by
Trevor Mearce
B.Sc., Humboldt State University, 2015
A Thesis Submitted in Partial Fulfillment
of the Requirements for the Degree of
MASTER OF SCIENCE
in the School of Earth and Ocean Sciences
Trevor Mearce, 2017
University of Victoria
All rights reserved. This thesis may not be reproduced in whole or in part, by photocopy or other
means, without the permission of the author.
landscape morphology, erosion rates, and river profiles. by
Trevor Mearce
B.Sc., Humboldt State University, 2015
Supervisory Committee
Dr. Kristin Morell, (School of Earth and Ocean Sciences)
Supervisor
Dr. Lucinda Leonard, (School of Earth and Ocean Sciences)
Departmental Member
Dr. Tom Gleeson (Department of Mechanical Engineering and SEOS)
Geodetic models suggest that much of the convergence across the Himalaya (~20 mm yr-1) is
taken up on the Main Himalayan Thrust, the main decollement beneath the Himalayan orogenic wedge. In Central Nepal and the majority of Northwest India, several geomorphic, geophysical and seismological datasets indicate that this decollement has a mid-crustal ramp that continues uninterrupted for hundreds of kilometers along strike from Nepal in the east to Uttarakhand in the west. In this study, I use spatial analyses of elevation, relief, channel steepness indices, and basin-wide erosion rates from cosmogenic
10Be concentrations to outline a potential large-scale change in the active fault configuration between the
Main Himalayan Thrust and Main Boundary Thrust near longitude 77°E in the Northwestern Indian Himalaya. The physiography in the areas to the east of 77ºE appears similar to that observed along much of the Himalaya where topographic relief, erosion rates, and river channel steepness (ksn <200) remain
relatively low in the areas to the south of a line known as the Physiographic Transition2. North of the
Physiographic Transition2, these metrics increase sharply within a 30-km zone due to higher rock uplift
rates above a mid-crustal ramp on the decollement or an unidentified out-of-sequence thrust fault that soles to the decollement. Either of these models are perceivable with a duplex growing by underplating of the Indian plate into the Himalayan orogenic wedge contributing to higher rock uplift rates north of the Physiographic Transition2. To the west of 77ºE, however, the landscape morphology indicates the Main
Boundary Thrust makes a northward bend coinciding with the along-strike termination of the
Physiographic Transition2 and an arc-perpendicular Bouguer gravity anomaly reflecting a trough on the
Indian plate nearlongitude 77°E. These data suggest that the Main Boundary Thrust merges along strike with the ramp or with an emergent fault soling into the Main Himalayan Thrust at this location,
Supervisory Committee ... ii
Abstract ... iii
Table of contents ... iv
List of Tables (Appendix) ... vi
List of Figures ... vii
Acknowledgments ... viii
1. Introduction ... 1
2. Background ... 2
2.1. Tectonostratigraphy and bedrock geology of the northwest Himalaya... 2
2.1.1. Uttarakhand ... 3
2.1.2. Western Himachal Pradesh ... 5
2.2. Along-strike changes in Neotectonic Structures ... 6
2.2.1. Uttarakhand ... 6
2.2.2. Western Himachal Pradesh ... 7
3. Methods ... 8
3.1. Topographic Analyses ... 9
3.2. Normalized Channel Steepness ... 10
3.2.1. Slope-Area Method... 11
3.2.2. Integral Method ... 12
4.1. Along-strike Variations ... 15
4.2. Basin-averaged
10Be Erosion Rate Analyses ... 19
4.3. Predicted Basin-averaged Erosion Rates... 23
4.4. Comparison of Channel steepness and erosion rates to the distribution of precipitation .. 24
5. Discussion ... 26
5.1. Geomorphology ... 26
5.2. Implications for active tectonic configuration ... 28
5.2.1. East of 77°E: Mid-crustal Ramp and Duplex model ... 29
5.2.2. East of 77°E: Out-of-sequence, emergent fault model ... 29
5.2.3. Tectonic configuration west of 77°E ... 30
5.2.4. Controlling factors on the change in tectonic configuration in Himachal Pradesh ... 32
5.2.5. Implications for tectonic segmentation... 36
5.2.6. Orographic effect on the Dhauladar Range – Response of erosion rates and channel
steepness to precipitation ... 37
6. Conclusions ... 38
Bibliography ... 41
Table 1: New Basin-averaged
10Be Erosion Rates for Himachal Pradesh ... 52
Table 2: Basin-averaged Landscape Metrics for Himachal Pradesh ... 53
List of Tables (Appendix)
Figure 1: Regional setting and geologic map of the northwestern Himalaya ... 4
Figure 2: Plan-v iew maps of elevation and relief ... 16
Figure 3: Topographic swath profiles ... 17
Figure 4: Plan-view maps and profiles of normalized channel steepness ... 18
Figure 5: Plan-view maps and profiles of basin-averaged erosion rates ... 21
Figure 6: Scaling relationships between relief and channel steepness, and
10Be erosion rates .... 22
Figure 7: Plan-view map showing annual precipitation for the northwest Himalaya... 25
Figure 8: Tectonic models for the northwest Himalaya... 31
I want to thank Dr. Kristin Morell for supervising this project, Dr. Lucinda Leonard and
Dr. Tom Gleeson for helping as committee members, and Dr. Lindsay Schoenbohm for acting as
external examiner. Dr. Talat Ahmad and Dr. Jwellys Samom were extremely helpful during field
work and provided additional support during the project. Also, thanks to Dr. Alexandru Codilean
and Dr. Reka Fulop at the University of Wollongong in Australia for their essential work in
calculating
10Be erosion rates and Dr. David Fink at the Australian Nuclear Science and
Technology Organisation (ANSTO) for measuring
10Be concentrations in the collected river
sands. Many thanks to the students, staff and faculty throughout the School of Earth and Ocean
Science at the University of Victoria for the outstanding support through the years. Finally, to the
love of my life, Caitlin Mearce, for her unparalleled emotional support and sacrifices taken for
me to be successful. The work presented here was supported by the National Sciences and
Engineering Research Council of Canada grant (K. Morell).
1. Introduction
A growing body of evidence suggests that along-strike changes in the geometry of seismogenic faults can influence a variety of processes including rupture behavior (Wells and Coppersmith, 1994; Wesnousky, 2006, 2008), and fault segmentation (Kiser and Ishii, 2011; Molnar and Pandey, 1989; Wesnousky, 1988, 2006), both of which are important controls on earthquake magnitude (Anderson et al., 1996; Klinger, 2010). For example, the geometry, rheology and frictional properties of the Andean subduction zone thrust have been shown to control lateral variations in the degree of plate interface coupling, which can influence seismic rupture propagation (Saillard et al., 2017). In Costa Rica, along-strike variations in the frictional properties and geometry of the megathrust due to subducting seamounts likewise correlate with regions of interseismic creep or locking that tend to respectively inhibit or promote rupture propagation during large earthquakes (Bilek et al., 2003; Ikari et al., 2013; Kanamori, 1986; Kyriakopoulos et al., 2015; Kyriakopoulos and Newman, 2016; Marone, 1998; Saillard et al., 2017; Yang et al., 2013). In particular, geodetic data show that the rupture associated with the 2012 Mw 7.6 Nicoya earthquake terminated along strike due to the presence of the Fisher seamount chain, a topographic rise that increases the degree of locking on the subducting Cocos plate interface (Kyriakopoulos and Newman, 2016).
In the Himalaya, the rupture extent of large earthquakes appears to be partially controlled by the along-strike geometry of the Main Himalayan thrust (Main Himalayan Thrust; Fig. 1), the decollement beneath the Himalayan orogenic wedge (Elliott et al., 2016; Hubbard et al., 2016; Qiu et al., 2016). Seismic reflection data and thermochronological data from the Indian plate highlight ridges and troughs in the Ganga basin (Bollinger et al., 2004; Denolle et al., 2015; Grandin et al., 2015), which have modified the local Main Himalayan Thrust geometry and correlate to the eastern and western rupture extents of the 25 April 2015 Mw 7.6 Gorhka earthquake in Nepal (Avouac et al., 2015; Bilham, 2015; Hayes et al.,
2015; Kobayashi et al., 2015; Parameswaran et al., 2015). Moreover, geophysical, geological and topographic data suggest that steeply dipping ramps surrounding the rupture patch of the Nepal
earthquake effectively blocked further propagation along the Main Himalayan Thrust (Hubbard et al., 2016; Qiu et al., 2016). Thus, identifying changes in subsurface fault geometries is important for understanding earthquake rupture behavior and fault segmentation along the Himalayan orogen.
