compared to observed outcrop patterns and 3-point problem calculations for internal consistency. Basin sediments are time transgressive and progressively onlap the north dipping basement toward the south (Lyon‐Caen and Molnar, 1985; Powers et al., 1998). Strata of the

Chakki sub-Himalaya have a total stratigraphic thickness of 8-10 km (Figure 3.2).

Structural geometries across the Chakki section are constrained by new field data and from published sections along strike. The Main Himalayan Thrust, the basal décollement beneath the sub-Himalaya, dips 2.0° - 2.5° toward the NE (Gavillot et al., 2018; Powers et al., 1998;

Yeats and Lillie, 1991). Thrusts mapped at the surface are projected to the décollement at depth using kink-band theory and surface dip measurements (Suppe, 1983). Detachment depth is inferred from the lowest stratigraphic unit exposed in hanging walls above thrusts and in fold cores. In the southern portion of the Chakki section, Lower Siwalik (LS) strata are the deepest exposed unit indicating that the detachment horizon lies either within or at the base of the LS.

Rocks of the Dharmsala Formation are exposed on the north, which suggests the detachment ramps to deeper stratigraphic levels towards the north. Anticlines are interpreted as fault- propagation folds except for the symmetrical SMA, which has a box fold geometry.

3.5.3 Results of Balanced Section Reconstruction

The Chakki transect (B-B’) is described in two sections: (1) the southern portion between the

Main Frontal thrust and the Surin-Mastgarh anticline; (2) a northern portion from the Surin-

Mastgarh Anticline (SMA) to the MBT in the north (Figure 3.3).

The southern part of the Chakki transect includes the MFT and the Soan Thrust (ST) sheets.

Structural geometry of the MFT and ST are similar. Both faults are interpreted as steeply- dipping near the surface (~45°) and shallowing northward to ~16° before merging with the

MHT at depths of 4.5 km and 5.5 km, respectively. Well data from the core of the Janauri

Anticline (JA) to the southeast of the section imply that the MHT occurs near the base of the

Lower Siwalik Formation or deeper (Powers et al., 1998). Dun basins on the north limbs of the

JA and Soan Anticline are positioned above hanging wall flats (0-5° dips) that constrain depths of the principal stratigraphic contacts. Restoration of the deformed section reveals 3 km of shortening on the MFT and 4 km on the ST.

The northern part of the Chakki transect includes structures from the SMA to the MBT on the north. Surface bedding measurements reveal that the SMA is symmetric and has a box fold geometry. Lower Siwalik strata crop out in the core. A back thrust on the north limb of the

SMA forms part of a triangle zone with the south-vergent Jawalamukhi Thrust (JT). Whereas the deep geometry of the triangle zone beneath the JT is unconstrained, dip data from surface exposures in the triangle zone provide geometric constraints. At this position on the section, the MHT is inferred to be located within shales of the Upper Dharmshala Formation at a depth of ~8 km. An area of excess space is thus created between the SMA and MHT at depth. The space beneath the SMA is interpreted as a horse in a duplex formed above a footwall ramp where the MHT steps from deeper to shallower stratigraphic levels from north to south, respectively. A duplex interpretation satisfactorily explains the presence of Lower Dharmshala and older rocks at the surface and is consistent with other interpretations of the subsurface at the SMA (Gavillot et al., 2018; Powers et al., 1998). Geometric constraints indicate that the JT forms a steeply dipping hanging wall ramp that shallows and becomes a hanging wall flat merging northward into the roof thrust of the duplex. Dip data from the north side of the JT hangingwall syncline are consistent with folding of the JT thrust sheet by a second horse of the duplex to the north in the MBT footwall. Whereas, the MBT is interpreted as merging with the roof thrust, no new data were collected that constrain MBT geometry to the north. Restoration reveals 3.75 km of shortening in the SMA and triangle zone and 11.25 km on the JT thrust.

3.6 Detrital Apatite (U-Th)/He Data

We employed detrital apatite (U-Th)/He thermochronology (AHe) to evaluate shallow crustal exhumation and cooling related to fold growth and thrust sheet emplacement across the deformed foreland (Ehlers and Farley, 2003; Farley, 2002; Flowers et al., 2009; Gavillot et al.,

2018; Sousa et al., 2016). (U-Th)/He thermochronology is a low-temperature method based on radiogenic decay of U, Th, Sm and He. Helium retention during exhumation occurs at

temperatures below 40-80°C for apatite, depending on grain size, cooling rate, and the effective

Uranium concentration (Farley, 2002; Flowers et al., 2009; Reiners et al., 2005; Sousa and

Farley, 2020). The low closure temperature of the AHe system enables direct constraints on the timing of cooling through the uppermost few kilometers of the crust. Detrital thermochronometry characterizes the thermal history of grains in clastic sedimentary rocks. To a first approximation, grain ages that are older than the deposit record the source area thermal history (Brandon et al., 1998; Ehlers and Farley, 2003). Thermal resetting of detrital grains requires burial to a depth sufficient for loss of pre-depositional helium from the apatite grain

(resetting the zero age). Thus, as a proxy, AHe dates older than the stratigraphic age of a host sediment are partially reset to completely unreset. AHe dates younger than the deposit are thermally reset. Reset AHe dates can be used to constrain the timing of a sample’s cooling through the AHe partial retention zone during exhumation.

3.6.1 Sampling Schema

Thrust-related exhumation provides constraints on the spatial and temporal evolution of shortening across the deformed Himalayan foreland. Sampling focused on stratigraphically and structural deep sites, such as the base of thrust sheets and the core of anticlines. Low structural and stratigraphic sampling maximizes the likelihood that grain ages record post-depositional resetting and structurally controlled exhumation. A total of 13 samples were collected from exposed hanging wall strata along three major fault traces and from the core of three folds across the Chakki study region (Figure 3.4; Appendix B - Table 3.1S). Multiple samples were taken from individual structures to assess along-strike variations in exhumation timing. Sample size ranged from 7-15 kg per sample locality to maximize the potential for discovery of high- quality apatite grains required for AHe analysis.

3.6.2. Apatite (U-Th/He) Laboratory Procedures

Standard magnetic and heavy liquid mineral separation procedures were used to isolate apatite in facilities at the University of Potsdam, Kurukshetra University, Oregon State University, and GeoSep Services. Individual apatite grains were hand-selected in alcohol under a stereographic microscope and scrutinized for quality. Only large grain size (>60µm), euhedral, and inclusion- and crack-free grains were selected for analysis. Selected grains were measured to determine alpha ejection correction values. In total, 64 individual apatite grains from the 13 samples were extracted, measured, and packed in Nb tubes for (U-Th)/He analysis. The analysis for Samples #17-23 were carried out at the University of Colorado-Boulder TRaIL

Lab and at the University of Potsdam for Samples #24-29 (Supplementary Table 3.2S).

Samples #1-16 can be found in Lamont et al. ((Chapter 2)).

The Nb packets were loaded into an ASI Alphachron He extraction and measurement line, placed in the UHV extraction line (~3 X 10-8 torr) and heated with a 25W diode laser to ~800-

145Nd tracer in HNO3, capped, and baked in a lab oven at 80°C for 2 hours. After dissolution, samples were diluted with 1 to 3 mL of doubly-deionized water. Sample solutions, along with normal solutions and blanks, were analyzed for U, Th, and Sm content using an Agilent 7900 quadrupole ICP-MS. After U, Th, and Sm measurement, He dates and all associated data were calculated on a custom spreadsheet using the methods described in Ketcham et al. (2011). The naturally occurring 238U/235U ratio used in data reduction is 137.818 after Hiess et al. (2012).

Every batch of samples included standards run sporadically throughout the process to monitor procedures and maintain run consistency. The calculated single grain ages are corrected using an alpha ejection correction (Farley et al., 1996). A summary of AHe ages is reported in

Appendix B - Table 3.2S.

3.6.3 Results of (U-Th)/He Analyses

Grain ages range from 0.8 ± 0.2 Ma to 6694.0 ± 122.5 Ma with the majority of grains yielding dates <8 Ma. Dispersion in grain ages is widely reported in the literature and can be explained by a variety of factors. Variations in crystal size in which larger grains lose less helium due to diffusion than smaller grains (Reiners and Farley, 2001). Radiation damage traps helium, which diffuses at a lower rate in the damage pathways created by the ejection of an alpha particle during decay of the parent atom (Flowers et al., 2009). Inclusions that contain U- and Th- lead to excess He through decay (Vermeesch, 2007). He implantation in grains yields excess helium through injection of an alien alpha particle during isotopic decay (Reiners et al., 2008). Four samples containing only trace amounts of U-, Th- and Sm- but yielding very old AHe dates were excluded from further analysis (* samples in Appendix B - Table 3.2S).

3.7 Thermal History Modeling

Ages obtained for individual apatite grains represent the integrated thermal history of that grain since the start of helium retention (Section 3.6). Inverse modeling is one approach to constrain the range of thermal histories consistent with AHe data and other geologic and geochronologic information. These models consider the complex relationship between the physical processes acting on a grain and the time-Temperature (t-T) dependence of helium diffusion (Flowers et al., 2015; Gallagher, 2012). In general, samples collected from stratigraphic levels from the upper 500-1000 m of the Middle Siwalik and shallower are not fully reset to unrest. Samples from Middle Siwalik and older strata yield fully reset dates (Figure 3.4.B.).

3.7.1 QTQt Inverse Thermal Modeling

We utilized the QTQt inverse modeling software v. PC64R5.6.0 (Gallagher, 2012) to determine t-T pathways for each multi-grain sample. QTQt employs a trans-dimensional Bayesian Monte

Carlo Markov Chain (MCMC) statistical approach to find the range of t-T thermal history pathways consistent with the AHe data. Model inputs include only (1) all single-grain (U-

Th)/He dates from a given sample and (2) the host sediment depositional age range. Model outputs are t-T pathways that are consistent with all grain ages. Each grain within a thermally reset sample experiences the same post-burial exhumation history. We used the RDAAM kinetic model of helium diffusion (Flowers et al., 2009) to model radiation damage effects and grain specific size controls of helium diffusion in the AHe system. Models yield the suite of equally acceptable t-T pathways for the given range of grain ages.

3.3.2 QTQt Model Constraints & Parameters

Model inputs include only the depositional age of the sampled strata, an estimate of surface temperature, and analytically derived, single-grain AHe dates. Depositional age ranges for the

Siwalik Group were derived from nearby published regional magnetostratigraphic sections

(Meigs et al., 1995; Ranga Rao, 1993). Modeled sample pathways originate from a domain that includes the stratigraphic age and the assumed surface temperature prior to burial. Mean modern and paleo-depositional sub-Himalayan surface temperatures were assumed as 17.5 ±

2.5°C, to be consistent with to other published sub-Himalayan studies (Gavillot et al., 2018;

Ghani et al., 2020; Khan and Raza, 1986; van der Beek et al., 2006). For all models, t-T boundary conditions were set to 80 ± 80 Ma for time and 85° ± 70° C for temperature, exceeding expected geologic t-T bounds for the sub-Himalayan geologic history. Model parameters are reported in Supplement Table 3.3S.

3.7.3 Results of Modeled Apatite Thermal Histories

The incorporation of AHe dates with known stratigraphic constraints in QTQt distinguishes grain ages characterized by full thermal resetting from those that experienced partial to no thermal resetting. Detrital samples yielding partial to no resetting record the thermal history of the grain prior to deposition. Consequently, we interpret only models characterized by fully reset grains. For reset samples, QTQt returns t-T models reflecting a grain’s suite of possible paths from burial to exhumation to the modern surface. We interpret the transition from heating, related to burial by sedimentary overburden, to cooling (shown graphically as an inflection point in the t-T pathways; Figure 3.4.C; Supplement Figure 3.1S) as indicative of thrust-related exhumation. We pick the path with the youngest inflection as the point in time by which cooling must have initiated on the associated structure. Initiation ages older than, but not younger than, that point are consistent with the input data.

Thermal modeling indicates that thrust-related exhumation across the Chakki study area largely occurred after ~4 Ma (Figure 3.4.C.; Supplement Figure 3.1S). Two samples were modeled from the ST. Sample 17 comes from the upper 500-1000 m of the Middle Siwalik Formation and is partially reset. Sample 18, which was extracted from a lower stratigraphic position within the Middle Siwalik Formation than Sample 17, is reset and the youngest t-T inflection point indicates exhumation was underway by 2.3 Ma on the ST. The SMA, the next structure to the north, has two samples extracted from Lower Siwalik rocks. SMA models consistently show active fold-related exhumation and growth by 3.5 Ma (Samples 22 & 23). Similarly, two

Lower Siwalik samples along the JT (Samples 20 & 21) indicate exhumation was underway by

1 Ma. Four modeled samples come from the hanging wall of the Basoli Thrust, a splay from the JT. Middle Siwalik rocks of Sample 24 from the western edge of the Kangra reentrant yields a >2.8 Ma exhumation age. Samples 25 and 29 were collected from near the top of the Middle

Siwalik Formation to the west of the Chakki Section and are both only partially reset. Sample

28, also from near the top of the Middle Siwalik Formation appears unreset and indicates no post-deposition burial. Given that the sample was collected from the immediate hanging wall of the Basoli Thrust, it is surprising that none of the grains experienced partial resetting similar to other upper Middle Siwalik samples. One possibility is that spatially variable thickness of the Upper Siwalik affected the degree to which Middle Siwalik grains are reset. Alternatively, the Basoli thrust may have initiated prior to substantial burial by younger sediment.

3.8 Discussion

3.8.1 Spatial Distribution of Shortening in the NW Sub-Himalaya

In the NW sub-Himalaya, the Chakki domain straddles the transition between structures of the wide Kangra reentrant and the narrow Jammu structural domains to the SE and NW, respectively. Balanced cross-sections consistently resolve 22-24 km of total shortening between the deformation front and the MBT despite along-strike differences in structural architecture (Gavillot et al., 2018; Powers et al., 1998). Overall, shortening is accommodated on a fewer number of faults and folds in the Jammu domain than in the Kangra resulting in higher slip rates and more deeply exhumed strata in the Jammu region. These variations indicate that structures of the Chakki study region transfer shortening between the Kangra reentrant on the SE and the Jammu section to the NW.