Here I use digital topography, river channel steepness, and basin-wide erosion rate data from cosmogenic nuclides in quartz to investigate potential along-strike changes in active fault geometry in the northwest Himalaya. Our data reveal a transition in topography from two-step topography in the
southeast, coinciding with the Main Frontal Thrust and a line known as Physiographic Transition2, to
one-step topography in the northwest coinciding with the active Main Boundary Thrust. These data reflect an increase in relative rock uplift rates ~100-km north of the Main Frontal Thrust in the southeast due to a ramp on the Main Himalayan Thrust or an emergent fault. In contrast, the sharp transition west of 77°E shows relatively high rock uplift rates along the active Main Boundary Thrust. We propose that either a steeply northward dipping splay fault or a mid-crustal ramp along the Main Himalayan Thrust merges with the Main Boundary thrust west of 77°E, with important implications for along-strike fault segmentation in the northwest Himalaya.
2. Background
Along the 2500-km long Himalayan arc, the arrangement of geologic units and evidence for active structures varies substantially between Himachal Pradesh and Uttarakhand in northwest India directly west of Nepal (Fig. 1; Deeken et al., 2011; Yin, 2006). As such, this study encompasses
Himachal Pradesh as well as a large section of northwestern Uttarakhand in the southeast (Fig. 1). In this section, I describe the prominent along-strike changes in active fault geometries and bedrock geology from Uttarakhand to Himachal Pradesh in the northwestern Himalaya (Fig .1a).
2.1. Tectonostratigraphy and bedrock geology of the northwest Himalaya
The tectonostratigraphy of the northwest Himalaya in the study area comprises four broad geologic units (Fig. 1) (Gansser, 1964; Hodges, 2000). From south to north these units are: 1. the
Sub-Himalaya, including Cretaceous to Cenozoic shallow-marine and continental deposits (DeCelles et al., 1998; Lavé and Avouac, 2000); 2. the Lesser Himalayan Sequence, consisting of Neoproterozoic
sedimentary rocks, Permian through Miocene sedimentary rocks (DeCelles et al., 2000; Kohn et al., 2010; Martin et al., 2011; Upreti, 1999; Webb et al., 2011), Proterozoic felsic intrusives, and Paleozoic to Mesoproterozoic metasedimentary rocks; 3. the Greater Himalayan Crystalline (GHC) sequence, consisting of upper Proterozoic to lower Paleozoic greenschist to amphibolite facies metamorphic rocks, Cambrian to Ordovician granites and orthogneisses, and Miocene leucogranites (Gansser, 1964; Gehrels et al., 2011; Hodges et al., 1996; LeFort, 1975; Parrish and Hodges, 1996; Scaillet et al., 1990, 1995; Searle et al., 1997; Searle and Godin, 2003; Yin, 2006) and; 4. the Tethyan Himalayan Sequence (THS), which consists of both Proterozoic to Eocene slightly metamorphosed siliciclastic and carbonate
sedimentary rocks interbedded with Paleozoic and Mesozoic volcanic rocks (Fig. 1) (Deeken et al., 2011; Gansser, 1964; Hodges, 2000; Steck et al., 1993; Yin, 2006).
2.1.1. Uttarakhand
In the Uttarakhand region of Northwest India (Fig. 1d), the units described above are broadly deformed by a series of northward dipping faults in a configuration similar to many areas described across the majority of the Himalaya (e.g. (Gansser, 1964; Yin, 2006). At the front of the range in Uttarakhand, the Main Frontal Thrust and Main Boundary Thrust transport folded and imbricated Siwalik Group sediments over the Indus-Gangetic foreland (DiPietro and Pogue, 2004; Fuchs and Linner, 1995; Gansser, 1964; Heim and Gansser, 1939; Kumar et al., 2001; Lavé and Avouac, 2000; Lyon‐Caen and Molnar, 1985; Nakata, 1989; Yeats and Lillie, 1991). North of the Main Boundary Thrust, the Lesser Himalayan duplex has developed through imbrication of the Lesser Himalayan Sequence since the Miocene (Fig. 1c and 1e) (Avouac, 2003; Célérier et al., 2009). The Lesser Himalayan Sequence is underthrust beneath the Greater Himalayan Crystalline sequence along the Main Central Thrust, which
Figure 1: A. Digital elevation model showing location of study area. B. Simplified geologic map
of Himachal Pradesh (modified from Deeken et al., 2011 and Celerier et al., 2009). Note the
rotated frame by 40° to the northwest. C. Geological cross-section from A-A’ with unconstrained
flat decollement geometry in western Himachal Pradesh (Deeken et al., 2011). D. Geological
cross-section from B-B’ with decollement geometry in Uttarakhand from geophysical data of
Caldwell et al., 2013 and with the grey box representing the position of the PT
2. E. Duplex model
with respect to the PT
2. F. Emergent fault model. C-E are modified from Wobus et al., (2006a).
D, E and F are at the same scale as C. MFT -- Main Frontal Thrust; MBT - Main Boundary
Thrust; MCT - Main Central Thrust; STD - South Tibetan Detachment; MHT - Main Himalayan
Thrust.
Figure 2:These physiographic metrics are based on the ~30 m Shuttle Radar Topography Mission (SRTM; jpl.nasa.gov/strm/) data set. A. Topography of Himachal Pradesh. B. Local Relief calculated using a moving circle with a 4.5-km radius. Red boxes and black boxes delimit across-strike and along-strike swath profiles in Figure 3, respectively. MFT - Main Frontal Thrust; MBT - Main Boundary Thrust; MCT - Main Central Thrust; PT2 - Physiographic Transition2; STD - South Tibetan Detachment.Figure 3:
A. Digital elevation model showing location of study area. B. Simplified geologic map of Himachal Pradesh (modified from
hosts rocks of the Greater Himalayan Sequence in its hanging wall (DiPietro and Pogue, 2004; Fuchs and Linner, 1995; Gansser, 1964; Heim and Gansser, 1939; Valdiya, 1980). The northernmost structure in this sequence, the South Tibetan Detachment, juxtaposes unmetamorphosed or low-grade Tethyan Himalayan Sequence rocks in the hanging wall against high-grade Greater Himalayan Sequence footwall rocks (Burchfiel et al., 1992; Burg et al., 1984).
2.1.2. Western Himachal Pradesh
Northwest of Uttarakhand, the tectonic configuration of the region north of the Dhauladar Range in western Himachal Pradesh (Fig. 1) varies from the geology to the southeast in several ways (Fig. 1c). First, structural mapping and geological cross-sections suggest the Lesser Himalayan duplex is less developed here than in areas to the southeast (Fig. 1b) (Deeken et al., 2011; Robyr et al., 2006; Steck et al., 1993). For this reason, the Main Boundary Thrust and Main Central Thrust are no more than ~10 km apart in this region, whereas in the southeast in Uttarakhand they are more than 75 km apart due to the development of the Lesser Himalayan duplex (Fig. 1). Second, although the details remain controversial, the geometry of the South Tibetan Detachment is also reported to change along strike from a
subhorizontal northeast dipping detachment near the Sutlej River to western Himachal Pradesh north of the Dhauladar Range (Fig. 1) (Herren, 1987; Searle, 1986). Here, 1-km thick mylonites show normal sense motion, dipping ~20° to the northeast (Dezes et al., 1999). Geologic observations suggest the South Tibetan Detachment and Zanskar Shear Zone’s western and eastern terminations, respectively, are linked by folding of the South Tibetan Detachment contemporaneous with folding of the Main Central Thrust (Yin, 2006). South of the Zanskar Shear Zone, the Main Central Thrust in the Dhauladar Range juxtaposes two low-grade units—the Haimantas Group of the Greater Himalayan Crystalline and the Lesser Himalayan Sequence (Deeken et al., 2011; DiPietro and Pogue, 2004; Frank et al., 1995; Thakur, 1998; Webb et al., 2007; Yin, 2006) that are likewise not present to the southeast. Finally, the range front in western Himachal Pradesh consists of Ordovician Dalhousie Granites intruded into the Dhauladar Range that are otherwise absent to the southeast (Deeken et al., 2011; Thakur, 1998).
2.2. Along-strike changes in Neotectonic Structures
The current activity on the major faults included in the Himalayan tectonic model also varies substantially from the southeast in Uttarakhand to Himachal Pradesh in the west and several studies have identified a prominent change in how strain accumulates across these portions of the Himalaya.
2.2.1. Uttarakhand
Neotectonic investigations in the Central Himalaya and Uttarakhand find that several, but not all, of the major unit bounding faults have been active into the late Quaternary (e.g. Célérier et al., 2009; Seeber and Gornitz, 1983; Valdiya, 1980; Yin, 2006). Paleoseismic investigations indicate the Main Frontal Thrust has produced large surface-rupturing earthquakes (Mw > 7.5) in the recent past, the most
recent occurring between 1200 A.D. and 1700 A.D. in the region between ~77°E and ~79°E along the Himalayan arc (Kumar et al., 2006). Additionally, paleoseismic trenching (Thakur et al., 2004) and uplifted terraces (Thakur and Pandey, 2004) indicate ~11 mm/yr of the total 40-50 mm/yr of relative convergence across the Uttarakhand is absorbed by faulting and folding along the Main Frontal Thrust. Further north, geologic and paleoseismic observations of folded Quaternary terraces and offset streams suggest strands of the Main Boundary Thrust have been reactivated during the late Quaternary and Holocene in western Uttarakhand (Gansser, 1964; Malik and Mathew, 2005; Malik and Nakata, 2003; Nakata, 1989; Valdiya, 1981).
While the two main faults at the front of the range show clear evidence for neotectonic activity, lack of historical seismicity suggests the Main Central Thrust is currently inactive (Ni and Baranzangi, 1984; Prasath et al., 2017). Nonetheless, GPS data and geomorphology suggest active strain accumulation occurs in the hinterland region 100 km north of the Main Frontal Thrust. Here, mean elevation and relief, and river channel steepness increase across a transition zone termed the Physiographic Transition2 (PT2;
Seeber and Gornitz, 1983; Morell et al., 2015, Harvey et al., 2015; Scherler et al., 2014); these increases are interpreted to reflect a northward increase in rock uplift rates. For reference, the first and third
physiographic transitions (i.e. PT1 and PT3) are north and south of the Physiographic Transition2
occurring across the South Tibetan Detachment and the Main Frontal Thrust, respectively (Hodges et al., 2001). While the other physiographic transitions follow the along-strike variations in the Main Frontal Thrust and South Tibetan Detachment, the Physiographic Transition2 is consistently located ~100-km
north of the mountain front between western Nepal (Harvey et al., 2015) and eastern Himachal Pradesh (Morell et al., 2015, 2017). Additionally, the Physiographic Transition2 correlates with: 1. a 50-km wide
zone of increased microseismicity (Mahesh et al., 2013); 2. hypocentral locations of thrust-type focal mechanisms (Mw 5-7; Ni and Baranzangi, 1984) and; 3. hinterland increases in10Be derived erosion rates
(Vance et al., 2003; Scherler et al., 2014; Morell et al., 2015, 2017). These results have been collectively interpreted to represent the surface expression of upper-plate structures such as a duplex or emergent fault (Morell et al., 2017) that spatially coincide with a geophysically imaged mid-crustal ramp along the Main Himalayan Thrust ~100-km north of the Main Frontal Thrust (Caldwell et al., 2013).
2.2.2. Western Himachal Pradesh
There have been fewer studies describing the neotectonic activity in the Northwest Himalaya compared to Uttarakhand; however, most results suggest Quaternary to Holocene activity on the Main Frontal Thrust and Main Boundary Thrust. North of the most recently activated Main Frontal Thrust (Nakata, 1989; Powers et al., 1998), a sinuous Main Boundary Thrust in Himachal Pradesh increases the width of the Subhimalaya to ~100 km, a zone termed the Kangra Reentrant (Powers et al., 1998).
Numerous thrusts and backthrusts cut the Kangra Reentrant, imbricating the Subhimalaya with estimated shortening rates of 14 ± 2 mm yr-1 since 1.9-1.5 Ma (Powers et al., 1998). Steepened longitudinal river
profiles, offset terraces, and back-tilted fluvial terraces in the Kangra Reentrant surrounding thrust faults accommodate 40-60% of the total shortening across the Subhimalaya over the past ~10 ka (Dey et al., 2016). Finally, geomorphology and longitudinal river profiles across the Northwestern Himalaya show major knickpoints and drainage reorganization patterns that suggest the Main Boundary Thrust has been active throughout the Quaternary (Thiede et al., 2009, 2017; Deeken et al., 2011).
Each of these major thrust systems are interpreted to sole into the Main Himalayan Thrust, which is thought to have hosted the 7.8 magnitude “Kangra” earthquake on 4 April 1905 in Himachal Pradesh (Middlemiss, 1910; Molnar, 1987). GPS-derived slip rates are consistent with paleoseismic investigations and other slip rate estimates determining that events such as the 1905 Kangra earthquake could rupture the Main Himalayan Thrust with a recurrence interval on the order of ~350 years (Yeats and Thakur, 1998). Historical Rossi-Forel Intensity maps (Middlemiss, 1910), earthquake fault plane solutions (Ekstrom, 1987; Molnar and Lyon-Caen, 1989) and rupture area estimations (Molnar, 1990) suggest the 1905 Kangra earthquake involved five meters of slip on a nearly horizontal decollement (~2.5°; Molnar and Lyon-Caen, 1989). Exhumation patterns from thermochronological data can also be useful for estimating the geometry of the Main Himalayan Thrust and across-strike patterns in rock uplift rates. For example, variability in thermochronological data north of the Dhauladar Range in western Himachal Pradesh suggests that the Main Himalayan Thrust is moderately dipping without a mid-crustal ramp or an active duplex system; meanwhile, rapid exhumation rates at the range front advocate for consistent activity on the Main Boundary Thrust (Fig. 1c; Deeken et al., 2011). In contrast to Uttarakhand, one model for western Himachal Pradesh infers that deformation across the Dhauladar Range is the result of sliding of the orogenic wedge gradually over a flat decollement before abruptly uplifting along the frontal ramp of the Main Boundary Thrust (Deeken et al., 2011), assuming a near steady-state landscape where tectonic and surface processes are dynamically coupled (Seeber and Armbruster, 1981; Willett and Brandon, 2002).
These studies point to an along-strike change in the long- and short-term geologic configuration in the Northwestern Himalaya. In this thesis, I assess potential along-strike changes in active strain accumulation across this region, specifically the spatial distribution of current rock uplift rates, using topography, stream profile analyses, and 10Be derived basin-averaged erosion rates.
To evaluate spatial patterns of rock uplift rates across the regions with contrasting geology and neotectonics, I investigated the spatial distribution of elevation, relief, channel steepness, and basin-averaged erosion rates from concentrations of 10Be in quartz within detrital river sand. We use these data
collectively as a proxy for rock uplift rate (Kirby and Whipple, 2001; Wobus et al., 2006) as used in other studies where conventional methods are limited by rarely or poorly exposed fault traces (e.g. Kumar et al., 2006), or poor preservation of deformed Quaternary sediments (Lavé and Avouac, 2000) .
3.1. Topographic Analyses
For actively uplifting landscapes that have reached a steady-state balance between rates of rock uplift and rates of erosion (Adams, 1985; Ahnert, 1970; Wager, 1933, 1937), decades of studies show that there is a positive correlation between rock uplift rates and topographic metrics such as relief and elevation as well as channel steepness (e.g. Seeber and Gornitz, 1983; Kirby and Whipple, 2012; Tucker and Whipple, 2002; Wobus et al., 2006). I use elevation and relief maps to calculate basin-averaged relief in addition to topographic swath profiles to compare these landscape metrics to our basin-averaged erosion rate data. First, after acquiring the digital elevation model based on NASA’s 1-arc-second (~30 m) Shuttle Radar Topography Mission (SRTM; http://www2.jpl.nasa.gov/srtm/) dataset, I calculate local relief using a ~4.5-km-radius moving circle to reduce topographic noise. Also, I measure relief for 626 individual basins by subtracting the lowest from the highest elevations for each basin (e.g. Ouimet et al., 2009) in order to compare relief to basin-averaged erosion rates. Next, accompanying each plan-view map of elevation and relief is a series of 30-km-wide swath profiles extending along and across-strike for the elevation and relief maps that condense an area of topographic data into a single spatially averaged profile; this significantly reduces topographic noise. For this study, I consider two ~200-km-long swath profiles east and west of 77°E perpendicular to the PT2 and Main Boundary Thrust, respectively (Fig. 2).