The deformation front within the Kangra reentrant is 40 km farther south than it is on the

Jammu section (Figures 3.1 and 3.4). The MFT and related Janauri anticline mark the thrust front in the Kangra reentrant (Powers et al., 1998). Both the MFT and the NW-plunging Janauri anticline lose surface expression towards the Chakki domain in the NW. The next structure to the NE, the Soan/Barwain Anticline, in the Kangra reentrant also plunges towards the NW and terminates to west of the Chakki section. Quaternary alluvium drapes both fold noses and is more modestly tilted than the underlying Siwalik rocks, which indicates that the Quaternary

sediments are growth strata. These growth strata and the fold plunge indicate fold growth toward the northwest. Lateral growth of these structures is also indicated by the westward deflection of the Beas and Chakki Rivers (Karunakaran and Ranga Rao, 1979; Powers et al.,

1998; Suresh and Kumar, 2020). Between the Kangra and the Chakki domains, the MFT accommodates 2.5-3.0 km of shortening along strike. The Soan thrust accommodates 3.7-4.0 km of shortening. The sum of the MFT and Soan thrust shortening is ~7 km (Figure 3.5)

(Powers et al., 1998). Neither the MFT nor the Soan thrust continue past the Chakki section into the Jammu domain, which implies that roughly 7 km of shortening must be absorbed by the Surin-Mastgarh anticline (SMA) and other structures in the Jammu domain.

The SMA is a regionally continuous structural culmination that extends from the Kangra reentrant to the Chenab River near the Pakistan-India border (Figure 3.1). The SMA lies north of the Soan Thrust in the middle of the Kangra reentrant. To the northwest, however, the SMA represents the deformation front in the Jammu section (Gavillot et al., 2018). Most interpret that a duplex explains well the fold structural relief and the space between the fold at the surface and the basal décollement at depth. A multi-horse duplex is interpreted to form the core of the

SMA on the Jammu section whereas a single-horse duplex explains the structural relief on the

SMA on the Chakki and Kangra sections. Shortening represented by the SMA is tied to the number and displacement of horses that comprise the duplex. Roughly 19 km of shortening are absorbed by an antiformal stack in the core of the SMA in the Jammu section (Gavillot et al.,

2018). A single horse on the Chakki section absorbs ~3.7 - 3.8 km of shortening (Figure 3.5).

Shortening of the SMA by a single-horse duplex on the Kangra section is ~1 km (Powers et al.,

1998). The SMA plunges to the SE and appears to die out in the footwall of the Jawalamukhi thrust to the southeast of the Kangra section.

Like the SMA, the Jawalamukhi thrust is a regionally extensive structure that plays a significant role in foreland deformation. Faults with a variety of names occupy a similar structural position and are physically connected to the Jawalamukhi thrust represent splays from a single thrust system along strike. A newly mapped branch point on the trace of the Jawalamukhi thrust near the western edge of the Kangra reentrant interprets that of the Barsoli thrust is a splay of the

Jawalamukhi thrust. Exposure of the lowest unit in the hanging wall changes from Lower

Siwalik in the Kangra reentrant to Upper Dharmshala on the Jammu section reflects progressively deeper stratigraphic exhumation along strike from SE to NW, respectively. The hanging wall of the Jawalamukhi thrust is an ~10 km-wide, gently north-dipping tilt block bounded by a broad synclinal dun basin on the northeast. The Lambargaon Syncline becomes narrower and more tightly folded to the northwest of the Kangra reentrant. Approximately 6.5 km of shortening is accommodated on the Jawalamukhi thrust on the Kangra section (Figure

3.5) (Powers et al., 1998). Nearly 11.25 km of shortening is suggested by the Chakki balanced cross-section. Several alternative explanations reconcile the northwestern increase shortening between the two sections. Folds and imbricate thrust sheets that are not present in the Chakki region, such as the Paror anticline and Palampur thrust, account for shortening to the north of the Jawalamukhi thrust in the Kangra reentrant. A greater inferred hanging wall ramp length on the Chakki section implies larger fault slip. Whereas shortening for the MKT, the along- strike equivalent to the Jawalamukhi thrust in the Jammu section, is unconstrained, more deeply exhumed rocks in hanging wall imply greater fault slip toward the northwest.

In summary, the Chakki section occupies a location between regions with contrasting structural styles, along-strike structural reorganization, and slip transfer. Similar amounts of shortening

(22-24 km) are suggested by the Kangra, Chakki, and Jammu balanced cross-sections. Strain is transferred from numerous widely spaced structures in the Kangra reentrant to a few structures in the narrower sub-Himalayan belt of the Jammu section to the NW. Because the

MFT and the Soan Thrust lose displacement to the west of the Chakki section, more shortening must be absorbed fewer structures in the Jammu domain.

3.8.2 Temporal Distribution of Shortening in the NW sub-Himalaya

Minimum ages of fault-related exhumation derived from (U-Th)/He thermal models in the NW sub-Himalaya indicate that frontal accretion at the toe of the orogen was followed by a protracted period of distributed internal deformation. Cooling ages for the MFT on the Kangra section indicate that exhumation at the deformation front was underway by ~4 Ma (Figure 3.6).

Near the Chakki section, no datable grains were extracted from the JA. The NW plunge of the

Janauri anticline and overlying syntectonic strata indicate that the fold has low structural relief and thus limited exhumation at the surface (Karunakaran and Ranga Rao, 1979; Powers et al.,

1998; Suresh and Kumar, 2020). The SMA represents the deformation front on the Jammu section and model ages indicate fold-related exhumation commenced by ~4 Ma. Thus, foreland accretion and deformation initiated in synchrony along strike at the thrust front (Chapter 2;

Gavillot et al., 2018). Models indicate a slightly later onset of exhumation on the SMA at ~3.5

Ma on the Chakki section, potentially reflecting progressive lateral growth of the structure.

Exhumation ages from the Soan thrust young to the west. Dates change from 2.4 Ma ± 0.1 Ma in the Kangra section to partially reset on the Chakki section 40 km to the west. Dates along the Jawalamukhi thrust vary less systematically. In the Kangra reentrant, ages on the JT decrease westward from 2.0 Ma to 1.0 Ma. Near the Chakki section, west of the branch point that demarcates the Barsoli splay thrust from the Jawalamukhi thrust. The ages are partially reset. However, one sample on the Barsoli Thrust indicates exhumation was underway by 2.8

Ma. This value closely matches the 2.4 – 2.7 Ma exhumation dates for the MKT further to the west near the Jammu section (Gavillot et al., 2018). Critical wedge theory predicts that frontal accretion occurs only when the wedge has critical taper. Distributed deformation follows

frontal accretion. These age data and the temporal and spatial pattern of shortening suggest that the orogen went from a critical or supercritical wedge state to a subcritical state by 4

Ma. The subsequent and ongoing internal deformation acts to rebuild wedge taper (Boyer,

1982; Dahlen and Suppe, 1988; Davis et al., 1983).

Shortening rates combine modeled exhumation ages and shortening on individual structures derived from the balanced cross-sections (Figure 3.5 and 3.6). Similar rates are derived for the

Kangra and Chakki sections. In the southern portion of the deformed foreland, the shortening rate on the MFT is ~0.6 mm/yr, ~1.5 mm/yr for the Soan thrust, and ~1.9 mm/yr on the SMA.

The cumulative shortening from these frontal structures is transferred to the SMA on the

Jammu section and accounts for approximately half of the 6-9 mm/yr SMA rate (Gavillot et al., 2016). From the Jawalamukhi thrust to the MBT, a cumulative shortening rate of ~9 mm/yr characterizes the shortening rate. Rate for the Jawalamukhi thrust is ~3.3 mm/yr, ~1.8 mm/yr for the Paror Fault, and ~3.8 mm/yr for the Palampur thrust (an absolute minimum estimate).

Assuming modern geodetic shortening rates of 11-14 mm/yr for the NW Himalaya (Banerjee and Bürgmann, 2002; Jade et al., 2007; Schiffman et al., 2013) reflect the long-term rate, shortening absorbed in the sub-Himalaya is less than the geodetic rate. Balanced cross-sections resolve 22-24 km of total shortening between the deformation front and the MBT in the NW sub-Himalaya since 4.0 Ma (This Chapter – Section 3.1.2; Chapter 2; Gavillot et al., 2018;

Powers et al., 1998). An average of 1.4-8.5 mm/yr of shortening is in deficit compared to the modern geodetic rate and must be accommodated elsewhere in the orogenic wedge. Candidate structures for the missing shortening include the MKT on the Jammu section, the Palampur thrust on the Kangra section. Greater slip could be absorbed by the SMA duplex, distributed penetrative strain across the region, or slip on the MBT could also account for additional shortening. Recent studies of rock exhumation in the Dhaladhar Range west of the Kangra

reentrant are interpreted to indicate that the MBT is active (Deeken et al., 2011; Thiede et al.,

2017). Shortening rates < 2.6 -3.5 mm/yr are inferred since 6-8 Ma (Thiede et al., 2017).

In sum, the NW sub-Himalaya is characterized by frontal accretion along the length of the undeformed foreland after ~4 Ma followed by internally distributed deformation to the present.

Localization of deformation on the SMA in Jammu and the MFT in Kangra was nearly synchronous (~4 Ma). The two structures propagated laterally to the Chakki region by ~3.5

Ma. By 2.5 Ma, exhumation on the Jawalamukhi and all other sub-Himalayan thrusts was underway. Whereas shortening rates in the Kangra are low on southern structures (0.6-1.9 mm/yr), shortening rates are high on northern faults (1.8-3.8 mm/yr) because they are apparently younger and characterized by greater offset. Individual shortening rates in the

Jammu region nearly double the highest individual rates in the Kangra region (~6 mm/yr) because the long term shortening in the Jammu section was localized in the SMA duplex.

Nonetheless, equal amounts of total shortening (22-24 km) characterize the region from the

Kangra reentrant to the Chenab River (Figure 3.5). Shortening across the sub-Himalaya balances because the redistribution of slip between structures in the Kangra and Jammu regions.

The long-term average shortening rate is lower than the geodetically determined rate implying a deficit that requires additional deformation on blind, unconstrained, or hinterland structures.

3.8.3 Diachronous foreland accretion along the Himalayan arc

The timing, rate, and distribution of shortening in the NW Himalaya differ fundamentally from those of the central Himalaya. Convergence rates along the Himalaya vary from 13 mm/yr in the Potwar Plateau, 11-14 mm/yr in northwestern, 18-20 mm/yr in central, and 22 mm/yr in eastern Himalaya (Banerjee and Bürgmann, 2002; Bilham et al., 1997; Lavé and Avouac, 2000;

Leathers, 1987; Wesnousky et al., 1999). The systematic decrease in convergence rate reflects increasing obliquity of convergence from east to west (Jouanne et al., 1999). Orogen normal shortening in the Nepal central Himalaya focuses the full convergence rate on the MFT

(DeCelles and Carrapa, 2021; Lavé and Avouac, 2000). In contrast, deformation is distributed on the MFT and the other sub-Himalayan faults in the NW Himalaya (Gavillot et al., 2016;

Gavillot et al., 2018; Thakur, 2013; Thakur et al., 2014; Vassallo et al., 2015). Exhumation associated with growth of the MFT suggests initiation after ~2 Ma in Nepal (van der Beek et al., 2006), ~2 M.y. later than the NW sub-Himalaya. Potential explanations for this behavior include different rates of wedge taper growth, climatically modulated taper development through focused material removal, and the redistribution and accumulation of sediment in sub-

Himalayan wedge-top basins dampening taper and stalling wedge widening.

Accretion of the foreland basin in Pakistan across the Potwar Plateau closely mirrors that of the NW sub-Himalaya. The Salt Range Thrust (SRT) represents the deformation front which propagated ~130 km to the south of the MBT. This distance was facilitated by localization of the basal décollement in Eocambrian salt (Baker et al., 1988; Jaumé and Lillie, 1988; Lillie et al., 1987; Pennock et al., 1989) by ~5.2 Ma (Burbank and Beck, 1989). AHe dates indicate that exhumation was underway by 4.0 Ma (Ghani et al., 2020), consistent with exhumation ages on the SMA in Jammu and on the Kangra segment of the MFT. Data and balanced cross-sections indicate that the SRT accommodates as much as 22 km of shortening with a minimum average shortening rate of 5-6 mm/yr since 4Ma (Baker et al., 1988; Ghani et al., 2020; Pennock et al.,

1989), similar to the amount and rate of SMA shortening (Gavillot et al., 2018). Formation of the modern, active thrust front and the associated accretion by ~4 Ma is broadly coeval from the Potwar Plateau to the Kangra reentrant, more than 500 km along the Himalayan arc. After

~2 Ma shortening across the NW Himalaya is distributed across the sub-Himalaya and the foreland in central Nepal is accreted to the orogen.

3.8.4 Implications for Orogen Mechanics and Hazard Assessment

Critical wedge models provide a theoretical framework for interpreting the behavior of evolving contractional orogens (Davis et al., 1983). Model behavior depends on the proportion

of material added to an orogen via tectonic accretion at the front, or by underplating (i.e. duplexing), and that removed by erosion. Wedge internal deformation and sliding are closely tied to the taper defined by the angle between the basal décollement and the surface. When those two boundaries obtain a specific angle, the wedge geometry has attained a critical taper.

Critical taper is governed by internal material properties, basal décollement strength, pore fluid pressure, and other variables (Dahlen and Suppe, 1988; Davis et al., 1983). Changes in material fluxes result in predictable changes to wedge geometry. For example, when a wedge is stable

(critical) or overthickened (supercritical), wedge widening by frontal accretion shifts the wedge to a wider, thinner subcritical state. Distributed internal deformation, wedge narrowing, and/or underplating rebuild wedge taper (Boyer, 1982; DeCelles and Mitra, 1995; Gavillot et al., 2018;

Lyon‐Caen and Molnar, 1985; Meigs and Burbank, 1997). Wedge narrowing and distributed internal deformation are also the response to excess material removal by erosion (Beaumont et al., 1992; Hilley and Strecker, 2004; Whipple, 2009; Whipple and Meade, 2004; Willett, 1999;

Willett et al., 1993). Spatial and temporal patterns of deformation across the NW sub-Himalaya indicate that the wedge was already subcritical at the start of the Plio-Quaternary climate transition, a period of inferred enhanced erosional efficiency, and that wedge narrowing by internal deformation and duplex development at depth was the product of wedge widening and not necessarily erosive mass removal.