We also positioned two ~300-km-long swath profiles north and south of the PT2 continuing along strike
3.2. Normalized Channel Steepness
The rate at which hillslopes adjust to changes in vertical rock uplift is reflected in longitudinal profiles of the local channel network (Whipple et al., 1999; Wobus et al., 2006). As channel steepness adjusts to changes in rock uplift rates, topographic relief likewise adjusts to reach a steady state, where erosion rates and rock uplift rates are balanced (Seeber and Gornitz, 1983). In actively uplifting landscapes, empirical data from river channels show that when using a power-law scaling relationship between local channel slope and contributing drainage area (e.g. Hack, 1973; Howard and Kerby, 1983), channel steepness can be a proxy for vertical rock uplift rates (Whipple and Tucker, 1999). Our
longitudinal river profile analysis follows two approaches based on the stream power law for steady-state mountain belts (Howard and Kerby, 1983; Whipple and Tucker, 1999): the slope-area method (Kirby and Whipple, 2001; Wobus et al., 2006) and the integral method (Royden et al., 2000; Harkins et al., 2007; Mudd et al., 2014; Perron and Royden, 2013).
The integral and slope-area methods consider graded river profiles (Davis, 1902; Mackin, 1948), where local slope, S, can be represented using the stream power law with respect to upstream drainage area, A (Flint, 1974; Hack, 1957; Howard and Kerby, 1983):
𝑆 = 𝑘𝑠 𝐴−𝜃
In this equation the exponent, 𝜃, is the concavity index and ks is the channel steepness (Flint, 1974). By
fixing the concavity to a regional mean concavity of 0.45 (commonly in the range of 0.3-0.6) (Brocklehurst and Whipple, 2002; Kirby et al., 2003; Kirby and Whipple, 2001; Snyder et al., 2000; Whipple and Tucker, 1999; Wobus et al., 2003), I can calculate the normalized channel steepness index, ksn, which allows for comparison of stream profiles with varying drainage areas (Duvall, 2004; Harvey et
al., 2015; Kirby et al., 2003; Kirby and Whipple, 2012; Lague, 2014; Morell et al., 2015; Ouimet et al., 2009; Wobus et al., 2006, 2003). However, when measuring channel steepness for actively uplifting
landscapes there are a few assumptions to be considered before interpreting spatial patterns of normalized channel steepness.
The main assumptions and disadvantages to be considered for the slope-area method and integral method in the channel steepness analyses include the possibility for: 1. nonlinearities in the incision process (Whipple et al., 2000; Whipple and Tucker, 1999), including erosion thresholds (Snyder et al., 2003a; Tucker, 2004; Tucker and Bras, 2000), 2. changes in principal erosional processes with increasing erosion rate (Whipple et al., 2000); 3. adjustments in channel width and sinuosity, or channel morphology (Harbor, 1998; Lavé and Avouac, 2000, 2001; Snyder et al., 2000; Snyder et al., 2003); 4. adjustments in the extent of alluvial cover, bed material grain size, bed morphology, and hydraulic roughness (Hancock and Anderson, 2002; Sklar and Dietrich, 1998, 2001; Sklar, 2003; Whipple and Tucker, 2002); 5. changes in the frequency of erosive debris flows (Stock and Dietrich, 2003); and 6. orographic enhancement of precipitation or climatic influences (Roe et al., 2002, 2003; Snyder et al., 2000; Snyder et al., 2003b). Considering the effects of these channel characteristics on channel steepness analyses, these methods remain better suited for identifying relative rock uplift rates due to blind structures compared to methods such as paleoseismic trenching that require a surface rupture. Therefore, these channel steepness analyses are used to evaluate spatial patterns in relative rock uplift rates throughout the northwest Himalaya. 3.2.1. Slope-Area Method
Using the slope-area method, I created plan-view maps showing normalized channel steepness for stream networks throughout the Northwest Himalaya (Fig. 4a) using Stream Profiler Tools
(http://geomorphtools.geology.isu.edu/Tools/StPro/StPro.htm) based on workflows between ArcMap™ and MATLAB™. First, I extracted unglaciated river networks from the digital elevation model for the study area with upstream drainages small enough (i.e. <100 km2) to capture discrete tectonic or climatic
signals yet large enough (i.e. >10 km2) to study established river networks only. Second, I smoothed our
were selected using 1-km moving windows where upstream drainage areas are greater than 107m2 (i.e.
>10 km2). Subsequently, local slopes for the individual 1-km river segments throughout the study area
were measured and plotted against upstream drainage area in log-log space forming the slope-area plot. Finally, force-regressions of the slope-area data based on the 0.45 concavity index allows for calculating the normalized channel steepness index for each 1-km segment.
3.2.2. Integral Method
Using the same ~30-m digital elevation model, MATLABTM and TopoToolbox (Schwanghart and Scherler, 2014; https://topotoolbox.wordpress.com/) I also calculate basin-averaged channel steepness values based on the integral method (Royden et al., 2000). By integrating the stream power derived from the relationship between erosion rate and either stream power per unit area (Seidl and Dietrich, 1992; Howard et al., 1994) or bed shear stress (Howard and Kerby, 1983), the upstream distance along the river profile is normalized for drainage area to construct the transformed variable, χ (dimension of length) (Harkins et al., 2007; Mudd et al., 2014; Perron and Royden, 2013). By plotting χ against channel
elevation for each discrete distance upstream of the river outlet in a given watershed (i.e. every 30-m for a 30-m resolution DEM), the slope of regression line on the χ-elevation plot yields the basin-averaged normalized channel steepness (Mudd et al., 2014; Perron and Royden, 2013).
While the integral method has many of the same disadvantages as the slope-area method, there are advantages to the integral method. First, the integral method allows for comparison between
normalized channel steepness and basin-averaged erosion rates (Perron and Royden, 2013). Second, this method reduces topographic noise when calculating local slope from topographic data and therefore holds the ability to accentuate changes from bedrock channels to channels dominated by alluvial sediment, colluvial processes, and debris flows as these may affect channel steepness (Perron and Royden, 2013). Lastly, watersheds with uniform lithology and without knickpoints are shown to have a linear relationship between χ and elevation, allowing us to test whether the basin contains knickpoints (i.e. if R2 is not equal
to 1) (Perron and Royden, 2013). Under these considerations, I chose tributaries drainage area between 10-km2 and 100-km2, with R2 values greater than 0.70 to indicate these river profiles have adjusted to
regional rock uplift rates and therefore show changes in relative rock uplift rates across the landscape. Next, I performed least-squares regression analyses to identify any power-law or linear relationships between basin-averaged channel steepness and 10Be derived erosion rate data. Finally, the resultant
relationship between basin-averaged channel steepness and 10Be erosion rate was used to predict
basin-averaged erosion rates for other basins throughout the study area. 3.3. Erosion Rate Analyses
We supplement our topographic and river profile analyses with 33 new basin-averaged erosion rate measurements from in-situ cosmogenic radionuclide 10Be concentrations in quartz. Basin-averaged
erosion rates derived from in-situ cosmogenic 10Be nuclides (Bierman and Steig, 1996; Granger et al.,
1996; Lal, 1991) have been used to quantify the rate of denudation over watersheds from actively uplifting landscapes around the world. The measurement of in-situ 10Be works well for basin-averaged
erosion rate calculation because 10Be is rarely found naturally in rock (von Blanckenburg, 2005) and
instead quartz accumulates 10Be when exposed to cosmogenic rays resulting in a concentration that is
directly related to the amount of time the sediment has been exposed at the Earth’s surface. Hence, as sediment travels downstream to the point of collection, the concentration of 10Be is inversely correlated to
erosion rate, whereby slowly eroding basins are predicted to have large concentrations of 10Be and rapidly
eroding basins should contain relatively lower concentrations of 10Be (Granger et al., 1996; Nishiizumi et
al., 1996; Portenga and Bierman, 2011; Schaller et al., 2001). Recent studies in eastern Tibet (Harkins et al., 2007; Ouimet et al., 2009), the San Gabriel mountains of southern California (DiBiase et al., 2010), and the Appenines of Italy (Cyr et al., 2010) show there is a power-law relationship between channel steepness and erosion rate based on characteristics of the channel including the rate of flow required to mobilize sediment and start stream incision (DiBiase and Whipple, 2011; Lague et al., 2005).