Understanding the active pattern of faulting is critical to assessing seismic hazard potential.

Great earthquakes in collisional mountain belts result from slip along the basal detachment

(Yeats and Thakur, 1999). Earthquakes such as the 1905 Kangra, 1833 Nepal, 1934 Bihar, 1950

Assam, 2005 Kashmir, and 2015 Gorkha earthquakes provide evidence for the potential of major damaging events of Mw >7.6 across the sub-Himalaya (Bilham, 2004; Bilham and

Ambraseys, 2005; Bollinger et al., 2014; Kaneda et al., 2008; Middlemiss, 1910; Sapkota et al., 2013; Seeber and Armbruster, 1981; Thakur, 2013; Yeats and Lillie, 1991). In the NW sub-

Himalaya extensive work has been done to reconstruct the paleoseismic record on structures at the deformation front (Kumahara and Jayangondaperumal, 2012; Kumar et al., 2006; Malik et al., 2010), but the paleoseismic record of other faults in the sub-Himalaya is not well constrained. Observations of active overthrusting of Siwalik and Dharmsala strata above

Quaternary fans and alluvium by internal, sub-Himalayan faults highlights the need for studies evaluating hazard potential on these internal structures (Delcaillau et al., 2006; Dey et al., 2016;

Jayangondaperumal et al., 2017; Malik and Nakata, 2003; Thakur et al., 2014). Higher shortening rates on faults such as the Jawalamukhi thrust suggest that slip is transferred more frequently from the basal décollement to those faults rather than to lower slip rate faults. Thus, relatively long recurrence intervals inferred for the MFT potentially masks a more complex and more frequent occurrence of large earthquakes.

3.9. Conclusions

Reconstructing the timing and pattern of accretion within a deformed foreland setting is critical to decoding the long-term evolution of mountain building processes and for assessing seismic hazards. Shortening accommodation along the length of the NW sub-Himalaya, including the deformed foreland of the Potwar Plateau, since the Plio-Quaternary is characterized by accretion of the undeformed foreland by ~4 Ma followed by a protracted period of distributed internal deformation since ~2-2.5 Ma. This pattern of accretion and subsequent internal deformation is predicted by critical wedge models to reflect a tectonically-driven, subcritical orogenic wedge state actively rebuilding wedge taper.

Despite uniform minimum shortening of 22-24 km accommodated across the entire NW sub-

Himalaya, lateral variations in the number faults and the width of the deformed foreland necessitate a zone of strain reorganization to transfer shortening along strike. In the Kangra reentrant, structures south of the Jawalamukhi thrust accommodate lesser shortening at lower

rates (0.6–1.9 mm/yr) than the Jawalamukhi and other faults to the north (1.8-3.8 mm/yr). In the Jammu sub-Himalaya, fewer structures accommodate shortening but at higher rates, ~6-9 mm/yr. Shortening rates across the NW sub-Himalaya from 4-2 Ma suggest a deficit in shortening of <8.5 mm/yr compared to the 11-14 mm/yr modern geodetic rate. After 2 Ma, nearly all shortening is accommodated within deformed foreland. Missing shortening is likely absorbed by on-going shortening on the MBT in the hinterland, by shortening on unconstrained or blind structures within the sub-Himalaya, and by distributed penetrative strain.

We interpret that orogenic growth within the NW Himalaya reflects a stepwise continuum where accretion at the deformation front widens or steps the wedge southward by 4 Ma and is followed by a pattern of distributed deformation that progressively concentrates shortening within the deformed foreland between 2 Ma and the present. Active distributed deformation inferred by our new AHe data is supported by observations of Quaternary faulting in internal sub-Himalayan outcrops suggesting potential for seismic hazards on dispersed sub-Himalayan structures and not just the orogenic deformation front.

3.10. Acknowledgements

This material is based upon work supported by the National Science Foundation under Grant

Nos. EAR-1759200 and EAR-1759353. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect views of the National Science Foundation. Thanks to the AGeS program for its support.

The Fulbright-Nehru Research Program of the US-India Education Foundation, the German

Academic Exchange Service, the Ryoichi Sasakawa Young Leaders Fellowship Fund, and

OSU provided support. Mineral separations supported by Dr. R. Patel (Kurukshetra University,

India), GeoSeps, Moscow, ID, and J. Worster. Shri P. Kumar and the directors at the Wadia

Institute of Himalayan Geology are thanked for their generous assistance.

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Figure 3.1. Location map of the NW Himalaya (India) and Potwar Plateau (Pakistan) regions with labeled geographic features and major cities. Thick white line – India-Pakistan political border. Black barbed lines – major thrust faults. Thin black lines with double arrows – major folds. Pale yellow arrow – estimate of the portion of Indo-Eurasian convergence absorbed across the NW Himalaya (Banerjee and Burgmann, 2002). Thick black lines A-A’, B-B’, and

C-C’ – balanced cross-sections of Powers et al. (1998), Figure 3.3 from this chapter, and

Gavillot et al. (2018), respectively. Dashed box – location of figures designated in the text.

Inset (top right) shows the relative positions of the orogenic hinterland, sub-Himalaya, and

undeformed foreland with respect to the major bounding faults. MFT – Main Frontal thrust;

MBT – Main Boundary thrust; SMA – Surin Mastgahr Anticline; JT – Jawalamukhi thrust;

SRT – Salt Range Thrust; JA – Janauri anticline; ST – Soan thrust; SA – Soan anticline; .PR –

Paror Fault; PAL – Palampur thrust; RT – Riasi thrust; MKT – Mandili-Kishanpur thrust; TT

– Tanhal thrust; MT Mundun thrust.

Figure 3.2. Stratigraphy of the Cenozoic foreland basin sequence involved in deformation in the Chakki study area, Himachal Pradesh. Sequence compiled from regional sedimentological and paleomagnetic studies and supported by field observations (Gavillot et al., 2018;

Karunakaran and Ranga Rao, 1979; Meigs et al., 1995; Powers et al., 1998; Raiverman, 2002;

Raiverman et al., 1983; Ranga Rao, 1993; Ranga Rao et al., 1988). Thickness values are an average of reported values and cross-checked with 3-point problem calculations. Thickness of the Upper Siwalik is a minimum estimate that includes the Upper Boulder Conglomerate as the thickness is not well constrained due to similarity between beds in the upper portion of the unit and Neogal sediments. Field relationships suggest that Upper Siwalik thickens toward the north. US – Upper Siwalik Formation; MS – Middle Siwalik Formation; LS – Lower Siwalik

Formation; UD – Upper Dharmshala Formation; LD – Lower Dharmsala Formation.

Surin

–

Quaternary Quaternary

–

Middle Siwalik; Siwalik; Middle

1,2

–

numbered lines are lines are numbered

section reconstruction across the Chakki the Chakki across section reconstruction

ped in the balanced section reconstructions. reconstructions. section balanced the in ped

lower Upper Siwalik; MS MS Siwalik; Upper lower

–

Himalaya. Himalaya. Geologic units: Qal

US US

Soan Anticline (a.k.a. Barwain Anticline); SMA Anticline); Barwain Anticline (a.k.a. Soan

–

deformed balanced cross balanced deformed

st. st. Folds are shown as solid black double arrowed lines. Faults are

–

Himalaya. Himalaya. A. map Geologic across the deformed foreland of sequence

upper Upper Siwalik; Siwalik; Upper upper

–

Soan Thrust; SA SA Soan Thrust;

–

section across the Chakki sub

sections in part B. B. part sections in

Main Main Boundary Thru

–

Lower Dharmsala. Note that the two US units are lum are units US two the that Note Dharmsala. Lower

–

Janauri Anticline; ST Anticline; Janauri

Quaternary terraces; uUS terraces; Quaternary

length length balanced cross

–

d section reconstruction. d section reconstruction.

restored, restored, line

–

Jawalamukhi Jawalamukhi Thrust; MBT

B’ cross of both location the is

Upper Dharmsala; LD LD Dharmsala; Upper

–

Main Frontal Thrust; JA Thrust; Frontal Main

–

and and part C.

on. Line B Line on. Qt deposits; fan Quaternary

–

. Structural framework . Structural of framework the Chakki study of area the NW sub

Lower Siwalik; UD UD Siwalik; Lower

Himalaya Himalaya

–

Figure 3.3 regi the Chakki sub Qf alluvium; LS MFT Structures: Mastagahr Anticline; JT bedding lines black shown with measured as barbed wall.dips. on teeth Blue are lines the side valued Small hanging tick with balance in the used lines and loose the pin

Figure 3.4. Location and analysis of apatite (U-Th)/He samples from the Chakki sub-

Himalaya. A. Location map of AHe samples across the NW sub-Himalaya from new (blue points -this study) and previous (black points - Lamont et al., Chapter 2; Gavillot et al., 2018) studies. White points represent samples that produced either partially to fully unreset grains or yielded no datable grains. Lines A-A’, B-B’, and C-C’ are location of cross-sections found in

Figure 3.5. Small black x –published paleomagnetic studies. B. Assessment of thermal resetting of individual apatite grains. In plotting individual grain AHe dates from lab analysis against age of the sampled stratigraphic unit constrained by magnetostratigraphy (Meigs et al., 1995),

AHe ages found to be younger than stratigraphic ages suggest thermal resetting and vice versa.

Samples that lie close to the age 1:1 line are likely partially thermally rest. Errors represent range of in age of stratigraphic unit (vertical) and reported AHe analytical errors (horizontal).

C. Example thermal history model for Sample 27 from the SMA. Dashed black line denotes youngest inflection age.

f f the SMA (grey

ly reset AHe samples) samples) AHe reset ly

from from across the NW Himalaya

Chapter Chapter 2; Gavillot et al., 2018). Blue ovals

e e youngest modeled AHe exhumation age for a given

Sections Sections are aligned along the core o

s s (Gavillot et al., 2018; Powers et al., 1998)

ront ront (MBT & SMA) yield older exhumation ages than those from more internal

section

related related exhumation initiation ages.

Note Note that ages near the deformation f

Th)/He Th)/He (AHe) fault and fold

Himalaya. Himalaya.

Compilation Compilation of new and published balanced cross

Figure 3.5 with modeled apatite (U (thermal circles black as shown are locations sample AHe comparison. for scales vertical and horizontal equal at line) dashed and white circles reset (partially or unreset samples). Ovals containing numbers threpresent fault or fold structure. While ovals represent AHe ages from previous studies (This Dissertation represent new AHe ages. sub the within structures

AHe AHe

regional regional

–

ure growth toward growth toward ure

Himalaya. Himalaya. Filled ovals

n grey double arrowed grey n lines double arrowed

faults teeth with the Thi hanging wall. on faults

–

map of actual sample locations modified from Figure 3.4.A. Figure modified from locations sample map of actual

–

B’ Black . lines barbed

section section B

. Summary map schematic of lateral structural variations as observed by AHe dates across the NW sub

Figure 3.6 struct show of lateral direction arrows Orange models. history thermal onset from ages by exhumation sample locations colored the near region cross Chakki Inset plunge. of axial the direction in arrow large folds with

APPENDIX B.

SUPPLEMENTAL MATERIALS

B.1 (U-Th)/He Sample and Analysis Data Tables

Table 3.1S: Sample Location and Stratigraphic Age Constraints

TABLE S1. SAMPLE LOCATION AND STRATIGRAPHIC AGE CONSTRAINTS Identification Location Stratigraphy Sample Sample Latitude Longitude Elevation Lower Age Range Upper Age Unit Name ID Name (°N)* (°E)* (m) (Ma) § Range (Ma) § 17 BW1 32.126267 75.78185 338 Middle Siwalik 7.091 11.052 18 BW2 32.056117 75.821983 324 Middle Siwalik 7.091 11.052 19 DF1 32.223933 76.316717 1343 Upper Dharmsala 18.000 23.000 20 JT10 32.112817 76.166067 592 Lower Siwalik 11.052 18.000 Lower Siwalik - Upper 21 JT11 32.2256 76.07105 651 11.000 23.000 Dharmsala 22 SMA6 32.29825 75.9055 590 Lower Siwalik 11.052 18.000 23 SMA7 32.17325 76.042933 483 Lower Siwalik 11.052 18.000 24 RT17-02 32.24499 76.14587 895 upper Middle Siwalik 7.091 11.052 25 RT17-16 32.40707 75.89376 557 Middle Siwalik 7.091 11.052 26 RT17-24 32.4316 75.74796 560 lower Middle Siwalik 7.091 11.052 27 RT17-25 32.41766 75.74167 455 upper Middle Siwalik 7.091 11.052 28 RT17-28 32.53627 75.79945 760 upper Middle Siwalik 7.091 11.052 29 RT17-30 32.49225 75.82964 570 lower Middle Siwalik 7.091 11.052 * Latitude & Longitude reported in decimal degrees in theWGS84 (G1762) reference system. Collected with a Garmin 64SX Handheld GPS. § Age range from Meigs et al., 1995 magnetostratigraphy using full US, MS, LS, LD age range. Cande & Kent 1992; 1995 magnetostratigraphic datum