Consequently, I analyzed basin-averaged erosion rates to test for potential relationships between rock uplift rates, channel steepness, and erosion rates throughout the Northwest Himalaya.
We collected 33 modern sand samples from the outlets of tributaries to measure cosmogenic radionuclide 10Be concentrations in quartz to estimate basin-averaged erosion rates (Bierman and Steig,
1996; Granger et al., 1996; von Blanckenburg, 2005). Sample locations were constrained to modern bedrock channels with uniform lithology under the criteria that the collected quartz was: 1. from active river channels without local landslides, fluvial terraces, and/or debris flows; 2. from the mouths of tributaries with drainage areas >10-km2 to avoid mixing from upstream landslide debris (Niemi et al.,
2005), while isolating drainages small enough (i.e. <100-km2) to identify changes in erosion rates both
along and across strike; and 3. from presently unglaciated watersheds, to avoid potential shielding of cosmic rays by glaciers (Wittmann et al., 2007). I assume, given the local relief and size of the sampled tributaries, that the sands have undergone minimal sediment storage and rapid transport throughout each watershed (Gosse and Phillips, 2001; Granger et al., 1996; Lal, 1991). Under these conditions, spatial patterns in both erosion rates and landscape morphology can elucidate the nature of a landscape’s response to tectonic forces (DiBiase and Whipple, 2011; Kirby and Whipple, 2012; Wobus et al., 2006).
The following procedure was used to estimate 10Be concentrations in river sand from the chosen
basins. Through hours of work and commitment the quartz was cleaned and processed at the University of Wollongong in Australia under the supervision of Dr. Alexandru Codilean and Dr. Reka-H Folup, and the
10Be concentrations were measured at the Australian Nuclear Science and Technology Organization
(ANSTO) under the supervision of Dr. David Fink. Using froth flotation, quartz was separated from feldspars (Kohl and Nishiizumi, 1992) before extracting 10Be from the clean quartz by ion
chromatography (Von Blanckenburg et al., 1996). The ratios of 10Be/9Be were measured using the 6MV
SIRIUS Tandem Accelerator at ANSTO (Fink and Smith, 2007; Wilcken et al., 2017). Measured ratios were normalized to the 2007 KNSTD standard KN01-5-2 under a nominal 10Be/9Be ratio of 8.558 × 10−12
10−15 and 314.74 ± 7.18 × 10−15. Errors were calculated for the resulting 10Be concentrations (atoms g−1)
by finding the sum in quadrature of the statistical error for the Accelerator Mass Spectrometry (AMS) measurement. These errors include 2% for reproducibility, 1% for uncertainty in the Be spike
concentration, and 3.2 to 5.7% for analytical errors.
We use the open source code CAIRN (Mudd et al., 2016) to estimate basin-averaged 10Be erosion
rates. Nuclides generated from neutrons and muons were computed using the four exponential
approximation (Braucher et al., 2009). Specifically, this study used the sea-level and high-latitude 10Be
production rate of 4.3 atoms g-1 y-1 (Mudd et al., 2016) under the time-independent Lal/Stone scaling
scheme (Stone, 2000). Next, the production of nuclides was estimated using the SRTM 1-arc-second digital elevation model similar to the landscape metrics presented here, followed by calculating atmospheric pressure by interpolation of the NCEP2 data (Compo et al., 2011). Finally, our data were corrected for topographic shielding using the digital elevation model from Codilean (2006) and an assumed 10Be half-life of 1.387 ± 0.012 Myr (Chmeleff et al., 2010; Korschinek et al., 2010).
4. Results
4.1. Along-strike Variations
Our topographic and geomorphic analyses reveal substantial along-strike changes in across-strike patterns of elevation (Fig. 2a), local relief (Fig. 2b) and channel steepness (Fig. 4). Based on spatial patterns in the data, I divide the study area into two regions, east and west of longitude 77°E.
Furthermore, channel steepness indices from the slope-area analysis and the integral method returned a total of 626 basins with R2 values >0.70 (Fig. 4a-b). Lastly, there is a near linear relationship (R2=0.71)
between basin averaged channel steepness and relief considering all 626 basins.
East of 77°E, elevation (Figs. 2-3), relief (Figs. 2-3) and channel steepness data (Fig. 4) reveal a two-step topography. There is a sharp increase in elevation from south to north at the Physiographic Transition2 (PT2) ~100 km across strike from the Main Frontal Thrust, from <2000 m between the Main
Figure 2: Physiographic metrics based on the ~30 m Shuttle Radar Topography Mission (SRTM;
jpl.nasa.gov/strm/) data set. A. Topography of Himachal Pradesh. B. Local Relief calculated
using a moving circle with a 4.5-km radius. Red boxes and black boxes delimit across-strike and
along-strike swath profiles in Figure 3, respectively. Relative plate motion vector shows plate
motion of India with respect to a stable Eurasia, calculated by UNAVCO using Morvel 2010
(DeMets et al., 2011). MFT - Main Frontal Thrust; MBT - Main Boundary Thrust; MCT - Main
Central Thrust; PT
2- Physiographic Transition
2; STD - South Tibetan Detachment.
Figure 23:Elevation and relief swath profiles showing maximum, minimum and mean elevation and relief. A. Elevation (top) and Relief (bottom) swath profiles for western Himachal Pradesh, orthogonal to the Main Boundary Thrust. B. Elevation (top) and relief (bottom) swath profiles for Uttarakhand, orthogonal to the Physiographic Transition2. C. Elevation (left) and relief (right) along-strike swath profiles for the Greater Himalaya, north of the Physiographic Transition2. D. Elevation (left) and relief (right) along-strike swath profiles for the Lesser Himalaya and Subhimalaya transition across the Main Boundary Thrust. MFT - Main
Figure 3:
Elevation and relief swath profiles showing maximum, minimum and mean elevation
and relief. A. Elevation (top) and Relief (bottom) swath profiles for western Himachal Pradesh,
orthogonal to the Main Boundary Thrust. B. Elevation (top) and relief (bottom) swath profiles
for Uttarakhand, orthogonal to the Physiographic Transition2. C. Elevation (left) and relief
(right) along-strike swath profiles for the Greater Himalaya, north of the Physiographic
Transition2. D. Elevation (left) and relief (right) along-strike swath profiles for the Lesser
Himalaya and Subhimalaya transition across the Main Boundary Thrust. MFT - Main Frontal
Thrust; MBT - Main Boundary Thrust; PT
2- Physiographic Transition
2.
Figure 39: A. Predicted (grey outline) and 10Be-derived basin-averaged erosion rates (black outline). B. Basin-averaged 10Be erosion rates across-strike of the Main Boundary Thrust in the west near the Dhauladar range. C. Basin-averaged 10Be erosion rates across-strike of the Physiographic Transition2 near the Sutlej River. D. Predicted Basin-averaged erosion rate versus distance across-strike from Main Boundary Thrust binned every 10-km (black). E. Predicted basin-averaged erosion rate versus distance across-strike from the Physiographic Transition2 binned every 10-km. Predicted values based on power-law relationship between 10Be erosion rate and basin-averaged normalized channel steepness from Figure 4. All basins range in drainage areas from 10 km2 to 100km2. Glaciated basins are excluded.Figure 40:Elevation and relief swath profiles showing maximum,
Figure 4: A. Normalized river channel steepness (k
sn) calculated using the slope-area method
outlined by Wobus et al. (2006). B. Basin-averaged normalized river channel steepness (k
sn)
calculated using the integral method. C. Across-strike profile of normalized channel steepness
centered on the Main Boundary Thrust with black dots showing mean values binned every
10-km and error bars showing the standard deviation of values within each bin. D. Across-strike
normalized channel steepness centered on the Physiographic Transition
2with black dots showing
mean values binned every 10-km. Slope-area data were calculated using 1-km long river
segments and both methods have contributing drainages greater than 10 km
2with θ
ref= 0.45.