Table 3.2S: Summary of (U-Th)/He Sample Analysis with corrected age

TABLE S2. SUMMARY OF (U-TH)/HE SAMPLE ANALYSIS WITH CORRECTED AGE Uncorrected Corrected U Th 147Sm He Mass rs Sample Name +/- (Ma) § +/- (Ma) § e (U) †† Th/U Ft*** Date (Ma) † Date (Ma) # (ppm)** (ppm)** (ppm)** (nmol/g)** (ug) §§ (um)## Sample 17 BW1_a01 3.9 0.1 5.9 0.2 143.8 19.6 71.5 148.43 0.14 3.1 1.5 40 0.65 BW1_a02 2.8 0.1 4.4 0.3 20.6 119.1 67.7 48.63 5.77 0.7 1.3 40 0.63 BW1_a03 3.6 0.1 5.3 0.3 55.4 7.1 42.4 57.03 0.13 1.1 1.8 42 0.68 BW1_a04 4.2 0.1 6.2 0.4 50.2 8.6 34.0 52.26 0.17 1.2 1.8 41 0.67 BW1_a05 2.3 0.0 3.4 0.1 47.7 81.9 39.4 66.98 1.71 0.8 2.2 43 0.67 Sample 18 BW2_a02 2.5 0.1 3.7 0.2 50.4 60.2 75.1 64.54 1.19 0.9 1.5 44 0.68 BW2_a03 1.7 0.1 3.0 0.5 8.1 42.2 29.8 18.05 5.18 0.2 0.8 35 0.58 BW2_a04 3.8 0.1 6.4 0.3 41.4 154.7 55.1 77.75 3.74 1.6 1.0 36 0.60 BW2_a05 6.3 0.1 8.9 0.4 72.3 43.5 41.9 82.54 0.60 2.8 2.0 47 0.71 BW2_a06 1.5 0.1 2.5 0.2 21.8 208.5 119.1 70.83 9.54 0.6 1.1 38 0.61 Sample 19 DF1_a01 2.2 0.1 3.5 0.3 20.1 72.1 20.1 37.09 3.58 0.4 0.8 39 0.62 DF1_a02 2.6 0.1 4.0 0.2 65.2 111.1 30.0 91.30 1.70 1.3 0.9 41 0.64 DF1_a03 0.5 0.1 0.8 0.2 2.8 72.2 10.0 19.77 25.84 0.1 1.6 50 0.68 DF1_a04 2.3 0.2 4.3 0.7 1.8 63.0 13.7 16.63 34.58 0.2 0.8 33 0.54 Sample 20 JT10_a01 2.8 0.1 4.7 0.5 48.6 20.9 43.3 53.47 0.43 0.8 0.7 34 0.59 * JT10_a02 281.1 38107.2 319.7 123511.7 0.0 0.0 0.1 0.00 0.00 0.0 0.8 36 0.62 JT10_a03 5.6 0.5 8.2 1.4 2.9 6.6 2.6 4.43 2.28 0.1 1.9 46 0.69 JT10_a04 1.2 0.1 1.8 0.3 9.5 27.7 23.7 16.03 2.91 0.1 1.4 44 0.67 JT10_a05 1.4 0.1 2.4 0.4 27.2 39.9 32.6 36.60 1.46 0.3 0.9 35 0.60 Sample 21 JT11_a01 3.5 0.1 5.1 0.4 14.3 44.1 29.8 24.65 3.09 0.5 2.5 46 0.69 JT11_a02 3.2 0.0 5.3 0.2 38.5 255.0 72.8 98.39 6.63 1.7 1.1 37 0.60 JT11_a03 7.3 0.1 10.4 0.4 52.4 82.5 31.3 71.81 1.57 2.8 2.1 48 0.70 JT11_a04 2.3 0.1 3.3 0.2 15.9 126.4 30.3 45.57 7.96 0.6 3.0 50 0.70 JT11_a05 0.9 0.0 1.3 0.1 4.4 145.0 10.5 38.50 32.66 0.2 3.0 56 0.72 Sample 22 SMA6_a01 1.9 0.3 2.8 0.8 3.3 6.1 11.1 4.68 1.87 0.0 1.7 47 0.70 SMA6_a02 3.4 0.6 5.5 1.7 12.2 19.1 21.0 16.66 1.57 0.3 1.4 38 0.62 SMA6_a03 8.1 0.5 12.4 1.5 5.0 3.2 8.7 5.74 0.63 0.3 1.5 40 0.65 SMA6_a04 88.7 3.9 112.1 10.0 4.7 4.6 28.2 5.76 0.99 2.9 4.7 66 0.78 Sample 23 SMA7_a01 28.4 1.7 41.0 5.1 8.9 2.6 25.4 9.54 0.29 1.5 1.6 44 0.69 SMA7_a02 12.5 0.2 15.9 0.6 43.1 3.4 23.3 43.85 0.08 3.0 4.8 65 0.79 SMA7_a03 2.8 0.1 3.8 0.1 19.6 48.2 18.4 30.94 2.46 0.5 3.2 57 0.74 SMA7_a04 44.4 1.0 60.9 2.9 8.1 6.3 28.2 9.55 0.78 2.3 2.4 51 0.72 SMA7_a05 6.9 0.1 9.6 0.4 51.7 7.2 17.1 53.42 0.14 2.0 2.2 48 0.72 Sample 24 RT17-02-a1 5.4 0.3 7.6 0.3 12.2 14.4 10.9 15.57 1.22 0.5 3.0 52 0.71 RT17-02-a4 2.8 0.3 4.1 0.3 19.2 4.5 13.8 20.25 0.24 0.3 2.0 45 0.67 RT17-02-a5 6.1 0.5 8.4 0.5 6.6 23.7 4.5 12.16 3.73 0.4 3.2 54 0.72 RT17-02-a6 1.2 0.0 1.7 0.0 13.1 280.3 70.8 79.00 22.08 0.5 3.4 54 0.72 RT17-02-a7 6.6 0.4 8.3 0.4 4.1 4.2 6.2 5.06 1.08 0.2 8.7 74 0.80 Sample 25 RT17-16-a1 2.2 0.2 3.4 0.2 77.1 68.2 14.4 93.12 0.91 1.1 2.0 44 0.66 * RT17-16-a2 426.5 44.6 538.1 44.6 0.1 0.2 0.0 0.10 3.07 0.2 8.1 72 0.79 * RT17-16-a3 4861.0 122.5 6694.0 122.5 0.3 4.4 0.1 1.37 13.28 44.6 3.6 55 0.73 RT17-16-a4 5.5 0.6 7.1 0.6 2.3 13.3 8.4 5.44 5.89 0.2 6.4 68 0.78 RT17-16-a5 0.8 2.0 1.3 2.0 4.6 36.0 7.5 13.07 8.05 0.1 1.0 37 0.59 * RT17-16-a6 1779.4 89.8 2574.2 89.8 0.8 14.4 0.2 4.19 18.66 43.4 2.6 49 0.69

Table 3.2S: Continued

TABLE S2. CONTINUED

Uncorrected Corrected U Th 147Sm He Mass rs Sample Name +/- (Ma) § +/- (Ma) § e (U) †† Th/U Ft*** Date (Ma) † Date (Ma) # (ppm)** (ppm)** (ppm)** (nmol/g)** (ug) §§ (um)##

Sample 26 RT17-24-a1 0.9 0.3 1.4 0.3 15.1 42.1 10.5 25.01 2.88 0.1 1.3 40 0.63 RT17-24-a2 2.4 0.7 3.9 0.7 6.1 28.3 21.9 12.72 4.83 0.2 1.3 39 0.62 RT17-24-a3 2.6 0.3 4.0 0.3 20.9 12.5 20.0 23.80 0.62 0.3 1.6 43 0.65 RT17-24-a4 1.2 0.2 1.7 0.2 22.1 47.8 14.5 33.36 2.23 0.2 2.2 47 0.68 RT17-24-a5 0.9 0.1 1.2 0.1 55.5 159.9 9.7 93.08 2.98 0.4 2.9 52 0.71 RT17-24-a6 1.6 0.1 2.3 0.1 53.1 129.9 14.8 83.57 2.53 0.7 3.0 50 0.70 Sample 27 RT17-28-a1 7.2 0.4 9.4 0.4 13.2 6.5 6.9 14.76 0.51 0.6 5.4 63 0.76 RT17-28-a2 7.0 0.8 12.1 0.8 12.2 24.0 20.7 17.78 2.04 0.7 1.0 36 0.58 RT17-28-a3 6.7 0.3 8.6 0.3 5.1 2.8 6.1 5.82 0.57 0.2 7.2 69 0.78 Sample 28 RT17-30-a1 4.5 0.1 5.5 0.1 9.5 5.0 36.3 10.64 0.54 0.3 10.6 79 0.81 RT17-30-a5 4.3 0.1 4.9 0.1 31.2 3.0 1.7 31.86 0.10 0.7 31.3 116 0.87 RT17-30-a6 11.0 0.4 12.9 0.4 2.8 6.5 3.8 4.30 2.43 0.3 21.9 103 0.85 RT17-30-a7 9.6 0.2 10.9 0.2 5.9 2.3 8.0 6.41 0.40 0.3 41.1 126 0.88 RT17-30-a8 2.7 0.1 3.3 0.1 7.7 1.7 13.2 8.14 0.23 0.1 12.1 84 0.82 RT17-30-a9 3.5 0.9 4.7 0.9 1.5 4.7 3.0 2.57 3.35 0.0 4.8 60 0.75

* Grains not used in thermal history modeling † Uncorrected Date (Ma): Calculated iteratively using the 4He production equation defined as Equation 1 in Wolf et al. (1998) and assuming secular equilibrium. # Corrected date (Ma): Calculated iteratively using the absolute values of He, U, Th and Sm, the isotope specific FT corrections, and Equation 34 in Ketcham et al. (2011) assuming secular equilibrium. § +/- Ma: = 2s propagated analytical uncertainty on the U, Th, Sm, and He measurements ** Concentrations of He, U, Th and Sm: computed from their absolute amounts and the estimated dimensional mass †† eU: effective uranium concentration calculated as U + 0.2375*Th + 0.0012*Sm §§ Mass: The mass of the crystal. Determined from the measured grain dimensions, the volume assuming the reported grain geometry, and the volume equations and mineral densities in Ketcham et al. (2011). ## rs: The radius of a sphere with an equivalent alpha ejection correction as the grain, calculated using Equation A6 in Cooperdock et al. (2019)

Model

RDAAM

He Kinetic Kinetic He

Gaussian

Selection Type Selection

(Proposal Birth) (Proposal

Scale Parameter Parameter Scale

Model Model

500,000

Model-Specific Parameters Model-Specific

Iterations

# Post Burn-in Post #

Model Model

500,000

# Burn-in #

Iterations

)

°C

(

Surface

17.5 17.5

Deposition Deposition

Temperature at Temperature

Stratigraphic Constraints Stratigraphic

Mean Age Mean

9.07 9.07

20.5 20.5

9.07 9.07

14.53 14.53

17.00 17.00

14.53 14.53

Deposition (Ma) Deposition

)

°C

17.5 17.5

Modern Surface Modern

Temperature ( Temperature

)

∂t

Myr

Parameters

1000

°C

(

Max ∂T Max

)

°C

Thermal History Bounds History Thermal

TABLE S3. (U-TH)/HEHISTORY MODELINPUT THERMAL PARAMETERS S3. TABLE

Range(

Temperature Temperature

± ±

(Myr)

Time Range Time

# of#

Grains Grains

Excluded

# of#

Used

Grains Grains

Modeled Grains Modeled

Th)/He Thermal History Model Input Model Input History Thermal Th)/He

Yes

Sample Sample

Modeled

: (U :

DF1

JT11

JT10

BW2

BW1

SMA7

SMA6

Name

Sample Sample

RT17-30

RT17-28

RT17-25

RT17-24

RT17-16

RT17-02

Samples

18 17

Sample Sample Table 3.3S Table

Table 3.3S: (U-Th)/He Thermal History Model Input Parameters (Continued)

Thermochronologic Data: Specific (U-Th)/He analytical inputs for each analyzed grain, including the U, Th, Sm, and He concentrations as well as grain radius, can be found in Table

S2. ‘# of grains used’ refers to the number of individual grain results from a single sample used for the analysis. Excluded grains were discarded due to erroneous dates related to very small

U, Th, Sm concentrations, though initial trials included all grains to test overall fit of all data.

Depositional ages of the sampled Siwalik Group were constrained by the magnetostratigraphic section of Meigs et al. (1995). The magnetostratigraphic age range for the entire section from which it was extracted was utilized. Because these samples were deposited at the surface and because paleo-surface temperature is poorly constrained, we assumed similar surface temperatures to the modern at time of deposition.

System & Model-Specific Parameters: Thermal history analyses were conducted using the

the thermal history. 500,000 iterations for each were conducted for samples interpreted to yield thermally reset (U-Th)/He ages to tightly constrain thermal histories. Samples indicating only partial or no resetting were analyzed using 100,000 iterations each. The selection of new time and temperature points between individual model iterations in the sampling chain is done using a Gaussian distribution. The RDAAM model of alpha radiation damage on the diffusivity of

He in apatite is employed for all models (Flowers et al., 2009).

B.2 QTQt Thermal History Models

Figure 3.1S: All modeled time-temperature pathways for samples along the Chakki Transect

Figure 3.1S: Continued

CHAPTER 4:

FAULT-RELATED PERMEABILITY IN OREGON’S CASCADIA BACKARC AND IMPLICATIONS FOR MAGMATIC AND GEOTHERMAL FLUID FLOW

4.1 Authors & Affiliations

Ellen A. Lamont 1, Andrew Meigs 1, Daniel O’Hara 2, Phillip Wannamaker 3, Drew Siler 4

1 Oregon State University, College of Earth, Ocean, and Atmospheric Sciences

2 University of Oregon, Earth Sciences Department

3 University of Utah, Energy & Geoscience Institute

4 USGS, Geology, Minerals, Energy, and Geophysics Science Center

4.2 Abstract

Fault geometry and the regional stress state are key factors that dictate fault control of fluid flow. Optimally oriented faults, those oriented orthogonal to the minimum principal stress, are more likely to slip and/or dilate and provide a conduit for fluid flow via an increase in subsurface permeability. Because faulting can connect the surface to depth, increased permeability and enhanced fluid flow can localize volcanism, impact groundwater flow, and concentrate geothermal resources. This study explores slip and dilation tendency, a proxy for permeability, across the highly faulted, transtensional Cascade arc-backarc region of Oregon.

The tendency analysis combines new, high-resolution mapping of fault traces and estimates of regional stress state across the arc-backarc transition. Published estimates of regional stress show a wide range of minimum principal stress azimuths that range between 60-80° from north.

Fault orientation clusters into three regional trends - NW, N, and NE. We test the sensitivity of slip and dilation tendency on faults of normal and strike slip affinities to variations in minimum stress orientation by dividing stress into four azimuthal families: 60°, 80°, 100°, and 120°.

Results of the tendency analyses indicate that a minimum principal stress azimuth of ~ 80° best yields favorable conditions for fluid flow along all three fault clusters. All other minimum stress azimuths produce overall higher tendencies for one or two fault orientations, but not all

three. The observation of localized Quaternary volcanism along faults of all three geometries supports the inference that an 80° minimum stress direction enables slip and dilation for each of the NW, N, and NE orientations. Whereas localization of volcanism on sub-optimal fault geometries due to local stress perturbations cannot be ruled out, local perturbations are an unlikely explanation of the regional occurrence of volcanism in all three fault clusters. We conclude that faulting is a significant contributor to regional permeability and has potential for localizing exploitable geothermal resources.

4.3 Introduction

Characterizing the permeability structure within landscapes is often difficult due to insufficient critical rock property and subsurface data (Caine et al., 1996). Regional-scale assessments of permeability is prohibitively costly and difficult to quantify. However, in highly faulted landscapes, analysis of fault tendency for slip and dilation under a given stress state can provide first-order estimates of regional potential for enhanced fluid flow along faults (Moeck et al.,

2009; Morris et al., 1996; Siler et al., 2019). Faulting, among other factors, plays a critical role in the development of subsurface permeability. Fault pathways connect the surface to depth.

Whether a fault serves as a conduit or acts as a barrier to fluid flow depends on the stress state, geometry, and other rock properties (Anderson and Fairley, 2008; Caine et al., 1996; Davatzes and Hickman, 2005; Faulds et al., 2011). Whereas sub-optimally oriented faults can act as flow barriers, fault networks striking approximately orthogonal to regional horizontal stress minima promote dilation and enhanced fluid flow (Anderson and Fairley, 2008; Morris et al., 1996).

This study explores the potential coupling between faulting and fluid flow across the Oregon

Cascade volcanic arc-backarc transition (Figure 4.1). Both the arc and backarc deform in response to plate boundary forces (Humphreys and Coblentz, 2007). The structural style of deformation includes extensional and oblique-slip faults that developed in synchrony with

subduction-related arc volcanism (Ingebritsen and Mariner, 2010; King and Metcalfe, 2013;

Schmidt and Grunder, 2009; Wells et al., 1998). The resulting irregular system of northwest, north-south, and northeast trending, normal-to-transtensional faults transect both the arc and backarc landscape (Blakely et al., 1997; Crider, 2001; Donath, 1962; Pezzopane and Weldon,

1993; Priest, 1990; Trench and Meigs, 2007).

Faults and fault intersections influence fluid flow within the upper crust (Curewitz and Karson,

1997; Faulds et al., 2011; Micklethwaite and Cox, 2004). Faults serve as conduits that support transport of groundwater and other fluids (Ingebritsen and Mariner, 2010), including magma.

On a large scale, for example, Newberry Volcano in central Oregon sits astride the intersection of the Sisters and Walker Rim fault zones (Grasso et al., 2012; Mark-Moser et al., 2016; Mckay et al., 2009; Sherrod et al., 2002). Similar coincidence of smaller scale vents, fissures, and hot springs aligned along known fault traces provide further demonstration of regional tectonic control of fluid mobility (Grasso et al., 2012; Ingebritsen and Mariner, 2010; Taylor, 1990).

Despite the regional fault density and high heat flow (Blackwell et al., 1982; Ingebritsen and

Mariner, 2010; King and Metcalfe, 2013), exploration and assessment of fluid flow in the context of geothermal resources has been limited (Wannamaker et al., 2016). We assess the likelihood for fluid flow associated with faults across the arc-backarc transition using fault slip and dilation tendency as a proxy for fault-controlled permeability (Moeck et al., 2009; Morris et al., 1996). Models of slip and dilation tendency combine fault geometry with estimates of regional stress orientation and magnitudes to assess fault propensity for slip and dilation, and thus potential for fluid transmissivity. New understanding of the potential for fluid flow associated faults in with the Oregon Cascade arc-backarc transition informs the potential for future geothermal energy exploration and fault-controlled magmatism.

4.4 Geologic Setting of the Oregon Cascades

4.4.1 Subduction Arc Volcanism

The Cascade Range is a Cenozoic volcanic arc complex extending from British Columbia,

Canada to northern California, USA (Sherrod and Smith, 2000). Subduction of the young, hot

Juan de Fuca (JDF) plate beneath the North American plate (Atwater, 1970) at a rate of 40-45 mm/yr toward N60E (Pezzopane and Weldon, 1993) and active extension (Schmidt and

Grunder, 2009) drives spatially and temporally variable volcanism (du Bray and John, 2011;

Priest, 1990; Sherrod and Smith, 2000). The Western Cascade and High Cascade provinces represent the older and the active volcanic arc, respectively (du Bray and John, 2011; Sherrod and Smith, 2000; Smith et al., 1987). Rocks of the Western Cascades range in age from ~43 to

>7 Ma. Magmatism shifted to the High Cascades province by ~7 Ma (Smith et al., 1987). The eastward shift in volcanism from the Western to the High Cascades resulted from clockwise rotation of the Oregon forearc (Wells and McCaffrey, 2013; Wells et al., 1998).

The High Cascades regional stratigraphy is characterized by ~2-3 km of late Miocene to

Holocene lava flows, large, rhyolitic to basaltic stratovolcanoes, and hundreds of monogenetic cinder cones and vents that sit above older Western Cascades basement (Schmidt and Grunder,

2009; Sherrod and Smith, 2000; Smith et al., 1987; Taylor, 1981; Taylor, 1990). Alignment of cinder cones, vents, and fissures with known transtensional faults indicates that volcanism is strongly influenced by extension (Schmidt and Grunder, 2009; Sherrod and Smith, 2000;

Taylor, 1981). This localization of Quaternary volcanism on fault structures suggests that regional faults act as fluid conduits that focus magmatism.

4.4.2 Regional Tectonics

Understanding the regional tectonics, pattern of faulting, and stress state is essential to constraining fault slip and dilation tendency. The High Cascades mark the western limit of the

100

Oregon Basin and Range extensional province (Blakely et al., 1997; Pezzopane and Weldon,

1993; Sherrod and Pickthorn, 1992). The Oregon Cascade arc-backarc is thus located where extensional fault structures and subduction arc volcanism developed in synchrony. Clockwise rotation of the Oregon forearc and northward translation of the Sierra Nevada to the south

(McCaffrey et al., 2013; Wells et al., 1998) coupled with Basin and Range extension and dextral shear along the Eastern California Shear zone and Walker Lane fault systems in the backarc

(Crider, 2001; Lawrence, 1976; Pezzopane and Weldon, 1993; Waldien et al., 2019) results in deformation that varies systematically along the length of the Cascade Range (Figure 4.1A).

Transpression in the north gives way to transtension in the south (Wells et al., 1998).

Transtension is manifested by fault orientations with a trimodal distribution of north, northeast, and northwest-trending, normal-to-oblique slip faults (Blakely et al., 1997; Crider, 2001;

McCaffery et al., 2007; Pezzopane and Weldon, 1993; Scarberry et al., 2010; Trench et al.,

2012; Waldien et al., 2019). Northwest directed Basin and Range extension drives thinning of the continental lithosphere east of the Cascade arc. Propagation of extension into south-central

Oregon (McCaffery et al., 2007; Pezzopane and Weldon, 1993)(Figure 4.1A and 4.1B), south of the latitude of Mt. Jefferson, caused extension to impinge on the Cascade arc on the western edge of the Basin and Range province (Lawrence, 1976). A discontinuous series of north- trending, down-to-the-east normal faults mark the western limit of extensional deformation

(Bacon and Robinson, 2019; Keach et al., 1989; Sherrod and Pickthorn, 1992; Speth et al.,

2019). Faulting of late Pleistocene to Holocene lacustrine, glacial, alluvial and colluvial deposits as well as late Pleistocene volcanic and pyroclastic flows attest to the active deformation (Pezzopane and Weldon, 1993).

Further to the east, faults of the Basin and Range extensional province die to the north at the

Brothers fault zone (Figure 4.1) (Lawrence, 1976; Pezzopane and Weldon, 1993). A discontinuous series of N60W trending faults that extends ~300 km across Central Oregon

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defines the Brothers fault zone. Short (km-length scale) faults dominate the structurally- controlled topographic fabric and 10’s of meters of relief attest to the modest vertical slip component (Trench et al., 2012). Pleistocene volcanic deposits from Newberry Caldera unconformably overlie the NW end of the Brothers fault zone (Walker and MacLeod, 1991).

The southeastern end of the fault zone intersects the NNE-trending Steens Mountain fault zone.

The Sisters fault zone marks the northwest corner of the Basin and Range extensional province

(Figure 4.1B) (Mark-Moser, 2018). A NW-trending series of dextral, left-stepping, en echelon normal faults project southeastward from near Mt Jefferson to Newberry Crater (MacLeod et al., 1995; Mckay et al., 2009; Pezzopane and Weldon, 1993; Sherrod and Smith, 2000). In the

SE, Sisters fault zone is interpreted to extend to Newberry crater where it potentially intersects

Walker Rim fault zone to create a pull-apart basin bounding the volcano (Mark-Moser, 2018;

Pezzopane and Weldon, 1993). In the NW, the Sisters Fault Zone appears to cross the axis of the arc into the eastern edge of the West Cascades province (Pezzopane and Weldon, 1993).

4.4.3 Permeability, Fluid Transport and Heat Flow

Permeability refers to the ability of a material to transfer energy and fluids, such as water, steam, liquid organics, and even magma (Blackwell et al., 1982; Curewitz and Karson, 1997;

Davatzes and Hickman, 2005; Ingebritsen and Mariner, 2010; Siler et al., 2019). Permeability is largely controlled by material properties, interconnected porosity, and fracture network density. Increasing compaction with depth, metamorphism, and mineral precipitation into pore space decrease permeability (Saar and Manga, 2004). In active tectonic settings, fault intersections, terminations and accommodation zones such as step-overs, overlaps, relay ramps and pull-apart basins represent potential zones of high permeability (Anderson and Fairley,

2008; Curewitz and Karson, 1997; Faulds et al., 2011; Faulds et al., 2013; Micklethwaite and

Cox, 2004). Despite widespread faulting across the Cascade arc-backarc transition, little focus has been given to the impact of faulting on regional permeability, infiltration and fluid flow.

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Permeable rocks underlying the young constructional landscape of the High Cascades volcanic arc facilitate groundwater transport (Jefferson et al., 2006). Isotopic analyses of groundwater emerging from springs along the Western Cascades in the west and from the Deschutes Basin to the east indicate that fluids originate at elevation in the High Cascades (Cashman et al., 2009;

Ingebritsen et al., 1994; James et al., 2000; Jefferson et al., 2010; Jefferson et al., 2006;

Jefferson et al., 2007; Lite and Gannett, 2002). The high topography of High Cascade volcanoes creates an orographic barrier that focuses ~3.8 m of annual precipitation on the west side of the range, and <0.5 m on the east side (Jefferson et al., 2010; Taylor and Hannan, 1999). More than half of that precipitation infiltrates and recharges the groundwater system (Ingebritsen et al.,

1992). Less permeable Paleogene basement rocks confine groundwater to flow paths within the permeable Plio-Quaternary volcanic deposits of the High Cascades. High discharge cold and hot springs reflect the control on flow paths exerted by the stacked extrusive volcanic deposits (Ingebritsen et al., 1992; Ingebritsen et al., 1994; Jefferson et al., 2010; Lite and

Gannett, 2002; Mariner et al., 1990). In the Western Cascades, thermal springs along a few fault traces suggests that these structures facilitate fluid movement toward the surface

(Ingebritsen et al., 1994; Jefferson et al., 2010).

Systematic latitudinal variations in heat flow, volcanism and thermal-springs along the length of the Oregon Cascade arc-backarc are thought to reflect extensional faulting and crustal thinning (Ingebritsen and Mariner, 2010). Conductive heat flow increases from west to east and from north to south between the Washington and California borders (Blackwell et al., 1982;

Ingebritsen and Mariner, 2010). North of 45° 15’N (near the latitude of Mt. Jefferson; Figure

4.1B and 4.1C), cumulative heat discharge from sources such as thermal springs and fumaroles is low (<50 MW). South of this latitude, conductive heat discharge increases by ~1.8 MW/km arc length (>300MW) toward the SE and the cumulative measured heat discharge totals nearly

1000MW, a value observed to be steady over the last 30+ years of measurement (Ingebritsen

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and Mariner, 2010). This pattern of southward increasing heat discharge mirrors an increase in the number of Quaternary vents, fissures, and monogenetic volcanoes (Figure 4.1C) (Grasso et al., 2012; Mckay et al., 2009; Sherrod and Smith, 2000; Taylor, 1981). Thermal springs similarly increase in abundance to the south (Ingebritsen and Mariner, 2010)

4.5 Data & Methods

4.5.1 Fault Mapping

A database containing mapped fault traces was compiled for the study area from published sources and new high-resolution mapping. Published fault traces were extracted from the

Oregon Department of Geology and Mineral Industries (DOGAMI) 2009 Oregon Geologic

Compilation v5 database, from the U.S. Geologic Survey (USGS) Quaternary Fault database, and from Grasso et al. (2012). Duplicate faults and those located outside of the study area were removed. Because many of the published fault features were mapped using older 10-90m resolution imagery, 0.9 m resolution LiDAR imagery housed by the Oregon LiDAR

Consortium between 2000-2016 were used to evaluate, revise, and add new faults to the published regional fault array (Figure 4.2).