Maps are oriented to display the along-strike behavior of the Physiographic Transition
2in central
Himachal Pradesh. Note that the footwall of the Main Boundary Thrust is primarily alluvial
channels and therefore channel steepness relationships vary and should be considered lightly
.
Figure 5: A. Predicted (grey outline) and
10Be-derived basin-averaged erosion rates (black
outline). B. Basin-averaged
10Be erosion rates across-strike of the Main Boundary Thrust in the
west near the Dhauladar range. C. Basin-averaged
10Be erosion rates across-strike of the
Boundary Thrust and PT2 to >3000 m north of the PT2 (Fig. 3b). Second, the landscape south of thePT2
has a moderate mean relief of ~1500 m north of the Main Boundary Thrust (Fig. 3b). However, the landscape north of the PT2 is dominated by deep canyons and narrow valleys as relief increases to greater
than ~2500 m within a 25-km zone across strike (fig. 3b). Third, the region south of the PT2 is defined by
moderate channel steepness indices between ~100 and 200 (Fig. 4) corresponding to the moderate elevation and relief of this region (Fig. 2). In contrast, channel steepness indices exceed 200 north of the PT2 where 37 of these basins are much higher, with values between 300 and 430 (Fig. 4). These spatial
patterns in elevation, relief, and channel steepness in the east show a two-step across-strike transition for each data set (Fig. 2-4), not seen to the west.
West of 77°E, spatial patterns in elevation (Fig. 2a), relief (Fig. 2b), and channel steepness (Fig. 4) exhibit a single transition across the Main Boundary Thrust where plan-view maps of elevation show the PT2 extends along strike until 77°E (Fig. 2). Here, elevation increases across the Main Boundary
Thrust from 1000 m to 4000 m over ~10-km across strike (Fig. 3). Likewise, plan-view relief maps (Fig. 2b) and swath profiles (Fig. 3a) show mean topographic relief increases from 500 m to 3000 m across the Main Boundary Thrust into the Dhauladar Range of western Himachal Pradesh. Channel steepness indices also increase from less than 100 throughout the Subhimalaya south of the Main Boundary Thrust to >200 across the Dhauladar Range with a maximum of ~415 for portions of the mountain front (Fig. 4). Moreover, the Subhimalaya primarily hosts transport-limited channels further supporting our results showing low channel steepness indices in both the slope-area analyses (Fig. 4a) and basin-averaged channel steepness analyses (Fig. 4b). Collectively, these observations reveal an along-strike change in spatial patterns of elevated channel steepness, elevation, and relief from south of the Physiographic Transition2 in the east to the Subhimalaya south of the Main Boundary Thrust west of 77°E.
In addition to the geomorphology data, the erosion rate data show a significant northward increase in 10Be derived erosion rates across the PT
2 east of 77°E and across the Main Boundary Thrust
west of 77°E in western Himachal Pradesh (Table 1; Fig. 5). Also, our data reveal strong correlations between 10Be erosion rates, and basin-averaged normalized channel steepness (Fig. 6a) and relief (Fig 6b),
suggesting the data collectively are reliable proxies for relative changes in vertical rock uplift rates across the actively uplifting landscape.
Similar to the spatial patterns in elevation (fig. 2a), relief (fig. 2b), and channel steepness (fig. 4) east of 77°E, 10Be derived erosion rates also increase north of the PT
2 (Fig. 5). 10Be erosion rates for
samples south of the PT2 (n=4) have a low mean erosion rate of 0.20±0.04 mm yr-1 and range from
0.09±0.02 to 0.31±0.06 mm yr-1 (Fig. 5c; table 1, samples IN14-15 and IN16-03, respectively).
Meanwhile, the mean erosion rate for watersheds north of the PT2 (n=17) nearly triples to 0.65±0.12 mm
yr-1 and values range from 0.28 ±0.06 to 1.95± 0.36 mm yr-1 (Fig. 5c; Table 1, samples IN14-20 and
IN16-05, respectively). Where the PT2 crosses sampled basins (n=3), mean 10Be erosion rates are
intermediate (0.31±0.06 mm yr-1) and values range between 0.13±0.02 mm yr-1 to 0.50±0.09 mm yr-1
(Fig. 5c; Table 1, samples IN16-20 and IN16-21, respectively).
West of 77.0°E, 10Be basin-averaged erosion rates increase across the Main Boundary Thrust into
the Dhauladar Range and western Himachal Pradesh (Fig. 5). The sample south of the Main Boundary Thrust in the Kangra Reentrant and Subhimalaya(n=1) has a low erosion rate of 0.07±0.02 mm yr-1 (Fig.
5b; Table 1, sample IN16-28). These values agree with the downstream change to alluvial channels from bedrock channels present upstream of the Main Boundary Thrust. The mean erosion rate for watersheds north of the Main Boundary Thrust (n=9) sharply increases to 0.95±0.18 mm yr-1 and values range from
0.28 ±0.05 to 1.65± 0.31 mm yr-1 (Fig. 5b; Table 1, samples IN16-29 and IN16-23, respectively).
However, four out of nine samples along the Dhauladar Range have rates less than ~0.50 mm yr-1 while
five out of nine samples have rates much greater than 1.0 mm yr-1indicating there is high variability in 10Be erosion rates for samples north of the Main Boundary Thrust in western Himachal Pradesh (Fig. 5b).
Figure 5: A. Predicted (grey outline) and
10Be-derived basin-averaged erosion rates (black
outline). B. Basin-averaged
10Be erosion rates across-strike of the Main Boundary Thrust in the
west near the Dhauladar range. C. Basin-averaged
10Be erosion rates across-strike of the
Physiographic Transition
2near the Sutlej River. D. Predicted Basin-averaged erosion rate versus
distance across-strike from Main Boundary Thrust binned every 10-km (black). E. Predicted
basin-averaged erosion rate versus distance across-strike from the Physiographic Transition
2binned every 10-km. Predicted values based on power-law relationship between
10Be erosion rate
and basin-averaged normalized channel steepness from Figure 4. All basins range in drainage
areas from 10 km
2to 100km
2. Glaciated basins are excluded
.
Figure 53:Comparison of basin-averaged 10Be erosion rates to basin-averaged channel steepness and relief. A. Basin-averaged channel steepness showing a positive power-law relationship (R2 = 0.44) with 10Be erosion rates. Grey ellipse shows higher
Figure 6:
Comparison of 33 new basin-averaged
10Be erosion rates to basin-averaged
channel steepness and relief. A. Basin-averaged channel steepness showing a positive
power-law relationship (R
2= 0.44) with
10Be erosion rates. Grey ellipse shows higher
erosion rates where channel steepness is relatively low, most likely from enhanced
precipitation along the Dhauladar Range, which increases sediment load in river channels.