Examination of the LiDAR data revealed a collection of previously unrecognized topographic lineaments. We utilized the ArcGIS 10.7.1 platform to manually locate and examine these topographic lineaments using a series of products of the LiDAR imagery (e.g. digital elevation models (DEMs), slope, slope shade, and aspect maps, as well as aerial imagery). Lineaments showing clear and continuous relief were interpreted as faults, several of which were field verified, and added to the database. In total, the database consists of 1,089 individual faults

(Figure 4.1B). Roughly 35% of the faults in the database (356) are previously mapped faults with refined locations and the other 732 faults are newly- recognized from the LiDAR data.

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Fault trends in the Oregon backarc fall into three clusters based on their strike (NW-, N-, and

NE-trending). NW-trending lineaments represent the dominant fault set and consist of a dense series of parallel to subparallel, en echelon, individual fault segments. N-trending faults constitute the second largest fault population and are marked by shorter along-strike continuity but include a number of individual long faults characterized by large topographic relief. NE- trending faults are the least numerous and are often linked to NW-trending structures as short segments. Whereas fault trace maps well represent the spatial extent of a fault at the surface, the geometry at depth is poorly known. Published literature and field reconnaissance shows that high fault dip angles (≥ 60°) with largely normal, but also oblique-normal, slip indicators characterize the faults (Crider, 2001; Mark-Moser, 2018; Pezzopane and Weldon, 1993). Thus, for the purpose of this study, all faults are assumed to have dips of 60°.

4.5.2 Regional State of Stress

In order to assess the potential for fault slip and dilation, the orientation of the least compressive principal stress (σ3) is of critical importance. The state of stress across the Cascade region is poorly constrained. Published estimates of principal stress directions vary substantially in both orientation and spatially. Zoback and Zoback (1989) made early estimates of the stress state in central Oregon using vent alignments and a limited number of upper crustal earthquake focal mechanisms, noting that the roughly north-oriented structural trend of the arc suggests an E-W orientation to the least compressive horizontal stress (Shmin). Others have used few borehole breakouts (Davatzes and Hickman, 2011; Werner et al., 1991), earthquake focal mechanisms and T-axes (Braunmiller et al., 1995; Crider, 2001; Pezzopane and Weldon, 1993), vent and dike alignments (Crider, 2001; Werner et al., 1991), and data from the World Stress Map model

(Humphreys and Coblentz, 2007) to infer stress state in the Cascade backarc. Published azimuths for the σ3 vector vary from 25-118° and from a 0-26° plunge from horizontal. The resultant stress state ranges from normal to strike-slip fault regimes. Given that most plunge

105

values are less than 5° from horizontal, we assume σ3 ≈ Shmin. A compilation of these stress estimates and their sources are presented in Table 1.

The published estimates of the principal stresses are divided into four bins separated by 20° to yield a Shmin range of 60-120° from North (Table 2). For each Shmin bin, stress tensor geometries for both normal and strike-slip end-member fault regimes are derived, consistent with the transtensive backarc kinematics. Andersonian stress tensor geometry, where one principal stress vector is assumed vertical (Anderson, 1951). The stress tensor geometries used in our analysis enable exploration of the implications of stress field variations on slip and dilation tendency for the three fault populations.

4.5.3 Slip & Dilation Tendency

Within a fault zone, faults that are critically stressed and close to failure potentially localize fluid flow (Morris et al., 1996; Townend and Zoback, 2000; Zoback and Townend, 2001).

Whether a fault is critically stressed depends on the magnitude of principal stress resolved parallel and perpendicular to the fault surface (Figure 4.3A). The degree to which a fault is stressed can be described by slip and dilation tendency (Ferrill and Morris, 2003; Ferrill et al.,

1999; Moeck et al., 2009; Morris et al., 1996).

Slip tendency (Ts) is defined as the product of resolved shear stress and normal stress acting on a surface (Moeck et al., 2009; Morris et al., 1996), given as equation (1):

휏 푇푠 = (1) 휎푛 where shear stress (τ) is the surface-parallel force vector that acts to drive fault slip motion, and normal stress (σn) is the surface-perpendicular force vector that inhibits motion (Figure 4.3A).

When the shear stress acting on a surface is small, slip tendency is low and fault motion is

106

inhibited (Figure 4.3B). Conversely, when normal stress acting on a surface is low, the value of slip tendency is high, and the probability of slip increases. In these calculations, slip tendency depends solely on the stress tensor geometry and the orientation of the fault surface (Morris et al., 1996) and does not account for rock material properties (Siler et al., 2019).

Dilation tendency (Td) refers to a fault’s ability to dilate and remain dilated within a stress regime. It is largely used as a measure of potential for fluid transmissivity and to identify places where mode 1 fracture failure is likely to occur. Dilation tendency is the normal stress normalized by the differential stress (Ferrill and Morris, 2003; Moeck et al., 2009) and is calculated in equation (2) by:

(휎1 − 휎푛) 푇푑 = (2) (휎1 − 휎3)

where σ1 and σ3 are the maximum and minimum principal stresses, respectively, and the differential stress is the difference between the two. Combined tendencies (Tsd) can also be characterized to assess where a surface is likely to both fail and dilate using equation (3):

(휎1 − 휎푛) 휏 푇푠푑 = + (3) (휎1 − 휎3) 휎푛

which sums the Td and Ts values (Ferrill and Morris, 2003; Moeck et al., 2009). Values of Ts and Td range from 0.0 - 1.0, whereas values of Tsd range between 0.0 – 2.0. Low values represent low tendency and vice versa. Sites with large combined tendency values have the greatest potential to enhance fluid flow (Faulds et al., 2013; Siler et al., 2019). Tendency analysis as a geothermal exploration tool is best combined with data that constrain heat flow, groundwater availability and transport, substate material properties, pore fluid pressure, and depth changes in fault zone properties (Faulds et al., 2011).

107

We use the 3DStress v.4.2 software package developed at the Southwestern Research Institute

(Morris et al., 1996) to calculate Ts, Td, and Tsd for the fault database from the Oregon Cascade arc-backarc transition. 3DStress derives tendency values resolved for all possible fault or fracture orientations from a regional stress tensor geometry and magnitude and presents the values in lower hemisphere stereoplots (Ferrill et al., 1999; Moeck et al., 2009; Morris et al.,

1996). Calculated tendency values for individual structures are extracted and plotted in both a geographic reference frame and in rose diagrams binned by fault orientations and colored by tendency.

Published stress vectors for the study area indicate a 60° range in σ3 stress azimuth, have a broadly east-west orientation, and define both normal and strike-slip fault stress regimes (Table

1 & 2). Consequently, tendency calculations assume Andersonian geometries (Anderson, 1951) and explore a range of σ3 oriented at 60°, 80°, 100°, and 120° from north and with a near horizontal plunge of 10° for both normal and strike slip stress regime scenarios (Table 2).

Because the principal stress tensor magnitudes are poorly known for this region, common average relative magnitudes of σ1 = 95 Mpa, σ2 = 65 Mpa, and σ 3 = 25 Mpa (Moeck et al.,

2009; Morris et al., 1996) were assumed for the maximum, intermediate, and minimum principal stresses, respectively. We assess the sensitivity of the tendency analysis to this assumption later in Section 4.6.2. Pore fluid pressure was set to hydrostatic.

4.6 Results

4.6.1 Tendency Analysis

Slip, dilation, and combined tendency analyses were conducted for the >1000 fault traces in the database using eight different stress tensor geometries (Table 2). A normal fault regime (σ1 oriented approximately vertical) and a strike slip fault regime (σ2 oriented approximately vertical) were tested with σ3 azimuths oriented 60°, 80°, 100°, and 120° clockwise from North.

108

Table 3 summarizes the results for normal and strike slip fault regimes of models for the range of azimuth of σ3. Azimuths that optimize for high tendencies on all observed fault orientations by minimizing the range between low and high modeled tendency while maintaining at least moderate tendency values, not just maximizing for a narrow orientation range, is considered a good model fit. In general, Ts and Td model results under a normal stress regime indicate consistently moderate (0.5) to very high (1.0) tendency values with smaller min-max ranges for σ3 azimuths between 80-100 ° from north. Models of Ts and Td under a strike slip stress regime, however, yield min-max values with a wider range from very high (1.0) to very low

(0.0). Combined tendencies represent the sum of the Ts and Td models and thus yield a more value-averaged range of tendency values for the tensor geometry.

Model results show that faults acting under an 80° azimuth for σ3 yield the smallest range of min-max tendency values and overall moderate tendencies for both normal and strike slip end member stress regimes. An 80° azimuth for σ3 yields favorable slip and dilation on faults of all orientations. Other modeled azimuths yield conditions on only one or two of the three fault groups. The 80° azimuth is most frequently cited in the literature as the orientation given by other proxies (Braunmiller et al., 1995; Crider, 2001). Stereoplots, and rose histograms for both the 80° azimuth for σ3 and normal and strike slip stress regimes are shown in Figures 4.4 and

4.5, respectively. Similar plots for models of all other azimuths can be found in the supplemental materials. Maps depict fault traces colored by their respective tendencies

(locations same as Figure 4.1B). The stereoplots illustrate the distribution of all possible tendency values for any oriented plane for the given stress tensor geometry. This makes it possible to quickly assess the tendency for faults with dips outside of the assumed 60° dip used in our models. Rose histograms bin counts of individual fault trends in 5° intervals and are colored by tendency. Model results for azimuths from 60° to 120° are discussed below.

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σ3 Orientation - 60°: In the normal regime, Ts and Td values are very high (1.0) for NW oriented faults with decreasing values for faults with more northerly and easterly orientations in a clockwise direction. NE oriented faults yield the lowest tendencies (0.2 - 0.3). In contrast,

Ts and Td values are high (0.9 – 1.0) for N to NW oriented faults and very low (0.0 – 0.3) for

NE oriented faults in the strike slip regime.

σ3 Orientation - 80°: In the normal fault regime, Ts and Td values are moderate to very high

(0.7 - 1.0) for N to NW oriented faults with decreasing values for orientations in a clockwise direction. NE oriented faults yield intermediate tendencies (0.4). For the strike-slip regime, Ts and Td values are relatively high (0.7 – 0.8) for N to NW oriented faults and relatively low

(0.3) for NE oriented faults.

σ3 Orientation - 100°: Ts and Td values in the normal regime are high to very high (0.8 - 1.0) for N-oriented faults and values decrease for orientations both in the clockwise and counter- clockwise directions. NE and NW oriented faults yield intermediate tendencies (0.4). Whereas

Ts and Td values are very high (1.0) for N to NE oriented faults in the strike slip regime, they are very low (0.0 - 0.2) for ENE oriented faults.

σ3 Orientation - 120°: In the normal regime, Ts and Td values are moderate to very high (0.8 -

1.0) for NNE oriented faults with decreasing values for orientations in the counter-clockwise direction. NW oriented faults yield intermediate tendencies (0.4). The Ts and Td values for the strike slip regime are very high (1.0) for NNE oriented faults and very low (0.1 - 0.2) for NW oriented faults. Values for either fault regime resolved for the 100° and 120° azimuth are indistinguishable.

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4.6.2 Analysis of Variation in Stress Tensor Magnitudes

Because the magnitudes of the principal stresses are not well known across the Cascade arc- backarc transition in Oregon, we tested the sensitivity of slip and dilation tendency to variations in the relative vertical and horizontal stress magnitude for σ3 orientations between 0 - 180°.

Figure 4.6 shows the results of this analysis for both slip and dilation tendency where the x- axis is the orientation of σ3 (≈ θHmin), the y-axis is the ratio of minimum to maximum horizontal stresses (σHmin / σHmax), and the z-axis is the ratio of vertical stress to maximum horizontal stress (σV / σHmax). The colors represent slip and dilation tendency calculated for the mean fault orientation weighted by the tendency value representative of the longest, and therefore most significant, segment of a fault trace with an assumed dip of 60°. Warm colors represent high tendency and cool colors are low tendency.

Stress relative magnitude affects the dilation tendency more than it does slip tendency

(Equation 2). Slip tendency is highest for a relatively narrow range of fault strike azimuth

(Figure 4.6a). For the stress state appropriate for normal faulting where σV > σHmax, faults with dip directions between ~70° and 140° (strike 340° to 50° from north) and σHmin / σHmax ratios of 0.4 to 0.6 have the highest slip tendency. Dilation tendency for σV > σHmax is high for a wide range in fault strike and for a σHmin / σHmax ratio > 0.5 (Figure 4.6b). A narrower range of fault strike and σHmin / σHmax ratio yield favorable slip and dilation tendency for the relative stress magnitude expected for a strike slip regime (σV < σHmax). When the ratio of σHmin / σHmax increases, slip and dilation tendency decrease, which is expected because differential stress ought to approach zero as σHmin / σHmax approaches 1. Both slip and dilation tendency reach maximum values as fault strike approaches N-S.

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4.7 Discussion

4.7.1 Faulting, Stress & Slip Tendency

Earthquake focal mechanisms, limited borehole breakout data, volcanic vent alignment, dikes, fissures, faults, and fractures suggest that an E-W directed extensional stress regime with a poorly-constrained azimuthal range from 60 – 120° E characterizes the Cascade arc-backarc transition (Braunmiller et al., 1995; Crider, 2001; Davatzes and Hickman, 2011; Humphreys and Coblentz, 2007; Pezzopane and Weldon, 1993; Werner et al., 1991). A pervasive system of NW-, N-S-, and NE-trending normal to oblique transtensive faults developed in response to regional crustal deformation (Blakely et al., 1997; Crider, 2001; Donath, 1962; Pezzopane and

Weldon, 1993; Priest, 1990; Trench et al., 2012). The motion of western Oregon relative to

North America as constrained by the geodetic velocity field changes from NNW-oriented at the California-Oregon border (~42°N) to N oriented at Bend, OR (~44°N) (McCaffrey et al.,

2013). Thus, oblique rifting characterizes the arc-back arc transition (Pezzopane and Weldon,

1993; Crider 2001), and the degree of obliquity increases from south to north (Figure 4.1b).

Thus, oblique rifting (distributed right-oblique normal faulting) ought to transition to transtension (right-lateral shear with subordinate normal faulting) in the arc-back arc region between 42° and 45° N (McCaffrey et al., 2013). Coeval development of faults with a range of orientations is common to regions characterized by oblique rifting (Brune et al., 2018; Withjack and Jamison, 1986).

Our analyses indicate that moderate to high slip and dilation tendency is favored on faults with

N and NW trends under a normal fault stress regime for a σ3 azimuth between 80-120° (Figure

4.4). Faults with a NE trend have the lowest tendency in this stress regime. Slip and dilation tendency resolved for a strike-slip stress regime where σ3 has an azimuth of approximately 80° produces moderate tendency (Figure 4.5). In this model, low slip and dilation tendency characterizes NE-trending faults and tendency values on NW to NNW trending faults are

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relatively larger, but smaller than in the case of an extensional stress regime. For all other modeled stress orientations in a strike-slip regime, NE-trending faults yield negligible slip and dilation tendency.

4.7.2 Tendency, Permeability & the Localization of Magmatism on Faults

Fault zones play a critical role in the development of regional permeability in tectonic landscapes (Anderson et al. 2008; Cain et al., 1996; Coolbaugh et al., 2002; Curewitz and

Karson, 1997; Davatzes and Hickman, 2005; Faulds et al., 2011; Moeck et al., 2009; Siler et al., 2018). Proxies for permeability include fault state of stress, the location of structural discontinuities, hot and cold springs, and host rock lithologic characteristics (Curewitz and

Karson, 1997; Faulds et al., 2011; Siler et al., 2019). Faults that are critically stressed are often conducive to fluid flow (Ferrill et al., 2020; Ito and Zoback, 2000). Slip and dilation tendency are products of the stresses acting on faults and reflect the potential of a fault to act as a fluid conduit (Morris et al., 1996; Siler et al., 2019; Townend and Zoback, 2000; Zoback and

Townend, 2001). Thus, faults characterized by moderate to high values of slip and dilation tendency can be reasonably interpreted as locations with potential for enhanced permeability.

In the Oregon Cascade backarc, fault-controlled fluid flow is indicated by the coincidence of

Quaternary volcanic monogenetic cones, flows, and fissures on faults of all orientations

(Section 5.2; Figure 4.7) (Grasso et al., 2012; MacLeod et al., 1995; Mckay et al., 2009). This observation suggests that enhanced slip and dilation tendency on N-S- and NNW-trending faults played a role in the localization of magmatic fluid flow on faults. The association of fissures and monogenetic cones with NE-trending structures (e.g. SW of Newberry – Figure

4.7D; Grasso et al., 2012), however, is not simply explained. Relatively low slip and dilation tendency values are resolved for structures with NE orientations for all but a narrow range of azimuths of the normal fault stress regime (Figure 4.4). The values are smaller to negligible for

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the strike-slip stress regime (Figure 4.5). Several potential alternative interpretations account for magmatism localized on less favorably oriented faults. One possibility is slip and dilation tendency have a minimum threshold value for fluid flow. Alternatively, a local perturbation to the stress field associated with magmatic fluid pressure (Becerril et al., 2013) or point-load sources related to the growth of volcanic edifices (e.g. shield volcanoes, stratovolcanoes, and monogenetic cinder cones) potentially causes a reorientation of the stress field in the region affected by the load (Blakely et al., 1997; Fiske and Jackson, 1972). Moreover, pore fluid pressure (Saar and Manga, 2004) and pre-existing faults imply that effective stress and/or slip on sub-optimally oriented structures facilitated fluid flow on the NE trending faults.

Structural discontinuities, including fault intersections, terminations, and accommodation zones, are favorable structural settings for high permeability (Anderson and Fairley, 2008;

Curewitz and Karson, 1997; Faulds et al., 2011; Faulds et al., 2013; Micklethwaite and Cox,

2004; Siler et al., 2019). Regions around structural discontinuities and other complexities facilitate upwelling of fluids from depth. Hotsprings concentrated near fault tips, for example

(Curewitz and Karson, 1997) or magmatism are often co-located with fault intersections or discrete fault step-overs relative to laterally continuous fault reaches (Faulds et al., 2011).

Across the Oregon arc-backarc, evidence exists that the link between structural discontinuities and the localization of magmatism occurs on a range of scales. Several large volcanic centers are located at the termination of long, dense fault zones, such as Crater Lake at the northern end of the west Klamath fault zone (Figure 4.7E). Mt. Jefferson lies at the northern end of the

Sisters fault zone (Figure 4.7B). Newberry Volcano is located at the intersection of the NE trending Walker Rim and NW-trending Sisters fault zone (Figure 4.7D). An excellent example of Holocene volcanism is revealed by the co-location of monogenetic volcanoes and faults along the margin of the Chemult Graben to the west of Walker Rim (Figure 4.7D). Late-stage

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magmatism at North Sister, one of the Cascade arc stratovolcanoes, localized along N-trending faults that parallel regional normal faults (Schmidt and Grunder, 2009).

4.7.3 Implications for Geothermal Energy Exploration in the Oregon Cascade Backarc

Modern demands for energy production and a rapidly growing and urbanized global population are placing a heavy burden on the already limited supply capacity of natural energy resources.

Coupled with the effects of a changing climate, exploration for new, cost-effective and renewable sources of energy is needed to mitigate the growing pressure for ‘green’ energy sources and to curb greenhouse gas emissions. Geothermal energy represents one promising alternative to sustainably addressing increasing consumer energy needs. Approximately 75% of geothermal power production worldwide occurs in subduction arc volcanic settings, yet only

~10% of these arc volcanic centers are being actively utilized for power generation (Stelling et al., 2016), which suggests potential for future growth.

The continuous, uninterrupted supply of naturally heated, geothermal fluids extracted from depth is an attractive alternative to traditional energy sources. However, evaluation of site- specific characteristics necessary for commercial grade geothermal energy production requires costly, invasive techniques to assess production potential, inhibiting widespread exploration.

Clearly a need exists for low-cost, non-invasive early exploration. Sites with intersecting faults and structural complexities targeted for further investigation must meet three key preconditions

(Siler et al., 2018): (1) high heat flow; (2) ample groundwater supply; (3) extensive substrate permeability (Curewitz and Karson, 1997; Jolie et al., 2012; Stelling et al., 2016).

Consequently, the Oregon Cascades backarc is well poised for geothermal exploration.

The arc-backarc transition reflects processes associated with Juan de Fuca plate subduction arc volcanism in the North American plate and Basin and Range extensional deformation. Both tectonic provinces are characterized by extensive Quaternary volcanism (Figure 4.7a; i.e. vents,

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cones, fissures, and flows) (Grasso et al., 2012; McKay et al., 2009; Sherrod and Smith, 1990;

Taylor, 1981). High conductive heat flow and hydrothermal discharge rates characterize the region from the Sisters fault zone to the south and east (Blackwell et al., 1982; Ingebritsen and

Mariner, 2010; King and Metcalfe, 2013; Mariner et al., 1990). High heat flow and the existence of numerous hotsprings along a ~200 km length of the Western Cascade-High

Cascade boundary and gravity anomalies suggest close proximity to shallow magma bodies or other heat source at depth (Blackwell et al., 1982; Keach et al., 1989).

As has been shown successfully in other parts of the Basin and Range extensional province

(Siler and Pepin, 2021), slip and dilation tendency provides an inexpensive, first order assessment of permeability potential using only mapped fault traces and estimates of stress state (Ferrill et al., 2020). Thus, the combination of the three criteria represents a potential prospecting tool in geothermal exploration (Wannamaker et al., 2016). Examples of potentially favorable localities around Mt. Jefferson near Breitenbush and Austin hotsprings (northern study area), around the northern portion of Newberry Volcano (central study area), and the area spanning from eastern Walker Rim to northern Klamath Falls (southern study area).

4.8 Conclusions

The geologic setting of the Oregon Cascade backarc is uniquely suited for geothermal resource exploration due to its high heat flow (>300 MW in the southern arc-backarc) (Blackwell et al.,

1982; Ingebritsen and Mariner, 2010; King and Metcalfe, 2013), ample fluid infiltration

(Jefferson et al., 2010; Saar and Manga, 2004), and dense network of intersecting, extensional faults deforming in synchrony with subduction-related arc volcanism (Ingebritsen and Mariner,

2010; King and Metcalfe, 2013; Schmidt and Grunder, 2009; Wells et al., 1998). We conducted a regional assessment of fault slip and dilation tendency across the arc-backarc transition to assess potential for the development of fault-related permeability. Regional stress orientation

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is uncertain, and we therefore tested a range of stress orientations to calculate the magnitude of slip and dilation on the N, NNW, and NE strike of the three regional fault systems (Moeck et al., 2009; Morris et al., 1996).

A range of possible stresses with minimum principal stress (σ3) orientations between 60° - 120° from north were tested for normal and strike slip stress regimes. Only a σ3 orientation of ~80° yielded moderate to high tendency values for all N, NNW, and NE fault orientations across the study area; a property reflected in observed field inferences of elevated fluid flow such as the localization of magmatism on all structure orientations. Structures oriented toward the north and NW yield particularly high values of tendency, indicating potentially high permeability.

Combined with published estimates of heat flow, fluid infiltration, and hydrothermal discharge, the variability of slip and dilation tendency at fault intersections and other structural complexities represents an opportunity to identify new potential sites for geothermal exploration across the region.

4.9 Acknowledgements

Support for this work comes from the U.S. Department of Energy (DOE) under Contract DE-

EEO006727. Any opinions, findings, conclusions, or recommendations expressed in this document are those of the authors and do not necessarily reflect the views of the DOE. Dr.

Alan Morris at the Southwest Research Institute is thanked for his generous assistance with the

3DStress software. Thanks to DOGAMI and the Oregon LiDAR Consortium for making the high-resolution imagery used in our analysis available free of cost to the public.

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Taylor, E.M., 1990. Volcanic history and tectonic development of the central High Cascade Range, Oregon. Journal of Geophysical Research: Solid Earth, 95(B12): 19611-19622.

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Waldien, T.S., Meigs, A.J. and Madin, I.P., 2019. Active dextral strike-slip faulting records termination of the Walker Lane belt at the southern Cascade arc in the Klamath graben, Oregon, USA. Geosphere, 15(3): 882-900.

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. .

Holocene Holocene

–

thrust faults. Black

–

location location of Figure 1B and 1C.

–

convergence convergence rate of the Juan de Fuca (JDF)

–

. . Generalized tectonic province map of the Pacific

strike slip strike fault. slip box Dashed

–

vents, cinder cones, and volcanic edifices (O’ edifices volcanic cones, and cinder vents, Ramsey et al., 2020; Hara

–

sided sided arrow

faults from Figure 1B. Figure from faults

–

the backarc the study backarc adapted area from Ingebritsen and Mariner (2010). Grey polygons

study study area. Major features labeled in brown. Cities labeled in red and designated by red stars. Black

rth rth American (NAM) plate (Pezzopane and Weldon, 1993). Black barbed lines

Wells Wells et al. (1998). Provinces shown as colored regions with kinematics labeled in brackets. Colored

. Heat flow across

normal Black line normal faults. double with

Geologic Geologic context of the Oregon Cascade backarc study area. C

–

compilation of compilation mapped fault trends from DOGAMI, USGS, Grasso et al. (2012), and new 1m resolution LiDAR mapping

–

Figure Northwest adapted from arrows correspond to motions generalized of tectonic provinces. Large black arrow plate with respect to the No ticked lines Location map of the backarc lines (this study). dots grey Dark volcanism (DOGAMI). and Quaternary lines grey Pale 2017). and Siebert,

124

125

Figure 4.2: Comparison of 10m vs. 1m resolution digital elevation models for fault mapping.

Black arrows indicate the same location along a fault scarp in both images (imagery from the

Oregon LiDAR Consortium collection held at Oregon State University).

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Table 1. Compilation of Stress State Data for the Study Area and Neighboring Regions

Normal Regime Strike-Slip Regime Author Location Data Source (σ1,σ2,σ3) (T/P) (σ1,σ2,σ3) (T/P) Klamath Falls, Braunmiller et al., 1995 Focal Mechanism Solutions 090/00 060/00 Southern OR Summer Lake, S. Focal Mechanism Crider et al., 2001 241/62 349/10 084/26 Central OR Inversion

Cinder Cone Alignments 244/00

Earthquake T-Axes 263/00

Dike Alignments 256/00

Newberry Crater, Davatzes et al., 2007 Borehole Breakouts 092/00 Oregon West Coast United Humphrey et al., 2007 World Stress Map 028/00 118/00 States

Northern and Focal Mechanism Pezzopane et al., 1993 281/71 044/10 137/15 012/20 206/70 104/05 Southern OR Inversion

Geologic Moment Tensor 231/86 115/02 025/03

Borehole Breakouts, Vent Central WA & NNW to Werner et al., 1991 Alignments, Focal Central OR N-S Mechanisms σ1 - maximum principle stress, σ2 - intermediate principle stress, σ3 - minimum principle stress. Principle stress values shown as azimuthal trend (degrees from north) and plunge (degrees from trend )

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Table 4.2. Stress Tensor Geometries

Principle Normal Faulting Strike-Slip Faulting

Deg. Stress Trend Plunge Trend Plunge

σ1 240 80 150 00

σ2 330 00 240 80 060 σ3 060 10 060 10

σ1 260 80 170 00

σ2 350 00 260 80 080 σ3 080 10 080 10

σ1 280 80 190 00

σ2 010 00 280 80 100 σ3 100 10 100 10

σ1 300 80 210 00

σ2 030 00 300 80 120 σ3 120 10 120 10

Stress tensor geometries used in individual tendency analyses. Deg. is the azimuthal trend of the minimum principle stress for each trial. Maximum principle stress - σ1, Intermediate principle stress - σ2, minimum principle stress - σ3. Trend is measured in degrees from north. Plunge is measured in degrees from horizontal.

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Figure 4.3: Visualization of stress state and slip tendency. A. (Left) Example of principal stresses acting on a body of rock under an Andersonian-style normal faulting regime. σ1 - maximum principal stress. σ2 - intermediate principal stress. σ3 - minimum principal stress.