B. Basin-averaged relief showing a positive power-law relationship (R
2= 0.48) with
10Be
We observe a positive power-law relationship between basin-averaged 10Be erosion rates, and
basin-averaged channel steepness indices (Fig. 6a) and mean basin relief (Fig. 6b). In particular, 10Be
erosion rates from both east and west of 77°E show the best relationship with basin-averaged channel steepness where erosion rates follow a positive, power-law fit (R2=0.44; Fig. 6a). When the regions east
and west of 77°E are analyzed separately, I find a relatively strong power-law fit (R2=0.53) for the region
east of 77°E whereas the region west of 77°E exhibits a poor power-law relationship (R2=0.33) showing
high variability between basin-averaged 10Be erosion rate and channel steepness along the Dhauladar
Range. Secondly, I similarly see an overall positive power-law scaling relationship between 10Be erosion
rate and basin relief (R2=0.48; Fig. 6b). Together, these relationships between the landscape
metrics and 10Be erosion rates imply each data set is an accurate proxy for vertical rock uplift rates and
can be further used to estimate predicted erosion rates throughout the northwest Himalaya. 4.3. Predicted Basin-averaged Erosion Rates
Using the power-law relationship between basin-averaged channel steepness and 10Be erosion
rates (Fig. 6a), I calculate predicted basin-averaged erosion rates for basins east of longitude 77°E in Himachal Pradesh that reveal the western limit of the PT2 (Fig. 5a). North of the PT2, 174 basins have
χ-elevation plots with a combined mean R2 value of 0.94 implying the selected basins satisfy the condition
of topographic steady-state using the integral method (Perron and Royden, 2013). These basins show a high mean predicted erosion rate of ~0.56 mm yr-1 (Fig. 5e), similar to 10Be erosion rates (Fig. 5c). Also,
normalized channel steepness indices for these basins scale with respect to erosion rates from 85 to 417 with a high mean channel steepness of ~232. South of the PT2, however, 170 basins have χ-elevation plots
with a collective mean R2 value of 0.95 that have predicted basin-averaged erosion rates that are less than
half the rates of basins north of the PT2 (Fig. 5b). Here, the mean predicted erosion rate reaches ~0.25 mm
yr-1 and values range from 0.007 mm yr-1 to 0.78 mm yr-1 (Fig 5e) corresponding to a moderate mean
Along strike into western Himachal Pradesh, predicted basin-averaged erosion rates and channel steepness indices west of 77°E show two distinct regions north and south of the Main Boundary Thrust (Fig. 5a, d). First, 92 basins north of the Main Boundary Thrust have a mean R2 value of 0.92 and a high
mean predicted erosion rate of ~0.64 mm yr-1 (Fig. 5e), comparable to predicted erosion rates north of the
PT2 in the east (Fig. 5c). Also, predicted erosion rates for these basins scale from 0.20 mm yr-1 to 1.13
mm yr-1 (Fig. 5d) where normalized channel steepness indices for these basins range from 144 to 417 with
a high mean channel steepness of ~292. South of the Main Boundary Thrust, 29 basins with a mean R2
value of 0.91 show the mean predicted basin-averaged erosion rate decreases to ~0.02 mm yr-1 where the
mean basin-averaged channel steepness index is as low as 32 (Fig. 4c). Field observations also note that channels south of the Main Boundary Thrust are alluvial, supporting the low predicted erosion rates and low channel steepness throughout the Subhimalaya.
4.4. Comparison of Channel steepness and erosion rates to the distribution of precipitation
East of 77°E, two 50-km wide bands of heightened mean annual precipitation (NASA’s Tropical Rainfall Measurement Mission data from 1998-2005 used by Bookhagen and Burbank (2006)) coincide across strike with the Main Boundary Thrust and PT2 ~50 and 100-km north of the Main Frontal Thrust,
respectively (Fig. 7). Each band has a mean annual rainfall of >2500 mm yr-1, whereas regions between
the bands collect less than ~ 2000 mm yr-1 of precipitation with smaller regions reaching to less than 1500
mm yr-1(Fig. 7). North of the PT
2 in Uttarakhand, a 30-km-wide swath profile shows mean precipitation
of ~2000 mm yr-1 that continues west along strike for ~100-km before increasing in western Himachal
Pradesh (Fig. 7b).
South of the PT2 in the region east of 77°E, a 30-km wide swath profile shows mean precipitation
between the two bands has a mean value of ~1700 mm yr-1 for ~250-km into the Kangra Reentrant (Fig.
7c). Here, channel steepness indices (Fig. 4) and erosion rates (Fig. 5) remain moderate (100-200 m0.9)
Figure 7: A. Annual precipitation raster for Himachal Pradesh from the Tropical Rainfall
Measuring Mission (TRMM; https://pmm.nasa.gov/trmm). Note band of intensified precipitation
along the Dhauladar Range correlating with high channel steepness indices (Figure 4) and
basin-averaged erosion rates (Figure 5). B. Along-strike swath profile showing increasing precipitation
in the west coinciding with the Dhauladar Range. C. Along-strike swath profiles south of the
Physiographic Transition
2showing relatively consistent precipitation from east to west across the
Main Boundary Thrust and southern Kullu window. Black lines on part A denote major faults:
MFT - Main Frontal Thrust; MBT - Main Boundary Thrust; MCT - Main Central Thrust; STD -
South Tibetan Detachment; PT
2- Physiographic Transition
2.
southern precipitation swath profile (Fig. 7c). The along-strike extent of these bands of higher
precipitation continues west until spatial patterns in annual rainfall become diffuse near the eastern extent of the Kangra Reentrant (Fig. 7).
West of 77°E, the notable variability between basin-averaged channel steepness indices and 10Be
erosion rates along the Dhauladar Range correlates to a band of increased annual precipitation (Fig. 7). Along the Dhauladar Range five out of nine basins have substantially high erosion rates (Fig. 5) but exhibit relatively low channel steepness indices (Fig. 6a). Here, the sharp topographic transition north of the Main Boundary Thrust (fig. 2) appears to concentrate monsoonal rainfall and annual precipitation to >3000 mm yr-1 and separates the Dhauladar Range from the semi-arid interior of western Himachal
Pradesh (Fig. 7). Although channel steepness indices here increase by nearly five times over the rivers throughout the Sub-Himalaya (Fig. 4-5), the enhanced precipitation could increase landslide activity which could affect 10Be concentrations and local channel steepness indices (Wobus et al., 2006).
Therefore, when analyzing basin-averaged erosion rates and channel steepness independently and together, I consider spatial patterns in annual precipitation as small influences on channel morphology. 5. Discussion
5.1. Geomorphology
There are significant along-strike changes in landscape morphology between Uttarakhand to the east and Himachal Pradesh to the west. The region east of 77°E is well-defined by two-step topography, with sharp northward increases in relief, channel steepness and basin-averaged erosion rates at: 1. the Main Frontal Thrust and 2. ~100-km north of the Main Frontal Thrust at the PT2 (Scherler et al. 2014,
Morell et al. 2015, 2017). West of 77°E, however, spatial patterns in channel steepness (Fig. 4c), elevation (fig. 2a, 3a), relief (Fig. 2b, 3a) and basin-averaged 10Be and predicted erosion rates (Fig. 5)
show one-step topography coinciding instead with the Main Boundary Thrust, which in this region is located directly along strike of the PT2.
Several observations suggest that the landscape metrics and erosion rates I calculate reflect relative differences in rock uplift rate. First, the study area is an actively uplifting region where erosion rates are modulated primarily by vertical rock uplift rates and thus assumed to be in steady state (Howard and Kerby, 1983; Whipple and Tucker, 1999), which is shown by the absence of knickpoints in
longitudinal profiles and further verified by the high R2 values for the χ-elevation plots in the chosen
basins (Perron and Royden, 2012). Second, at least for the case of the region east of 77°E, the changes in landscape metrics across the PT2 show no relationship to lithologic contacts or fault traces but can be
interpreted to reflect differences in rock uplift rate on either side of a blind structure beneath the PT2
where higher rock uplift rates are exposed north of the PT2 by relatively high channel steepness indices
and erosion rates (Figs 4 and 5). West of 77°E, the spatial patterns in channel steepness, topography, and erosion rates coinciding with the Main Boundary Thrust do occur across a change in lithology between the Subhimalaya to the south and the Lesser Himalayan Sequence to the north of the Main Boundary Thrust (Figs. 2-5). Third, our data show that there is a relationship between channel steepness and erosion rate (Fig. 6a) as well as between relief and erosion rate (Fig. 6b) for the 33 new 10Be erosion rates
presented here. Together, these relationships between the metrics and geology in addition to the absence of knickpoints show the predicted response of actively uplifting landscapes where there is a northward increase in rock uplift rate across the PT2 east of 77°E and the Main Boundary Thrust west of 77°E.
The two-step topography observed east of 77°E (Fig. 2) represents the northward increase in rock uplift rates across of the Main Frontal Thrust and PT2. From the Ganga foreland basin to north of the
Main Frontal Thrust, elevation and relief increase with channel steepness and erosion rates and remain moderate until ~100 km north of the Main Frontal Thrust. Second, the region north of the PT2 has much
higher elevations and relief while maintaining steepened channels and increased basin-averaged erosion rates. These data show that the region south of the PT2 has adjusted to accommodate moderate rock uplift
or structure on the subducting or upper plate underlying the PT2 that is modifying the across-strike
variation in rock uplift rates.
West of 77°E, however, there are substantial along-strike changes in how rock uplift rates are distributed with respect to active structures. First, elevation and relief (Fig 2-3) show one-step topography coinciding with the Main Boundary Thrust ~100 km north of the Main Frontal Thrust. South of the Main Boundary Thrust, the Subhimalaya exhibits the lowest values in channel steepness and erosion rates for the region where topography is relatively mild. Further north, across the Main Boundary Thrust there is a sharp increase in elevation, relief, and channel steepness indicating a significant increase in rock uplift rates most likely associated with Quaternary uplift along the Main Boundary Thrust (e.g. Deeken et al., 2011; Powers et al., 1998). More importantly, the spatial patterns in rock uplift rates defining the trace of the PT2 appear to merge with the Main Boundary Thrust into western Himachal Pradesh near 77°E (Fig.