(Right) Normal (σn) and shear (τ) stress vectors resolved on the fault plane under the given stress state. B. Hypothetical values of slip tendency (Ts) for four surfaces oriented within a fixed 3D stress space (borrowed from the 3DStress Software Manual). Red – High Ts. Green

– Moderate Ts. Blue – Low Ts.

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Stereoplot of of all Stereoplot

–

cy. 60° dip assumed for all faults. Bottom left all cy. Bottom for left faults.60° dip assumed

) tendency for a minimum principal stress stress (σ3) ) tendency a minimum for of principal 80° direction in

low tenden low

–

Map Map of faults in the Oregon Cascade back arc (Figure 1B) colored by plane

–

rose rose histogram showing summed fault trace trends binned in 5° internals and colored by

–

high tendency. Blue tendency. high

–

like like stress regime. Top

: A. : Slip (Tsd (Ts), A. B. (Td), dilation and C. combined

Figure Andersonian normal fault values. Red tendency specific possible tendencies. Bottom right values. tendency

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131

Stereoplot of of all Stereoplot

–

low tendency. 60° dip assumed for all faults. Bottom left all Bottom for left faults.60° dip tendency. assumed low

Map of faults in the Oregon Cascade back arc (Figure 1B) colored by plane by colored 1B) (Figure arc back Cascade the Oregon in Map of faults

–

rose rose histogram showing summed fault trace trends binned in 5° internals and colored by

–

high tendency. Blue tendency. high

–

like stress regime. Top Top regime. stress like

dilation (Td), and C. combined (Tsd) tendency for a minimum principal stress stress (σ3) tendency (Tsd) a minimum for (Td), of principal dilation 80° direction and C. in combined -

. Red

: A. : Slip (Ts), A. B.

Figure fault slip strike Andersonian values tendency specific possible tendencies. Bottom right values. tendency

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Dir.

ENE

NNE

Tsd

0.5

1.7

0.2

1.7

0.6

1.4

0.3

1.7

1.0 for Ts and Ts for 1.0

–

(0.0 -2.0) (0.0

Dir.

ENE

NNE

NNW

0.1

1.0

0.0

1.0

0.3

0.8

0.0

1.0

alues alues of low tendency. Warm

(0.0 -1.0) (0.0

StrikeSlip Regime

Dir.

ENE

NNE

0.2

1.0

0.2

1.0

0.3

0.7

0.3

0.9

(0.0 -1.0) (0.0

Dir.

NNE

NW/NE

Tsd

0.8

1.7

0.8

1.7

0.8

1.7

0.5

2.0

(0.0 -2.0) (0.0

Dir.

NNE

NNW

NW/NE

0.4

0.8

0.4

0.8

0.4

0.7

0.3

1.0

NormalRegime

(0.0 -1.0) (0.0

Dir.

NNE

NW/NE

0.4

1.0

0.4

1.0

0.4

1.0

0.2

1.0

(0.0 -1.0) (0.0

Min Max Min Max Min Max Min Max

Range

2.0 for Tsd. Tsd. for 2.0

–

σ3 σ3

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100

3. 3. Summary of tendency analysis results for all tested orientations of σ3 under both normal and strike slip stress regime

Azimuth

Table Table endmembers.

Values reflect the maximum and minimum calculated tendency for a given stress configuration with a range of 0.0 range a with configuration a given for stress tendency calculated minimum and the maximum reflect Values Td and 0.0 Colors represent the tendency value and match those defined in Figures 4 and 5. Cool colors are v tendency. of values high are colors values. tendency maximum or minimum the given with associated trend the fault Dir. denotes

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Figure 4.6: Plot of slip (left) and dilation (right) tendency with respect to variations in stress magnitudes for different orientation of σ3 (≈ θHmin). Tendency values derived from the mean of fault orientations weighted by the value of the longest segments of a fault trace. σV – vertical stress. σHmax – maximum horizontal stress. σHmin – minimum horizontal stress.

135

t t

over at Newberry Volcano. at Newberry over

t. Jefferson and Crater Lake, respectively. B. Example of fluid flow at fault intersections. C. Fluid flow

tion of permeability in the Oregon Cascade backarc. (Left) Map of Slipσ3 for (Left) = backarc. 80° Tendency superimposed tionCascade the Oregon inof permeability

7. Interpreta

gure gure

upon Quaternary volcanics (Walker and McLeod, 1991) and vent locations (OHara et al., 2020). A & C. Fluid flow concentrated a concentrated flow Fluid C. & A al.,2020). et (OHara locations vent and 1991) McLeod, and (Walker volcanics Quaternary upon fault terminations at M step at a fault concentration Fi

136

137

APPENDIX C.

SUPPLEMENTAL MATERIALS

138

Stereoplot Stereoplot of all

–

low low tendency. 60° dip assumed for all faults. Bottom

Map of faults in the Oregon Cascade back arc (Figure (Figure arc by 1B) back plane colored Cascade Map the of faults Oregon in

–

high high tendency. Blue

–

like stress regime. Top like stress regime.

Slip (Ts), B. dilation (Td), and C. combined (Tsd) tendency for a minimum principal stress (σ3) direction of 60° in 60° of direction stress (σ3) principal minimum a for tendency (Tsd) combined C. (Td), and B. dilation Slip (Ts),

: A. : A. dency values. Red

4.1S

Figure normal fault an Andersonian specific ten tendencies. possible

139

140

Stereoplot Stereoplot of

–

stress (σ3) direction of 60° in 60° of direction stress (σ3)

l faults. Bottom

low low 60° tendency. dip for assumed al

–

Map Map of faults in the Oregon Cascade back arc (Figure 1B) colored by

–

high high Blue tendency.

–

like like stress regime. Top

fault

slip

strike

: A. Slip (Ts), B. dilation (Td), and C. combined (Tsd) tendency for a minimum principal minimum a for tendency (Tsd) combined C. (Td), and B. dilation Slip (Ts), : A.

4.2S

Figure Figure an Andersonian plane specific tendency values. Red all tendencies. possible

141

142

0° 0°

Stereoplot Stereoplot of

–

0° 0° dip for assumed all faults. Bottom

ndency. ndency.

) ) tendency for a minimum stress principal (σ3) direction of

low low te

–

Map Map of faults in the Oregon Cascade back arc (Figure 1B) colored by

–

high high Blue tendency.

–

like like stress regime. Top

: : Slip A. (Ts), B. (Td), dilation and C. combined (Tsd

4.3S

Figure Figure in an Andersonian normal fault plane specific tendency values. Red all tendencies. possible

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144

0° 0°

Stereoplot Stereoplot of

–

0° 0° dip for assumed all faults. Bottom

) ) tendency for a minimum stress principal (σ3) direction of

ow ow tendency.

Map Map of faults in Cascade back the Oregon arc 1B) (Figure by colored

–

high high Blue tendency.

–

like like stress regime. Top

fault

slip

strike

: : Slip A. (Ts), B. (Td), dilation and C. combined (Tsd

4.4S

Figure Figure in an Andersonian plane specific tendency values. Red all tendencies. possible

145

146

0° 0°

Stereoplot Stereoplot of

–

ndency. ndency. 60° dip for assumed all faults. Bottom

) ) tendency for a minimum stress principal (σ3) direction of

low low te

–

Map Map of faults in the Oregon Cascade back arc (Figure 1B) colored by

–

high high Blue tendency.

–

like like stress regime. Top

: : Slip A. (Ts), B. (Td), dilation and C. combined (Tsd

4.5S

Figure Figure in an Andersonian normal fault plane specific tendency values. Red all tendencies. possible

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148

0° 0°

Stereoplot Stereoplot of

–

) ) tendency for a minimum stress principal (σ3) direction of

ow ow 60° tendency. dip for assumed all faults. Bottom

Map Map of faults in Cascade back the Oregon arc 1B) (Figure by colored

–

high high Blue tendency.

–

like like stress regime. Top

fault

slip

strike

: : Slip A. (Ts), B. (Td), dilation and C. combined (Tsd

4.6S

Figure Figure in an Andersonian plane specific tendency values. Red all tendencies. possible

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150

CHAPTER 5:

CONCLUSIONS

151

The purpose of this dissertation is to explore the role of deformation processes in the development of the NW Himalaya and Oregon Cascade Cenozoic mountain belts. The work addresses how orogens evolve and how that growth can potentially impact resource development, hazard mitigation, and other issues tied to structural properties. Motivating questions include:

(1) How does deformation in mountain ranges develop in space and time?

(2) How does the spatial and temporal evolution relate to drivers such as global

climate, regional stress, and other variables?

(3) Can characteristics of deformed orogens be used as a prospecting tool for earth

resources?

(4) What are the implications of patterns, styles, and rates of faulting for the associated

hazards?

In Chapter 2, I combined a new suite of detrital apatite (U-Th)/He (AHe) data with stratigraphic ages and a well-constrained balanced cross-section to investigate the timing, style, and rate of shortening across the NW Himalayan foreland. Timing of accretion and subsequent deformation of the sub-Himalaya has implications for the role of the Plio-Quaternary climate transition in the growth of the Himalaya over the last ~5 Ma. Modeled AHe samples collected from every thrust sheet hanging wall and fold core across the widely exposed sub-Himalaya in the Kangra reentrant constrain the timing of fault related exhumation on individual structures.

AHe ages were evaluated in the context of widely accepted critical wedge material flux models of orogenic evolution to address the spatial and temporal evolution of the deformed foreland in the face of Plio-Quaternary climate change.

Accretion of the sub-Himalaya to the orogenic wedge occurred by 4 Ma in the Kangra region.

Distributed deformation as revealed by structure-related exhumation initiated by 2.0-2.5 Ma on internal structures within the accreted foreland basin material. Shortening rates resolved for

152

individual structures show that an increasing amount of shortening is accommodated in the sub-Himalaya following frontal accretion. Shortening rates across the Kangra sub-Himalaya total ~8-10 mm/yr since ~2 Ma. This observation suggests that although an increasing amount of shortening is focused in the foreland with time, shortening on hinterland structures are required to balance the 11-14 mm/yr convergence rate measured geodetically. These results also indicate that frontal accretion preceded the Plio-Quaternary climate transition, a period inferred to correspond with an increase in erosional efficiency. We interpret that the NW

Himalaya had a sub-critical wedge taper prior to the climate transition and that observed distributed deformation is a wedge rebuilding response to widening by frontal accretion.

Enhanced erosion associated with the shift in global climate likely acted to sustain distributed deformation resulting from foreland accretion.

Chapter 3 expands upon the work of Chapter 2 to explore the spatial and temporal evolution of the NW sub-Himalaya regionally. The NW sub-Himalaya is characterized by along-strike changes in width and structural style of the deformed foreland. This chapter uses new and published geologic mapping, balanced cross-sections, stratigraphic data, and AHe ages to explore the 3-Dimensional timing and structural evolution of the NW Himalayan deformed foreland. In addition to addressing questions (1) and (2) above, this analysis has implications for hazard assessment (4).

Distributed deformation following frontal accretion characterizes the entire NW sub-Himalaya.

Modeled AHe data from along the length of the deformation front show a near-synchronous onset of structurally controlled exhumation by 4 Ma along more than 300 km of the NW sub-

Himalaya. In detail, the structural style varies from southeast to northwest. The eastern region includes an emergent frontal thrust, called the Main Frontal Thrust (MFT), and a series of widely spaced thrust sheets. In contrast, the western region is narrow and marked by a blind deformation front - the regionally extensive, fault-cored Surin-Mastgarh Anticline (SMA). The

153

SMA is located ~40 km further north relative to the position of the MFT. The transition from the wide and widely spaced emergent thrusts of the east and the narrow, blind thrusted region of the west occurs in a narrow zone between the two regions. This narrow zone serves to transfer shortening between structures in the eastern and western regions. Balanced sections indicate that a minimum of 22-24 km of shortening are accommodated on deformed foreland structures regardless of regional variations in structural configuration. A cumulative shortening rate of ~5.5 – 9.6 mm/yr since 2.5 Ma is estimated for the NW sub-Himalaya. Similar timing of deformation front initiation (5.2-4.0 Ma) and total shortening (23 km) are reported further to the NW along the southern edge of the Potwar Plateau in Pakistan. We interpret that distributed deformation is a fundamental mechanism of orogenic evolution. The observation of young activity on the deformation front and internal structures indicates that both the fault at the deformation front and internal faults are possible sources of earthquakes. High shortening rates, such as along the Jawalamukhi and Barsoli thrust systems in the central NW sub-

Himalaya, are likely to have the shortest earthquake repeat times.

Finally, in Chapter 4, I combine published estimates of regional stress state and high-resolution mapping of fault traces to explore the potential coupling between faulting and fluid flow in the highly deformed Oregon Cascade arc-backarc region. Faulting plays a critical role in the development of subsurface permeability. Whether a fault serves as a conduit or acts as a barrier to fluid flow depends on the regional stress state and fault geometries, which has implications for geothermal potential in the Oregon Cascade backarc. Analyses of fault tendency for slip and dilation under a given stress state provide first-order estimates of potential for enhanced fluid flow in understudied landscapes. This study addresses questions (1) and (3) above.

Faults in the backarc cluster around NNW, NNE, and NE orientations. Co-location of volcanic features (fissures, vent alignment, and edifaces) and faults of all three orientations suggest that fluid flow is associated with faulting. A minimum principal stress oriented ~80° yields

154

favorable fault dilation and tendency analyses for all three fault orientations. Whereas other published stress orientations yield higher overall dilation and slip tendency for at least 1-2 of the fault orientations, only the 80° minimum stress orientation is consistent with fluid flow for all fault orientations. Local stress perturbations could cause magmatism on faults of less favorable geometries. We find that key structural configurations correlate with localized magmatism on a range of scales across the Oregon Cascade arc-backarc transition. Regionally, stratovolcanoes and volcanic craters are associated with fault tips and major fault intersections.

Monogenic cinder cones frequently align along individual fault traces. Thermal springs locally concentrate at fault exposures. The fact that faulting and fluid are coupled across the Oregon

Cascade arc-backarc indicates that faults play an important role for regional permeability, which has implications for both future volcanism and the exploration for geothermal resources.