2-5). In particular, these interpretations suggest the active structure controlling rock uplift rates north of the PT2 east of 77°E, merges into the Main Boundary Thrust, showing the study area encompasses at least
two different tectonic models potentially revealing a tectonic segment boundary in the northwest Himalaya.
5.2. Implications for active tectonic configuration
We consider several hypotheses for the active tectonic architecture in the region to the east and west of 77°E. East of 77°E, our results suggest the increase in rock uplift rates across the second step represents: 1. A mid-crustal ramp with an active duplex, or 2. An emergent, out-of-sequence thrust fault and active duplex. West of 77°E, I hypothesize the single, sharp increase in rock uplift rate north of the Main Frontal Thrust is controlled by Quaternary uplift along the Main Boundary Thrust that soles into the decollement with (Fig. 8a) or without (Fig. 8b) a mid-crustal ramp. When taken together, the tectonic models presented here suggest the active structure east of 77°E merges westward along strike with the Main Boundary Thrust.
5.2.1. East of 77°E: Mid-crustal Ramp and Duplex model
East of 77°E, the relatively high rock uplift rates north of the PT2 compared to moderate rock
uplift rates south of the PT2 may be caused by the transition from a ramp to a flat along the decollement,
with a growing duplex, where rock uplift rates increase across the leading edge of the ramp (Fig. 8c). Similar to models that have been proposed for Central Nepal (e.g. Wobus et al., 2006; Harvey et al., 2015), the mid-crustal ramp may be growing by underthrusting and underplating of material from the Indian plate into a duplex (Fig. 8c) (Bollinger et al., 2004; Herman et al., 2010; Robert et al., 2009). Consequently, previous studies based on relationships between 10Be erosion rates and long-term
thermochronology (Morell et al., 2017) propose the PT2 has remained fixed ~100-km north of the Main
Frontal Thrust throughout the Quaternary. Therefore, the ratio of underthrusting and adding material to the orogenic wedge to overthrusting along the Main Himalayan Thrust must be closely balanced to maintain the position of the PT2. Additionally, the presence of a blind duplex above the mid-crustal ramp
would provide an explanation for the PT2 cross-cutting lithologic contacts since the duplex has yet to
reach the surface. Essentially, our results east of 77°E could reflect the westward continuation of the mid-crustal ramp and active duplex that has remained roughly 100 km north of the Main Frontal Thrust for hundreds of kilometers along strike from Nepal to Himachal Pradesh (e.g. Morell et al., 2015, 2017; Harvey et al., 2015; Qiu et al., 2016; Caldwell et al., 2013)
5.2.2. East of 77°E: Out-of-sequence, emergent fault model
Similar to other studies focusing on tectonic models east of 77°E (Whipple et al., 2016; Wobus et al., 2005), an out-of-sequence, emergent fault stemming from either a shallowly dipping decollement or a mid-crustal ramp could generate a rapid increase in rock uplift rates across the PT2 (Fig. 8d).
Additionally, a growing duplex north of the PT2 with internal thrusting could introduce the emergent fault
by reaching the surface from sustained slip during earthquakes marking the PT2. Similar to the discussion
and thermochronology that the PT2 has remained at the same relative position north of the Main Frontal
Thrust throughout the Quaternary (Morell et al., 2017). However, evidence for a surface-breaking fault along the PT2 is yet to be identified in field investigations and the PT2 appears to not coincide with
surface traces of nearby faults or lithologic contacts (Fig. 1). While the high amounts of vegetation could hide such evidence for an emergent fault in such a rugged landscape, slip on the fault that produces large magnitude earthquakes would likely form a scarp eventually to be identified in the field.
5.2.3. Tectonic configuration west of 77°E
Along strike into western Himachal Pradesh, our results indicate the PT2 terminates into the Main
Boundary Thrust (fig. 2-5) and therefore I hypothesize two tectonic models for the region west of 77°E (Fig. 8a-b). The tectonic configuration west of 77°E might be explained by sustained thrusting and uplift through the Quaternary on the Main Boundary Thrust with a mid-crustal ramp (Fig. 8a) or a nearly horizontal decollement (Fig. 8b).
Assuming the PT2 represents a mid-crustal ramp terminating into the Main Boundary Thrust near
77°E, one explanation for the tectonic arrangement west of 77°E is that the Main Boundary Thrust soles into the leading edge of the same ramp (Fig. 8a). The consistent along-strike spatial patterns presented in this thesis between the region north of the Dhauladar Range and the region north of the PT2 (Figs. 2-5)
support the minimal change rock uplift rates potentially reflecting a minimal change in decollement geometry or active structures. However, since our data cannot directly confirm the presence of a ramp extending downdip of the Main Boundary Thrust, this hypothesis relies solely on the PT2 representing the
continuation of the ramp into western Himachal Pradesh.
In the case the PT2 represents an unidentified, emergent fault, another option for the tectonic
Figure 8:
Cartoons representing kinematic models for west and east of 77°E in the northwest
Himalaya. A. Main Boundary Thrust soling to a mid-crustal ramp on the Main Himalayan Thrust
beneath the Dhauladar Range in western Himachal Pradesh. B. Main Boundary Thrust soling to a
nearly horizontal decollement extending beneath northwestern Himalaya (Deeken et al., 2011).
C. Mid-crustal ramp model with active duplex due to underplating beneath the PT
2(Caldwell et
al., 2013; Bollinger et al., 2005; Herman et al., 2009; Morell et al., 2015, 2017). D.
Out-of-sequence, emergent fault model with or without a mid-crustal ramp beneath the PT
2for the east
of 77°E (Wobus et al., 2006; Whipple et al., 2016; Morell et al., 2017). MHT - Main Himalayan
Thrust; MFT - Main Frontal Thrust; MBT - Main Boundary Thrust; MCT - Main Central Thrust;
PT
2- Physiographic Transition
2.
with the emergent fault at 77°E (Fig. 8d). In the region west of 77°E, high rock uplift rates concentrated along the Main Boundary Thrust where exhumation rates based on bedrock thermochronology (Deeken et al., 2013; Thiede et al., 2006) in addition to geophysical imaging (Powers et al., 1998) support a nearly horizontal decollement north of the Main Boundary Thrust (Fig. 1c). Furthermore, these studies indicate the Main Boundary Thrust extends for ~40 km northward before soling into a gently northward dipping Main Himalayan Thrust near a depth of ~8-10 km (Powers et al., 1998; Thiede et al., 2017). Accordingly, our results show consistently high channel steepness indices (Fig. 4), predicted erosion rates (Fig. 5), and mean topographic relief (Fig. 2-3) north of the Main Boundary Thrust, which may reflect the lack of across-strike changes in rock uplift rates and active structures along a gently dipping decollement. Considering the competing tectonic models for the regions east and west of 77°E, the preferred model for the change in tectonic configuration in the northwest Himalaya is the mid-crustal ramp model as the termination of the PT2 into the Main Boundary Thrust could simply represent the Main Boundary Thrust
soling into the ramp, whereas the data presented here hold little evidence for a change in decollement geometry as described in the second model (Figs. 8b and d).
5.2.4. Controlling factors on the change in tectonic configuration in Himachal Pradesh
A variety of structural factors could be controlling the along-strike changes in rock uplift rates in the Northwest Himalaya. These factors include the along-strike spatial distribution of subducting
basement ridges and troughs on the Indian plate. There is a striking coincidence between the along-strike trace of the PT2 (Fig. 1-5) and features on the downgoing plate as revealed by gravity anomalies (Fig. 9)
(Godin and Harris, 2014). In particular, the Shimla Lineament (Fig. 9), as identified by mid-wavelength Bouguer gravity anomalies (Godin and Harris, 2014), coincides with the along-strike changes that I have highlighted across longitude 77°E (Fig. 9), namely the western extent of the PT2 terminating into the
Main Boundary Thrust (Fig. 2-5). For example, the along-changes in channel steepness, topography, and erosion rates from the region south of the PT2 in the east to the region south of the Main Boundary
